photochemistry of saturn’s atmosphere · the photochemistry of oxygen compounds is discussed in a...

Icarus143, 244–298 (2000)

doi:10.1006/icar.1999.6270, available online at http://www.idealibrary.com on

Photochemistry of Saturn’s Atmosphere

I. Hydrocarbon Chemistry and Comparisons with ISO Observations

Julianne I. Moses

Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas 77058-1113E-mail: [email protected]

Bruno Bezard and Emmanuel Lellouch

DESPA, Observatoire de Paris, 92195 Meudon, France

G. Randall Gladstone

Space Sciences Department, Southwest Research Institute, San Antonio, Texas 78228-0510

Helmut Feuchtgruber

Max-Planck Institut fur Extraterrestrische Physik, 85740 Garching, Germany

and

Mark Allen

Earth and Space Science Division, Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, California 91109;and Division for Geological and Planetary Sciences, Caltech 170-25, Pasadena, California 91125

Received February 18, 1999; revised August 18, 1999

To investigate the details of hydrocarbon photochemistry onSaturn, we have developed a one-dimensional diurnally averagedmodel that couples hydrocarbon and oxygen photochemistry, molec-ular and eddy diffusion, radiative transfer, and condensation. Themodel results are compared with observations from the InfraredSpace Observatory (ISO) to place tighter constraints on molecu-lar abundances, to better define Saturn’s eddy diffusion coefficientprofile, and to identify important chemical schemes that control theabundances of the observable hydrocarbons in Saturn’s upper atmo-sphere. From the ISO observations, we determine that the columndensities of CH3, CH3C2H, and C4H2 above 10 mbar are 4+2

−1.5×1013 cm−2, (1.1± 0.3)× 1015 cm−2, and (1.2± 0.3)× 1014 cm−2, re-spectively. The observed ISO emission features also indicate C2H2

mixing ratios of 1.2+0.9−0.6× 10−6 at 0.3 mbar and (2.7± 0.8)× 10−7 at

1.4 mbar, and a C2H6 mixing ratio of (9± 2.5)× 10−6 at 0.5 mbar.Upper limits are provided for C2H4, CH2CCH2, C3H8, and C6H2.The sensitivity of the model results to variations in the eddy diffu-sion coefficient profile, the solar flux, the CH4 photolysis branchingratios, the atomic hydrogen influx, and key reaction rates are dis-cussed in detail. We find that C4H2 and CH3C2H are particularlygood tracers of important chemical processes and physical condi-tions in Saturn’s upper atmosphere, and C2H6 is a good tracer of theeddy diffusion coefficient in Saturn’s lower stratosphere. The eddy

diffusion coefficient must be smaller than∼3× 104 cm2 s−1 at pres-sures greater than 1 mbar in order to reproduce the C2H6 abundanceinferred from ISO observations. The eddy diffusion coefficients inthe upper stratosphere could be constrained by observations of CH3

radicals if the low-temperature chemistry of CH3 were better under-stood. We also discuss the implications of our modeling for aerosolformation in Saturn’s lower stratosphere—diacetylene, butane, andwater condense between ∼1 and 300 mbar in our model and willdominate stratospheric haze formation at nonauroral latitudes. Ourphotochemical models will be useful for planning observational se-quences and for analyzing data from the upcoming Cassini mission.c© 2000 Academic Press

Key Words: Saturn, atmosphere; photochemistry; atmospheres,composition; organic chemistry; infrared observations.

1. INTRODUCTION

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244

0019-1035/00 $35.00Copyright c© 2000 by Academic PressAll rights of reproduction in any form reserved.

Photochemistry plays a central role in the determinationhe physical and chemical state of planetary atmospheres.orption of solar radiation and photodissociation of atmosphases into their constituent molecules, radicals, and atoms

owed by subsequent chemical reactions between the photo

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HYDROCARBON PHOTOCHEMIS

products and other atmospheric molecules help control the cposition of the “visible” portion of planetary atmospheres. Tchemical composition in turn affects many physical aspectthe atmosphere such as its thermal structure, radiation baladynamical processes, ionospheric structure, and the formaof clouds and hazes. The upper atmospheric photochemistJupiter, Saturn, Uranus, and Neptune is reasonably straighward because methane alone of all the major atmosphericstituents is volatile enough and abundant enough on the oplanets that it can diffuse or be dynamically injected into theper atmospheres where it can interact with solar and corpuscradiation and instigate photochemistry.

The qualitative picture of hydrocarbon photochemistry onouter planets has been understood for years (see the revieStrobel 1975, 1983; Atreya 1986). The most recent major stof hydrocarbon photochemistry in outer-planetary atmosphis the comprehensive work of Gladstoneet al. (1996). TheirJupiter model, which is based in large part on the Titan moof Yung et al. (1984), consists of a fairly complete study of thproduction and loss mechanisms of C–C4 hydrocarbons (i.e.,molecules containing from one to four carbon atoms). The sbasic photochemical processes operating on Jupiter are belto pertain to Saturn as well (Strobel 1975, 1978), althoughferences in solar flux, stratospheric temperatures, methane adance, and the degree of vertical mixing can affect the quatative details. Few published Saturn models exist; those thaare generally included in studies of other outer-planetary atspheres in which Jupiter (and sometimes Uranus and Neptare the primary focus (e.g., Strobel 1975, 1983; Atreya andmani 1985; Atreya 1986). For details on Saturn photochemisone must extrapolate from the more detailed studies of the oouter planets (e.g., Gladstoneet al. 1996; Summers and Strobel 1989; Romani and Atreya 1988, 1989; Moses 1991; Moet al. 1995; Romaniet al. 1993; Bishopet al. 1998; Dobrijevicand Parisot 1998) or Titan (Yunget al. 1984, Toublancet al.1995, Laraet al. 1996). Moreover, most of the reaction ratand photolysis pathways in the Jupiter model of Gladstoneet al.(1996) are based on pre-1990 laboratory studies. Our knowleof the appropriate hydrocarbon reaction rates, absorption csections, and photolysis quantum yields has improved greatthe past decade, and the hydrocarbon photochemistry needsupdated for both Jupiter and Saturn. For a very recent discusof Saturn photochemistry, see Ollivieret al. (1999).

Saturn is now a particularly attractive target for theoretimodeling because recent observations have provided usmany new constraints on molecular abundances. Observering the Infrared Space Observatory (ISO)1 have identified strato-spheric CH3, C2H2, C2H6, CH3C2H, C4H2, CO2, and H2O in theinfrared spectrum of Saturn (de Graauwet al. 1997, Feuchtgrube
et al. 1997, Bezardet al. 1998). The ISO data have proven to be
1 ISO is an ESA project with instruments funded by ESA Member Sta(especially the PI countries: France, Germany, the Netherlands, and the UKingdom) with the participation of NASA and ISAS.

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TRY IN SATURN’S ATMOSPHERE 245

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valuable tool in allowing us to deduce the stratospheric comption of Saturn not only because of the large number of molecobserved but because the extensive wavelength coverage aall species to be observed simultaneously, helping to minimuncertainties in retrieved abundances. The high-quality ISOwill allow us to better constrain the eddy diffusion coefficiein Saturn’s atmosphere and enable us to examine and impspecific details of atmospheric chemistry. WithCassinion itsway to Saturn, model predictions regarding abundance variatwith altitude and latitude in Saturn’s atmosphere will be parularly useful, and the new observations and laboratory measments make it an opportune time to update the photochemmodels.

We have developed a one-dimensional steady-state mfor Saturn’s atmosphere that couples hydrocarbon and oxyphotochemistry, condensation, vertical diffusion, and radiatransport. The reaction list of Gladstoneet al. (1996) is ourstarting point for the hydrocarbon reactions, but we add atocarbon, C3, C3H, metastable excited diacetylene (C4H∗2), oxygenreactions, and a micrometeoritic or ring-derived source of ogen to the model and update virtually all the reaction ratessome reaction pathways based on recent laboratory meaments. The rate constant information in the extensive databof Baulchet al. (1992, 1994) and Mallardet al. (1994) havebeen very useful in compiling the necessary information. Tresults of our photochemical models are used to create syntspectra that are then compared directly with ISO observatio

The main purpose of this paper is to understand the quantive details of hydrocarbon photochemistry in Saturn’s strasphere and to examine how recent laboratory measuremhave changed our understanding of the important hydrocareaction schemes. The photochemistry of oxygen compouis discussed in a companion paper (Moseset al. 2000). In thefollowing sections, we discuss the details of the photochecal model, its numerical approach, and the various inputsboundary conditions required to run the model. We look atresults in terms of the photochemistry of the major hydrocbon compounds, and we examine the sensitivity of the resto various input parameters. We then use a radiative trancode to create synthetic spectra to directly compare the mresults with ISO observations. When necessary, we updateconstraints on the hydrocarbon abundances provided byComparisons withVoyagerUVS data (e.g., the variation of CH4

with altitude and the emission of H and He) are also presentehelp constrain the upper stratospheric eddy diffusion coefficprofile. Finally, we discuss the major implications of our modwith regard to atmospheric photochemistry, the eddy diffuscoefficient profile, the formation of stratospheric hazes, andrelevance of the model to theCassinimission.

2. OBSERVATIONAL CONSTRAINTS

We focus our modeling on Saturn’s nonauroral stratosph
and upper troposphere, where several hydrocarbon molecules

246 MOSES ET AL.

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TABLE ISaturn Observations

Species Mixing ratio Pressure (mbar) Latitude Date Technique Reference

CH3 1.0× 10−7 2× 10−4 global ave. 12/97 ISO–SWS B´ezardet al. (1998)

CH4 4.3× 10−3 0.2–10 (2) 28◦–42◦ 11/80 IRIS Courtinet al. (1984)2.5× 10−3 50–2000 various 1979 Pio. IPP Tomasko and Doose (1984)4.2× 10−3 50–2000 S. temperate 1970–1983 vis/near-IR Trafton (1985)2.2× 10−3 40–2000 ±18◦ 1980 vis/Voy. SSI Killen (1988)3.0× 10−3 1–3000 0◦–90◦ 1986–1989 vis/near-IR Karkoschka and Tomasko (191.7× 10−3 100–1500 global ave. 3/78 KAO near-IR Kerolaet al. (1997)9.1× 10−5 3.6× 10−5 29.5◦, 3.8◦ 8/81 UVS Smithet al. (1983)6.0× 10−5 2.6× 10−5 29.5◦, 3.8◦ 8/81 UVS Smithet al. (1983)5.8× 10−6 1.4× 10−5 29.5◦, 3.8◦ 8/81 UVS Smithet al. (1983)1.4× 10−4 2.6× 10−5 3.8◦ 8/81 UVS Festou and Atreya (1982)

C2H2 2.0× 10−7 0.6–15 (3) 28◦–42◦ 11/80 IRIS Courtinet al. (1984)3.6× 10−7 0.1–10 (1.1) ±45◦ 6/80 IRFP Nollet al. (1986)1.2× 10−7 2–22 (7) global ave. 1978–1980 IUE Chenet al. (1991)3.5× 10−6 0.1 global ave. 6/96 ISO–SWS de Graauwet al. (1997)2.5× 10−7 1 global ave. 6/96 ISO–SWS de Graauwet al. (1997)

C2H6 2.9× 10−6 0.16–4 (0.8) 28◦–42◦ 11/80 IRIS Courtinet al. (1984)8.0× 10−6 0.1–10 (1.1) ±45◦ 6/80 IRFP Nollet al. (1986)7.1× 10−6 0.1–8 (2) ±45◦ 11/95–12/95 IRGS Sadaet al. (1996)4.0× 10−6 0.15–5 (1) global ave. 6/96 ISO–SWS de Graauwet al. (1997)

CH3C2H 4.0× 10−10 0.3–10 (2) global ave. 6/96 ISO–SWS de Graauwet al. (1997)

C4H2 5.0× 10−11 0.3–10 (2) global ave. 6/96 ISO–SWS de Graauwet al. (1997)

CO 1.0× 10−9 2–9 bar (5 bar) global ave. 3/81 IR FTS Noll and Larson (1990)

CO2 3.0× 10−10 0.3–10 (2) global ave. 6/96 ISO–SWS de Graauwet al. (1997)

H2O 1.5× 10−7 0–2 global ave. 1978–1980 IUE Chenet al. (1991)6.0× 10−9 0.07–0.7 (0.2) global ave. 1996–1997 ISO–SWS Feuchtgruberet al. (1997)

Note.Pressure ranges are estimated, and central pressures are given in parentheses. Latitudes are positive northward.ratios quoted for CH3C2H and C4H2 are lower than those reported by de Graauwet al. (1997) due to an error in the way the intensitieof the hot vs fundamental bands were handled in the latter paper. The pressure levels quoted for the Courtinet al. (1984) C2H2 andC H IRIS measurements are roughly where most of the emission originates when assuming a constant mixing ratio pro
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and oxygen compounds have been identified from Earth-batelescopes and spacecraft instruments. Table I lists the set ovations that we use for our initial model-data comparisons. Tlist is not meant to be comprehensive; we have merely chosfew representative ultraviolet and infrared observations to hus make a first attempt at reproducing the data. Because Srepresents a small target to Earth-based telescopes, observare generally of the whole disk of Saturn and thus correspto global averages weighted toward middle and low latitudWe therefore develop our nominal model for the 30◦ N latituderegion. Note that the measurements in Table I were obtaiat different dates, using different techniques, and focuseddifferent latitude regions. For consistency, subsequent mowere developed to specifically make comparisons with the Iobservations.

The mixing ratios shown in Table I cannot always be takenface value. One difficulty is that most of the observational anyses assume a constant mixing ratio profile with altitude, whin reality, the mixing ratio profiles have a height variation th

from transport processes (e.g., diffusion and winds)

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from photochemical production and loss. The contribution futions of the observations thus change with the assumed sof the mixing ratio profile, and the altitude or pressure levapplicable to the observations are difficult to determine withresorting to more detailed modeling. Rather than fitting mels to the reported mixing ratios, a better way of checkingthe reliability of the photochemical models is to use forwamodeling techniques to simulate the observations based oresults of the photochemical models. We use this method inanalysis; the photochemical model results concerning moleconcentrations as a function of altitude are run through a rative transfer code to create synthetic infrared spectra. Thesethetic spectra are compared directly with the ISO spectra fSaturn (see Section 5). Details of the ISO observations are gbelow.

2.1. ISO Observations of Saturn

The Infrared Space Observatory recorded spectra of S
andbetween 2.4 and 16.2µm, using the grating mode of the

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HYDROCARBON PHOTOCHEMI

Short-Wavelength Spectrometer (SWS).2 Descriptions of theISO satellite and of the SWS spectrometer can be founKessleret al. (1996) and de Graauwet al. (1996) respectively. Afirst spectrum of the whole range, recorded on 15 June 1996presented in de Graauwet al. (1997). In the short-wavelengtrange (SW;λ<12.5µm) of the spectrometer, the resolvinpower allowed by the observing mode (AOT 01) increased fr∼1100 at 7.7µm to∼1700 at 12.5µm. In the long-wavelengthrange (LW;λ>12.5µm), the resolving power varied from approximately 1100 at 12.5µm up to 1500 at 16.2µm. On 6 De-cember 1996, a spectrum of Saturn covering the range 116.2µm was recorded in the AOT 06 observing mode, providan improved spectral resolution of 1350 at 12.5µm and 1750 at16.2µm. The total integration time was 32 min. In addition, oservations of a smaller interval (16.42–16.58µm) were carriedout on 30 December 1997 to search for emission from meradicals. The results were reported in B´ezardet al. (1998).

In the present analysis, we use the SW range of the 151996 spectrum (de Graauwet al. 1997), the more recentDecember 1996 spectrum for the LW region, and the CH3 ob-servations from B´ezardet al. (1998) near 16.5µm. In all datasets, the aperture (14′′ × 20′′ below 12.5µm, 14′′ × 27′′ above)was centered on Saturn’s disk with the long axis oriented acelestial north. Saturn’s equatorial diameter was 17′′–18′′, onlyslightly larger than the slit width. The ISO observations threpresent approximately disk-averaged conditions. The absaccuracy of the flux scale is≈20%.

3. PHOTOCHEMICAL MODEL INPUTS

3.1. Numerical Approach

We use the Caltech/JPL chemical kinetics and diffusion cto solve the coupled one-dimensional continuity equationsfunction of time and altitude for H, He, and all the carbon- aoxygen-bearing compounds in our Saturn model atmosp(e.g., Gladstoneet al. 1996, Yunget al. 1984, Allenet al. 1981).We allow the solutions to the coupled continuity equationsreach steady state, and we consider diurnally averaged quanfor the flux and the production and loss terms. Both eddymolecular diffusion are considered in the transport terms.

The continuity equations are solved using finite-differentechniques with 80 atmospheric levels and a vertical resoluof at least three altitude levels per scale height. Newton’s meis used to solve nonlinear chemistry. Calculations are perforuntil successive iterations differ by no more than 0.1%. A toof 62 different species are allowed to vary with vertical traport and with 431 different chemical reactions. The molecuhydrogen number density is not calculated from the continequations but is assumed to be equal to the bulk atmospdensity minus the calculated densities of H, He, and CH4.

At the lower boundary of the model (5 bar), the volume mixi
ratios of He and CH4 are fixed at 3.25× 10−2 and 4.5× 10−3,
2 The SWS instrument is a joint project of the SRON and the MPE.

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respectively (cf. Conrathet al. 1984, Courtinet al. 1984), andthe CO mixing ratio is fixed at 1.0× 10−9 (Noll and Larson1990). All other species are assumed to have a zero contration gradient at the lower boundary so that the speciestransported through the lower boundary at a maximum poble velocity given by the generalized diffusion coefficientvided by the generalized scale height. Note that becauselower boundary is so far below the tropopause, our choicthe boundary condition at the lower boundary has little effecconcentration profiles in the stratosphere. At the upper boun(3.8× 10−8 mbar), most atmospheric constituents are too heto escape thermally; chemical sources and sinks are expecbe negligible as well. Therefore, zero flux is assumed as an uboundary condition for most of the species in the model.one exception to this rule is atomic hydrogen, which is produby chemistry higher up in the thermosphere. For the atomicdrogen upper boundary condition, we impose a downwardof 1.1× 109 atoms cm−2 s−1, and we will discuss the sensitivitof our results to this choice in a later section.

Our photochemical model results pertain primarily tostratosphere. Although we present concentration profilesextend into the troposphere, the results become increasinglcertain at higher pressure levels due to our neglect of aescattering and absorption, multiple scattering by H2 and othergases, and photochemistry of NH3 and PH3.

3.2. Background Atmospheric Structure

The first step in developing a photochemical modelSaturn is to generate a hydrostatic-equilibrium backgromodel atmosphere that accurately depicts Saturn’s densitytemperature variations with altitude. Uncertainties in the tperature profile, planetary shape, rotation rate (including wspeeds), gravitational field, and variation of mean molecmass with altitude make this task challenging. Figure 1 shour adopted temperature profile. Details on the generation obackground atmospheric structure are given in Appendix A

3.3. Diffusion Coefficients

Large-scale vertical motions help homogenize the bulk ofvisible atmosphere of Saturn, allowing chemically inert speto be mixed uniformly with height. In the frame of a ondimensional model, vertical mixing processes can be paramized by a single macroscopic “eddy” diffusion coefficientK thatis variable with altitude. As the atmospheric density decreahowever, molecular diffusion begins to dominate. At this pothe composition varies greatly with altitude as the heavier cstituents become diffusively separated. The altitude at whichmolecular diffusion coefficient equals the eddy diffusion coecient is termed the homopause. Because the molecular diffucoefficients are different for different constituents, each spehas its own homopause level. Allusions to the “homopausegion in outer-planetary literature generally refer to the meth
homopause, as CH4 is the dominant photochemical constituent.

resultsfrom

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248 MOSES ET AL.

FIG. 1. The temperature profile adopted for our 30◦ N model for Saturn. The solid line describes our nominal model, the dashed line represents thefrom theVoyager 2RSS ingress occultation (Lindalet al. 1985, Lindal 1992) adjusted for a helium abundance of 3.3%, the dotted line is the profile obtainedthe 28 Sgr occultation data (Hubbardet al. 1997), and the triangles are from theVoyagerUVS occultation results of Smithet al. (1983) and Festou and Atreya(1982). The pressure levels for the latter data were obtained by converting the UVS radius–temperature information to the pressure at that radius obained fromour hydrostatic model (see Appendix A). Our profile, which was designed to mimic global-average behavior, is warmer than the RSS profile below∼6 mbar to

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better fit the ISO observations of the H2–He continuum beyond 13µm. The pro

Both molecular and eddy diffusion coefficients are modelput parameters that need to be specified before the contiequations can be solved.

Marrero and Mason (1972) have used experimental daderive expressions for the molecular diffusion coefficientsvarious atoms and molecules in a hydrogen atmosphereuse these experimentally derived expressions whenever pble. The molecular diffusion coefficient of CH4, the other hy-drocarbon molecules, and some of the oxygen molecules i2

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wheren is the total number density andT is the temperature othe atmosphere at any particular altitude, andmi is the mass ofthe diffusing species. The diffusion coefficients for He, H, Cand CO2 are taken directly from Marrero and Mason (1972). Tthermal diffusion factorsαi are estimated from experimental atheoretical information given in Chapman and Cowling (19and Grew and Ibbs (1952). For CH4 and other heavy moleculewe assume thatαi ≈ 0.25. For He,αi ≈ 0.145(1− ni /n) andfor H, αi ≈ −0.1(1− ni /n), whereni is the number density ospeciesi .

The eddy diffusion coefficient profile is one of the main frparameters in the model, although ground-based, ISO, andVoy-
agerdata help constrainK at several altitude levels in the atmo
le follows a dry adiabat below∼1 bar.

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sphere. In the lower stratosphere, we use the ISO hydrocaobservations to constrain the eddy diffusion coefficient proThe long-lived species such as C2H6 are particularly useful forthis purpose. At high altitudes, we consider the UVS CH4 oc-cultation results to be the most reliable indicator of the diffuscoefficients near the methane homopause, and we use the4

concentration profiles presented by Smithet al. (1983) to helpconstrain the eddy coefficient (both the slope and the magnitin the upper stratosphere. The major drawbacks to this chare the model dependence of the UVS analysis (cf. Jupiter Uanalyses of Yelleet al. 1996 and Festouet al. 1981) and uncertainties in the absolute altitude/pressure scale of the UVS d

In our nominal model, we adopt

K = 6.843× 106(1.941× 1013/n)0.3

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= 6.843× 106(1.941× 1013/n)0.69

for 1.941× 1013 ≤ n < 1.343× 1017 cm−3

= 1.5335× 104(1.343× 1017/n)0.87

for 1.343× 1017 ≤ n < 4.67× 1018 cm−3

= 7.0× 102

for 4.67× 1018 ≤ n ≤ 3.03× 1019 cm−3

= 2.5× 104

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for K in squared centimeters per second andn in per cubiccentimeters. The selection of the tropospheric diffusion cocient (2.5× 104 cm2 s−1) is based on the analysis of Edgingtet al. (1997) and has little effect on the stratospheric hydcarbon abundances. The selection of the stratospheric proexplained further in Section 4.5.

3.4. Solar Flux

The solar flux values used in our models were compiled fra variety of sources. To provide general predictions conceratmospheric chemistry, we use values that are typical of aage conditions during the solar cycle. The fluxes were binin 2-nm intervals at wavelengths below 122.5 nm (exceptindividual solar lines, which were assumed to be 0.1 nm wid5-nm intervals between 122.5 and 402.5 nm, and 10-nm invals at wavelengths longer than 402.5 nm. From 5 to 105the flux was taken from the solar minimum (July 1976) valuof Torr and Torr (1985); from 105 to 120 nm, we interpolatein Gladstoneet al. (1996); and from 120 to 305 nm, we use flvalues from the 12 May 1983 measurements of the Solar Mspheric Explorer satellite (R. T. Clancy, personal communicato M. Allen, 1989). Beyond 305 nm, we use values compiledthe World Meteorological Organization (WMO 1985). The soH Ly-α line at 121.6 nm is responsible for a large percentagthe methane dissociation on the outer planets; the Ly-α flux (at1 AU) in our nominal model is 3.21× 1011 photons cm−2 s−1 ina 0.1-nm interval centered at 121.57 nm. The model calculatwere for 30◦ N latitude with diurnally averaged fluxes and tseason held fixed under conditions relevant to mid-1996 (∼200days past southern vernal equinox).

3.5. Photochemical Reactions

Our modern view of outer-planetary methane photochemiwas first established by Strobel (1969), who emphasized theportance of transport of the heavier hydrocarbons to deeperhotter and denser) atmospheric regions where thermal deposition of these molecules can allow methane to be recycAlthough the quantitative details have changed since that tthe qualitative picture of Strobel (1969) is still viable. Moern photochemical models (e.g., Gladstoneet al. 1996) trackthe transport and kinetics of hydrocarbon molecules contaifrom one to as many as six carbon atoms.

Our model contains H (atomic hydrogen), He (helium),(atomic carbon), H2 (molecular hydrogen); major hydrocarbomolecules CH4 (methane), C2H2 (acetylene), C2H4 (ethylene),C2H6 (ethane), CH3C2H (methylacetylene), CH2CCH2 (allene),C3H6 (propylene), C3H8 (propane), C4H2 (diacetylene), C4H∗2(diacetylene in a metastable excited state), C4H4 (both viny-lacetylene and butatriene), 1-C4H6 (ethylacetylene), 1,2-C4H6

(methylallene), 1,3-C4H6 (bivinyl), C4H8 (butylene and otheforms), C4H10 (both iso- andn-butane), C6H6 (1,5-hexadiyne,
1,2-hexadien-5-yne, benzene, and other forms); hydrocarradicals CH (methylidyne),1CH2 (methylene in thea1A1
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bon

state),3CH2 (methylene in the ground state,X3B1), CH3 (me-thyl), C2, C2H (ethynyl), C2H3 (vinyl), C2H5 (ethyl), C3, C3H,C3H2, C3H3, C3H5, C3H7, C4H, C4H3, C4H5, C4H9, C6H, C6H2,C6H3, C8H2; major oxygen-bearing molecules O2 (molecularoxygen), H2O (water), CO (carbon monoxide), CO2 (carbondioxide), H2CO (formaldehyde), CH3OH (methanol), H2CCO(ketene), CH3CHO (acetaldehyde); and oxygen-bearing racals O, O(1D), OH, HCO, CH2OH, CH3O, HCCO, CH3CO,and C2H4OH. Also included in the model are water, diacetlene, and butane molecules in the condensed phase (i.e., H2O(s),C4H2(s), and C4H10(s)). Note that aside from C3H4 and C4H6, wedo not distinguish between different molecules with the sachemical formula; i.e., due to a lack of laboratory data, allpossible isomers are considered together. Meteoritic oxygeintroduced to the atmosphere in the form of H2O, CO, CO2,and CH3OH at high altitudes. Details are given in Moseset al.(2000).

Table II lists the set of 89 hydrocarbon photodissociationactions included in our model. Also presented in the tablethe photolysis rate coefficients for each reaction (determiat the top boundary of the model and at 1.1× 10−3 mbar), thewavelength region in which the photodissociation cross sectare relevant for each reaction, and the references for thosesections.

The branching ratios of the various possible CH4 photodis-sociation pathways at H Ly-α (and other wavelengths) are nwell determined. From measuring the kinetic energy spectof H atoms produced from methane photolysis, Mordauntet al.(1993) determine that CH4

hν−→CH3+H (R5) is the dominantphotolysis channel at Ly-α. However, Mordauntet al. also es-timate that∼25% of the CH3 fragments produced from thiprocess possess sufficient internal energy that they will rapundergo unimolecular decay into either CH+H2 or1,3CH2+H,with the relative efficiencies of these secondary decay procebeing unknown. Hecket al. (1996) confirm these results: thefind that the formation of H atoms from the photolysis chanR5 (CH4

hν−→CH3+H) is six times more important than the suof all the other channels that produce H atoms (R7, R8, andIn addition, from a study of the H2 photofragments, Hecket al.determine that channel R6 (CH4

hν−→ 1CH2+H2) is a factor of∼2 more important than channel R9 (CH4

hν−→CH+H+H2).By combining this information and by assuming that channR7 and R8 are unimportant, Hecket al. conclude that channeR6 is twice as important as R9, and channel R5 is six times mimportant that R9. A reasonable estimate for the initial braning ratios of CH4 photolysis at H Ly-α is thus 69% R5, 20%R6, 0% R7 and R8, and 11% R9. If R7 and R8 are also optive initially, then channels R6 and R9 will need to be reducwhile still keeping all the branching ratios consistent with tguidelines of Hecket al. (1996). For our nominal model, wassume that some of the CH3 formed in R5 ends up decayininto CH+H2 (Mordauntet al. 1993) so that our final branchinratios at Ly-α are 48% R5, 20% R6, 0% R7 and R8, and 32% RAlthough we have allowed the above branching ratios to h
some wavelength dependence, the dominance of Ly-α photons

250 MOSES ET AL.

TABLE IIHydrocarbon Photolysis Reactions

Photolysis rateJ(s−1)

Reaction at 10−8 mbar at 1.1× 10−3 mbar Wavelength (nm) Reference

R1 H2hν−→ 2H 2.4× 10−10 0 69≤ λ≤ 113 a

R2 3CH2hν−→ CH+H 5.5× 10−7 5.5× 10−7 99≤ λ≤ 198 a

R3 CH3hν−→ CH+H2 3.7× 10−9 2.6× 10−9 147≤ λ≤ 223 a

R4hν−→ 1CH2+H 1.4× 10−6 1.4× 10−6 147≤ λ≤ 153 a

R5 CH4hν−→ CH3+H 2.0× 10−8 3.6× 10−11 97≤ λ ≤ 163 a, b, c

R6hν−→ 1CH2+H2 1.0× 10−8 3.2× 10−11 75≤ λ≤ 163 a

R7hν−→ 1CH2+ 2H 1.2× 10−9 1.2× 10−13 75≤ λ≤ 129 a

R8hν−→ 3CH2+ 2H 1.5× 10−9 1.1× 10−13 75≤ λ≤ 133 a

R9hν−→ CH+H+H2 1.3× 10−8 3.9× 10−12 79≤ λ≤ 135 a

R10 C2H2hν−→ C2H+H 1.9× 10−8 2.5× 10−9 67≤ λ≤ 223 a, d, e, f, g, h

R11hν−→ C2+ H2 8.0× 10−9 2.1× 10−9 69≤ λ≤ 203 a, d, e, f, g, h

R12hν−→ C2H∗2 0 0

R13 C2H3hν−→ C2H2+H 2.3× 10−6 2.3× 10−6 415≤ λ≤ 425 a

R14 C2H4hν−→ C2H2+H2 7.3× 10−8 5.4× 10−8 93≤ λ≤ 203 a, i

R15hν−→ C2H2+ 2H 1.1× 10−7 6.6× 10−8 93≤ λ≤ 203 a, i

R16hν−→ C2H3+H 7.2× 10−9 6.8× 10−9 142≤ λ≤ 203 a, i

R17 C2H5hν−→ CH3+ 1CH2 1.3× 10−6 1.3× 10−6 232≤ λ≤ 256 a

R18 C2H6hν−→ C2H4+H2 2.5× 10−9 1.1× 10−10 93≤ λ≤ 163 a

R19hν−→ C2H4+ 2H 1.7× 10−8 1.4× 10−10 93≤ λ≤ 163 a

R20hν−→ C2H2+ 2H2 1.8× 10−8 3.3× 10−10 93≤ λ≤ 163 a

R21hν−→ CH4+ 1CH2 1.2× 10−8 8.1× 10−11 93≤ λ≤ 158 a

R22hν−→ 2CH3 3.7× 10−9 2.2× 10−11 93≤ λ≤ 158 a

R23 C3H2hν−→ C3+H2 1.0× 10−9 1.0× 10−9 (Est.)

R24 C3H3hν−→ C3H2+H 9.9× 10−6 9.9× 10−6 247≤ λ≤ 305 a, j

R25hν−→ C3H+H2 4.1× 10−7 4.1× 10−7 247≤ λ≤ 305 a, j

R26 CH3C2Hhν−→ C3H3+H 7.5× 10−8 7.2× 10−8 142≤ λ≤ 223 a, k, l

R27hν−→ C3H2+H2 1.2× 10−7 1.2× 10−8 105≤ λ≤ 193 a, k, l

R28hν−→ 1CH2+C2H2 1.3× 10−9 1.3× 10−9 192≤ λ≤ 223 a, k, l

R29 CH2CCH2hν−→ C3H3+H 6.8× 10−7 5.7× 10−7 120≤ λ≤ 253 a

R30hν−→ C3H2+H2 1.6× 10−7 1.3× 10−7 120≤ λ≤ 253 a

R31 C3H5hν−→ CH3C2H+H 5.0× 10−6 5.0× 10−6 197≤ λ≤ 256 a

R32hν−→ CH2CCH2+H 2.0× 10−5 2.0× 10−5 197≤ λ≤ 256 a

R33hν−→ C2H2+CH3 2.2× 10−6 2.2× 10−6 197≤ λ≤ 256 a

R34 C3H6hν−→ C3H5+H 2.4× 10−7 2.4× 10−7 162≤ λ≤ 203 a

R35hν−→ CH3C2H+H2 1.9× 10−8 6.4× 10−9 105≤ λ≤ 203 a

R36hν−→ CH2CCH2+H2 3.3× 10−8 8.5× 10−9 105≤ λ≤ 203 a

R37hν−→ C2H4+ 1CH2 1.6× 10−8 9.4× 10−9 105≤ λ≤ 203 a

R38hν−→ C2H3+CH3 1.8× 10−7 1.6× 10−7 105≤ λ≤ 203 a

R39hν−→ C2H2+CH4 2.4× 10−8 1.8× 10−8 105≤ λ≤ 203 a

R40 C3H8hν−→ C3H6+H2 1.7× 10−8 1.9× 10−9 120≤ λ≤ 168 a

R41hν−→ C2H6+ 1CH2 5.8× 10−9 2.0× 10−10 120≤ λ≤ 158 a

R42hν−→ C2H5+CH3 2.8× 10−8 8.3× 10−10 120≤ λ≤ 158 a

R43hν−→ C2H4+CH4 1.8× 10−8 4.6× 10−10 120≤ λ≤ 168 a

R44 C4H2hν−→ C4H+H 7.3× 10−8 5.6× 10−8 120≤ λ≤ 217 a, mhν −8 −8
R45 −→ C2H2+C2 3.8× 10 3.0× 10 120≤ λ≤ 217 a

e

),

HYDROCARBON PHOTOCHEMISTRY IN SATURN’S ATMOSPHERE 251

TABLE II—Continued

Photolysis rateJ(s−1)

Reaction at 10−8 mbar at 1.1× 10−3 mbar Wavelength (nm) Referenc

R46hν−→ 2C2H 1.3× 10−8 9.5× 10−9 120≤ λ≤ 217 a

R47hν−→ C4H∗2 9.2× 10−7 8.7× 10−7 120≤ λ≤ 260 a

R48 C4H4hν−→ C4H2+H2 8.8× 10−6 9.8× 10−6 167≤ λ≤ 233 a

R49hν−→ 2C2H2 2.4× 10−6 2.4× 10−6 167≤ λ≤ 233 a

R50 1-C4H6hν−→ C4H4+ 2H 7.5× 10−8 2.5× 10−8 105≤ λ≤ 208 a

R51hν−→ C3H3+CH3 7.8× 10−8 6.3× 10−8 105≤ λ≤ 223 a

R52hν−→ C2H5+C2H 2.8× 10−8 1.5× 10−8 105≤ λ≤ 223 a

R53hν−→ C2H4+C2H+H 2.3× 10−8 8.8× 10−9 105≤ λ≤ 188 a

R54hν−→ C2H3+C2H+H2 3.9× 10−8 6.0× 10−9 105≤ λ≤ 163 a

R55hν−→ 2C2H2+H2 1.5× 10−8 1.6× 10−9 105≤ λ≤ 163 a

R56 1,2-C4H6hν−→ C4H5+H 7.4× 10−8 7.2× 10−8 167≤ λ≤ 233 a

R57hν−→ C4H4+ 2H 3.2× 10−7 3.1× 10−7 167≤ λ≤ 203 a

R58hν−→ C3H3+CH3 4.0× 10−7 3.9× 10−7 167≤ λ≤ 233 a

R59hν−→ C2H4+C2H2 2.2× 10−8 2.2× 10−8 167≤ λ≤ 233 a

R60hν−→ C2H3+C2H2+H 3.2× 10−8 3.1× 10−8 167≤ λ≤ 213 a

R61hν−→ C2H3+C2H+H2 1.1× 10−8 1.0× 10−8 167≤ λ≤ 188 a

R62hν−→ 2C2H2+H2 4.6× 10−8 4.5× 10−8 167≤ λ≤ 233 a

R63 1,3-C4H6hν−→ C4H5+H 5.8× 10−6 5.8× 10−6 167≤ λ≤ 233 a

R64hν−→ C4H4+H2 1.0× 10−6 1.0× 10−6 167≤ λ≤ 233 a

R65hν−→ C3H3+CH3 8.4× 10−6 8.3× 10−6 167≤ λ≤ 233 a

R66hν−→ C2H4+C2H2 3.5× 10−6 3.5× 10−6 167≤ λ≤ 233 a

R67hν−→ 2C2H3 2.1× 10−6 2.1× 10−6 167≤ λ≤ 233 a

R68 C4H8hν−→ 1,3-C4H6+ 2H 1.4× 10−7 1.1× 10−7 105≤ λ≤ 203 a

R69hν−→ C3H5+CH3 3.8× 10−7 3.5× 10−7 105≤ λ≤ 203 a

R70hν−→ CH3C2H+CH4 1.6× 10−8 1.3× 10−8 105≤ λ≤ 203 a

R71hν−→ CH2CCH2+CH4 2.9× 10−8 8.9× 10−9 105≤ λ≤ 173 a

R72hν−→ C2H5+C2H3 5.7× 10−8 2.3× 10−8 105≤ λ≤ 183 a

R73hν−→ 2C2H4 3.9× 10−8 3.6× 10−8 105≤ λ≤ 203 a

R74hν−→ C2H2+ 2CH3 1.8× 10−8 1.4× 10−8 105≤ λ≤ 183 a

R75 C4H10hν−→ C4H8+H2 5.5× 10−8 4.6× 10−9 120≤ λ≤ 168 a

R76hν−→ C3H8+ 1CH2 2.7× 10−9 4.1× 10−11 120≤ λ≤ 143 a

R77hν−→ C3H6+CH4 5.5× 10−9 1.4× 10−10 120≤ λ≤ 168 a

R78hν−→ C3H6+CH3+H 1.3× 10−8 4.0× 10−10 120≤ λ≤ 168 a

R79hν−→ C2H6+C2H4 2.8× 10−8 1.1× 10−9 120≤ λ≤ 168 a

R80hν−→ 2C2H5 2.0× 10−8 8.4× 10−10 120≤ λ≤ 168 a

R81hν−→ C2H4+ 2CH3 1.4× 10−8 3.7× 10−10 120≤ λ≤ 168 a

R82 C6H2hν−→ C6H+H 7.3× 10−8 5.6× 10−8 =J44 (Est.)

R83hν−→ C4H+C2H 1.3× 10−8 9.5× 10−9 =J46 (Est.)

R84 C6H6hν−→ H+Prod 9.1× 10−8 9.0× 10−8 163≤ λ≤ 198 Est., n, o

R85hν−→ C4H2+C2H4 9.1× 10−9 9.0× 10−9 163≤ λ≤ 198 Est., n, o

R86hν−→ 2C3H3 4.6× 10−8 4.5× 10−8 163≤ λ≤ 198 Est., n, o

R87hν−→ 3C2H2 7.6× 10−7 7.5× 10−7 163≤ λ≤ 198 Est., n, o

R88 C8H2hν−→ C6H+C2H 1.3× 10−8 9.5× 10−9 =J46 (Est.)

R89hν−→ 2C4H 1.3× 10−8 9.5× 10−9 =J46 (Est.)

Note. References: (a) Gladstoneet al. (1996), (b) Mordauntet al. (1993), (c) Hecket al. (1996), (d) R. Wu, personal communication 1997, (e) Smithet al.(1991), (f) Benilanet al. (1995), (g) Segallet al. (1991), (h) Satyapal and Bersohn (1991), (i) Balkoet al. (1992), ( j) Fahret al. (1997), (k) Seki and Okabe (1992
(l) Payne and Stief (1972), (m) Fahr and Nayak (1994), (n) Pantoset al. (1978), (o) Malkin (1992).

252 MOSES ET AL.

TABLE IIIHydrocarbon Chemical Reactions

Reactiona Rate constantb Reference

R90 H+HM−→ H2 k0= 2.7× 10−31 T−0.6 Baulchet al. (1994)

R91 H+CH −→ C+H2 1.3× 10−10 e(−80/T) Hardinget al. (1993)R92 H+ 1CH2 −→ CH+H2 2.0× 10−10 EstimateR93 H+ 3CH2 −→ CH+H2 2.66× 10−10 Boullart and Peeters (1992)

R94M−→ CH3 k0= 3.4× 10−32 e(736/T) Est. based on 0.1×R140

k∞= 7.3× 10−12 Est. based on 0.1×R140

R95 H+CH3M−→ CH4 k0= 2.3× 10−17 T−4.03 e(−1366/T) Brouardet al. (1989) and

k0= 2.52× 10−29, T ≤ 300 K estimate; see textk∞= 1.14× 107 T−5.72 e(−1644/T) Brouardet al. (1989) andk∞= 3.23× 10−10, T ≤ 280 K estimate; see text

R96 H+CH4 −→ CH3+H2 6.4× 10−18 T2.11 e(−3900/T) Rabinowitzet al. (1991)

R97 H+C2HM−→ C2H2 k0= 1.26× 10−18 T−3.1 e(−721/T) Tsang and Hampson (1986)

k∞= 3.0× 10−10 Tsang and Hampson (1986)R98 H+C2H2 −→ C2H+H2 1.0× 10−10 e(−11200/T) Tsang and Hampson (1986)

R99M−→ C2H3 k0= 8.2× 10−31 e(−352/T) Hoyermannet al. (1968) and

k∞= 1.4× 10−11 e(−1300/T) Gordonet al. (1978)Baulchet al. (1994)

R100 H+C2H3 −→ C2H2+H2 2.0× 10−11 Baulchet al. (1994)

R101M−→ C2H4 k0= 5.5× 10−27 Fahret al. (1991) and

k∞= 1.82× 10−10 Monkset al. (1995); see text

R102 H+C2H4M−→ C2H5 k0= 1.3× 10−29 e(−380/T) Baulchet al. (1994)

k∞= 6.6× 10−15 T1.28 e(−650/T) Baulchet al. (1994)R103 H+C2H5 −→ 2CH3 1.25× 10−10 Sillesenet al. (1993)R104 −→ C2H4+H2 3.0× 10−12 Tsang and Hampson (1986)

R105M−→ C2H6 k0= 5.5× 10−22 T−2 e(−1040/T) 10×Gladstoneet al. (1996)

k∞= 2.6× 10−10 Est. based on Sillesenet al. (1993)R106 H+C2H6 −→ C2H5+H2 2.35× 10−15 T1.5 e(−3725/T) Baulchet al. (1992)

R107 H+C3H2M−→ C3H3 k0= 2.52× 10−28 Est. based on 10×R95

k∞= 5.0× 10−11 Estimate

R108 H+C3H3M−→ CH3C2H k0= 5.5× 10−27 Est. based on R101

k∞= 1.15× 10−10 e(−276/T) Homann and Wellmann (1983)

R109M−→ CH2CCH2 k0= 5.5× 10−27 Est. based on R101

k∞= 1.15× 10−10 e(−276/T) Est. based on R108R110 H+CH3C2H −→ CH3+C2H2 9.63× 10−12 e(−1560/T) Wagner and Zellner (1972a)

R111M−→ C3H5 k0= 2.0× 10−29 Est., Whytocket al. (1976)

k∞= 3.98× 10−11 e(−1152/T) Whytocket al. (1976)R112 H+CH2CCH2 −→ CH3C2H+H 4.0× 10−12 e(−1006/T) Est., Wagner and Zellner (1976b)

R113M−→ C3H5 k0= 2.0× 10−29 Est. based on R111

k∞= 1.0× 10−11 e(−1006/T) Est., Wagner and Zellner (1972b)R114 H+C3H5 −→ CH3C2H+H2 1.4× 10−11 Est. based on Tsang (1991)R115 −→ CH2CCH2+H2 1.4× 10−11 Est. based on Tsang (1991)R116 −→ CH3+C2H3 1.4× 10−11 Estimate

R117 H+C3H5M−→ C3H6 k0= 2.0× 10−28 Est. based on 10×R111

k∞= 2.8× 10−10 Hanning-Lee and Pilling (1992)R118 H+C3H6 −→ C3H5+H2 2.87× 10−19 T2.5 e(−1254/T) Tsang (1991)R119 −→ CH3+C2H4 2.2× 10−11 e(−1641/T) Tsang (1991)

R120M−→ C3H7 k0= 1.3× 10−28 e(−380/T) Est. based on 10×R102

k∞= 2.2× 10−11 e(−785/T) Tsang (1991)R121 H+C3H7 −→ C3H6+H2 3.0× 10−12 Tsang (1988)R122 −→ C2H5+CH3 6.0× 10−11 Tsang (1988)

R123M−→ C3H8 k0= 5.5× 10−22 T−2 e(−1040/T) Est. based on R105

k∞= 2.49× 10−10 Munk et al. (1986)R124 H+C3H8 −→ C3H7+H2 2.2× 10−18 T2.54 e(−3400/T) Tsang (1988)

R125 H+C4HM−→ C4H2 k0= 1.26× 10−18 T−3.1 e(−721/T) Est. based on R97

k∞= 3.0× 10−10 Est. based on R97

5)


TABLE III—Continued


R126 H+C4H2M−→ C4H3 k0= 1.0× 10−28 Est., Yunget al. (1984)

k∞= 1.39× 10−10 e(−1184/T) Navaet al. (1986)R127 H+C4H3 −→ 2C2H2 1.5× 10−11 Est. based on R100R127a −→ C4H2+H2 5.0× 10−12 Est. based on R100

R128M−→ C4H4 k0= 6.0× 10−30 e(1680/T) Est. based on 0.1×R158

k∞= 8.56× 10−10 e(−405/T) Duranet al. (1988)

R129 H+C4H4M−→ C4H5 k0= 6.0× 10−31 e(1680/T) Est. based on 0.01×R158

k∞= 3.3× 10−12 Est., Schwanebeck and Warnatz (197R130 H+C4H5 −→ C4H4+H2 2.0× 10−11 Est. based on R100

R131M−→ 1-C4H6 k0= 6.0× 10−30 e(1680/T) Est. based on 0.1×R158

k∞= 1.0× 10−10 Gladstoneet al. (1996)R132 H+C4H9 −→ C4H8+H2 1.5× 10−12 Tsang (1990)

R133M−→ C4H10 k0= 6.0× 10−30 e(1680/T) Est. based on 0.1×R158

k∞= 6.0× 10−11 Tsang (1990)

R134 H+C6H2M−→ C6H3 k0= 1.0× 10−28 Est. based on R126

k∞= 1.39× 10−10 e(−1184/T) Est. based on R126R135 H+C6H3 −→ C6H2+H2 2.0× 10−11 Est. based on R127R136 H+C8H3 −→ C8H2+H2 2.0× 10−11 Est. based on R127

R137 C+H2M−→ 3CH2 k0= 7.0× 10−32 Husain and Young (1975)

k∞= 2.06× 10−11 e(−57/T) Hardinget al. (1993)

R138 C+C2H2M−→ C3H2 k0= 1.0× 10−31 Estimate

k∞= 2.1× 10−10 Haider and Husain (1993)R139 CH+H2 −→ 3CH2+H 3.75× 10−10 e(−1662/T) Beckeret al. (1991)

R140M−→ CH3 k0= 3.4× 10−31 e(736/T) Beckeret al. (1991)

k∞= 7.3× 10−11 Beckeret al. (1991)R141 CH+CH4 −→ C2H4+H 5.0× 10−11 e(200/T) Berman and Lin (1983)R142 CH+C2H2 −→ C3H2+H 3.49× 10−10 e(61/T) Bermanet al. (1982)R143 CH+C2H4 −→ C2H2+CH3 2.23× 10−10 e(173/T) Bermanet al. (1982)R144 CH+C2H6 −→ C3H6+H 1.8× 10−10 e(132/T) Berman and Lin (1983)R145 1CH2+H2 −→ 3CH2+H2 1.26× 10−11 Braunet al. (1970); andR146 −→ CH3+H 9.24× 10−11 Langfordet al. (1983)R147 1CH2+CH4 −→ 3CH2+CH4 1.2× 10−11 Bohlandet al. (1985b)R148 −→ 2CH3 5.9× 10−11 Bohlandet al. (1985b)R149 23CH2 −→ C2H2+ 2H 1.8× 10−10 e(−400/T) Baulchet al. (1992)R150 3CH2+CH3 −→ C2H4+H 7.0× 10−11 Baulchet al. (1992)R151 3CH2+CH4 −→ 2CH3 7.1× 10−12 e(−5051/T) Bohlandet al. (1985a)R152 3CH2+C2H2 −→ C3H2+H2 5.0× 10−12 e(−3332/T) Bohlandet al. (1986)R153 −→ C3H3+H 1.5× 10−11 e(−3332/T) Bohlandet al. (1986)

R154M−→ CH3C2H k0= 6.0× 10−29 e(1680/T) Est. based on R158

k∞= 2.0× 10−12 e(−3330/T) Estimate based on R153R155 3CH2+C2H3 −→ C2H2+CH3 8.0× 10−11 EstimateR156 3CH2+C2H5 −→ C2H4+CH3 8.0× 10−11 EstimateR157 CH3+H2 −→ CH4+H 6.6× 10−20 T2.24 e(−3220/T) Rabinowitzet al. (1991)

R158 2CH3M−→ C2H6 k0= 6.0× 10−29 e(1680/T) MacPhersonet al. (1983)

k∞= 6.0× 10−11 Baulchet al. (1992)R159 CH3+C2H3 −→ CH4+C2H2 3.4× 10−11 Fahret al. (1991)

R160M−→ C3H6 k0= 6.0× 10−28 e(1680/T) Est. based on 10×R158

k∞= 1.2× 10−10 Fahret al. (1991)R161 CH3+C2H5 −→ CH4+C2H4 2.0× 10−12 Baulchet al. (1992)

R162M−→ C3H8 k0= 1.01× 10−22 e(341/T), T ≤ 200 K Gladstoneet al. (1996)

k0= 2.22× 10−26 e(2026/T), T > 200 K Gladstoneet al. (1996)k∞= 6.64× 10−11 Sillesenet al. (1993)

R163 CH3+C3H3M−→ 1,2-C4H6 k0= 6.0× 10−28 e(1680/T) Est. based on 10×R158

k∞= 4.2× 10−12 Est., Wu and Kern (1987)

R164M−→ 1-C4H6 k0= 6.0× 10−28 e(1680/T) Est. based on 10×R158

k∞= 4.2× 10−12 Est., Wu and Kern (1987)−12 −0.32 (66/T)
R165 CH3+C3H5 −→ CH4+CH3C2H 2.5× 10 T e Tsang (1991)

254 MOSES ET AL.



R166 −→ CH4+CH2CCH2 2.5× 10−12 T−0.32 e(66/T) Tsang (1991)

R167M−→ C4H8 k0= 7.12× 10−22 e(715/T), T ≤ 200 K Est. based on R171

k0= 4.57× 10−24 e(2184/T), T > 200 K Est. based on R171k∞= 6.5× 10−11 Garland and Bayes (1990)

R168 CH3+C3H6 −→ CH4+C3H5 2.32× 10−13 e(−4390/T) Kinsman and Roscoe (1994)


k∞= 1.34× 10−13 e(−3330/T) Kinsman and Roscoe (1994)R170 CH3+C3H7 −→ CH4+C3H6 1.9× 10−11 T−0.3 Tsang (1988)

R171M−→ C4H10 k0= 7.12× 10−22 e(715/T), T ≤ 200 K Lauferet al. (1983)

k0= 4.57× 10−24 e(2184/T), T > 200 K Lauferet al. (1983)k∞= 3.2× 10−10 T−0.32 Tsang (1988)

R172 CH3+C3H8 −→ CH4+C3H7 1.5× 10−24 T3.7 e(−3600/T) Tsang (1988)R173 CH3+C4H5 −→ CH4+C4H4 3.4× 10−11 Est. based on R159

R174M−→ prod k0= 7.12× 10−22 e(715/T), T ≤ 200 K Est. based on R171

k0= 4.57× 10−24 e(2184/T), T > 200 K Est. based on R171k∞= 3.2× 10−10 T−0.32 Est. based on R171

R175 C2+H2 −→ C2H+H 1.77× 10−10 e(−1469/T) Pittset al. (1982)R176 C2+CH4 −→ C2H+CH3 5.05× 10−11 e(−297/T) Pittset al. (1982)R177 C2H+H2 −→ C2H2+H 1.2× 10−11 e(−998/T) Opansky and Leone (1996b)R178 C2H+CH4 −→ C2H2+CH3 1.2× 10−11 e(−491/T) Opansky and Leone (1996a)R179 C2H+C2H2 −→ C4H2+H 1.1× 10−10 e(28/T) Pedersenet al. (1993)R180 C2H+C2H4 −→ C4H4+H 7.8× 10−11 e(134/T) Opansky and Leone (1996b)R181 C2H+C2H6 −→ C2H2+C2H5 3.5× 10−11 e(3/T) Opansky and Leone (1996b)R182 C2H+C4H2 −→ C6H2+H 1.1× 10−10 e(28/T) Est. based on R179R183 C2H+C4H10 −→ C4H9+C2H2 1.0× 10−11 Tsang (1990)R184 C2H+C6H2 −→ C8H2+H 1.1× 10−10 e(28/T) Est. based on R179R185 C2H+C8H2 −→ prod 1.1× 10−10 e(28/T) Est. based on R179R186 C2H∗2+H2 −→ C2H2+H2 0R187 C2H∗2+CH4 −→ C2H2+CH4 0R188 C2H∗2+C2H2 −→ C4H2+H2 0R189 C2H∗2+C4H2 −→ C6H2+H2 0R190 C2H3+H2 −→ C2H4+H 5× 10−20 T2.63 e(−4298/T) Fahret al. (1995)R191 C2H3+C2H2 −→ C4H4+H 3.31× 10−12 e(−2516/T) Fahr and Stein (1988)


k∞= 4.17× 10−19 T1.9 e(−1058/T) Weissman and Benson (1988)R193 2C2H3 −→ C2H4+C2H2 2.4× 10−11 Fahret al. (1991)

R194M−→ 1,3-C4H6 k0= 6.0× 10−28 e(1680/T) Est. based on 10×R158

k∞= 1.2× 10−10 Fahret al. (1991)R195 C2H3+C2H4 −→ 1−C4H6+H 1.05× 10−12 e(−1559/T) Fahr and Stein (1988)R196 C2H3+C2H5 −→ 2C2H4 8.0× 10−13 Tsang and Hampson (1986)R197 −→ C2H6+C2H2 8.0× 10−13 Tsang and Hampson (1986)R198 −→ CH3+C3H5 see text Tsang and Hampson (1986)

R199M−→ C4H8 k0= 6.0× 10−28 e(1680/T) Est. based on 10×R158

k∞= see text Tsang and Hampson (1986)R200 C2H5+H2 −→ C2H6+H 5.1× 10−24 T3.6 e(−4253/T) Tsang and Hampson (1986)R201 2C2H5 −→ C2H6+C2H4 2.4× 10−12 Baulchet al. (1992)

R202M−→ C4H10 k0= 1.55× 10−22 e(586/T), T ≤ 200 K Lauferet al. (1983)

k0= 5.52× 10−24 e(1253/T), T > 200 K Lauferet al. (1983)k∞= 1.4× 10−11 e(35/T) Gladstoneet al. (1996)

R203 C3+H2 −→ C3H+H 1.0× 10−14 EstimateR204 C3H+H2 −→ C3H2+H 1.0× 10−14 Estimate

R205 C3H2+C2H2M−→ prod k0= 6.0× 10−31 e(1680/T) Est. based on R158

k∞= 2.0× 10−11 e(−3330/T) Estimate based on 0.01×R154R206 C3H2+C2H3 −→ C3H3+C2H2 8.0× 10−11 EstimateR207 C3H2+C2H5 −→ C3H3+C2H4 8.0× 10−11 Estimate

R208 2C3H3M−→ C6H6 k0= 6.0× 10−28 e(1680/T) Est. based on 10×R158

−10
k∞= 1.2× 10 Morteret al. (1994)

)




R209 C3H5+H2 −→ C3H6+H 5.25× 10−11 e(−9913/T) Allara and Shaw (1980)R210 C3H7+H2 −→ C3H8+H 3.0× 10−21 T2.84 e(−4600/T) Tsang (1988)R211 C4H+H2 −→ C4H2+H 1.2× 10−11 e(−998/T) Est. based on R177R212 C4H+CH4 −→ C4H2+CH3 1.2× 10−11 e(−491/T) Est. based on R178R213 C4H+C2H2 −→ C6H2+H 2.5× 10−11 Brachholdet al. (1988)R214 C4H+C2H6 −→ C4H2+C2H5 3.5× 10−11 e(3/T) Est. based on R181R215 C4H+C4H2 −→ C8H2+H 1.1× 10−10 e(28/T) Est. based on R179R216 C4H+C6H2 −→ prod 1.1× 10−10 e(28/T) Est. based on R179R217 C4H+C8H2 −→ prod 1.1× 10−10 e(28/T) Est. based on R179R218 C4H∗2 −→ C4H2 1.0× 103 Zwier and Allen (1996)R219 C4H∗2+H2 −→ C4H2+H2 1.4× 10−15 Zwier and Allen (1996)R220 C4H∗2+CH4 −→ C4H2+CH4 1.4× 10−15 Zwier and Allen (1996)R221 C4H∗2+C2H2 −→ C6H2+H2 1.75× 10−13 Zwier and Allen (1996)R222 −→ C6H2+ 2H 1.75× 10−13 Zwier and Allen (1996)R223 C4H∗2+C2H4 −→ H+ prod 9.8× 10−14 Zwier and Allen (1996)R224 −→ H2+ prod 3.69× 10−13 Zwier and Allen (1996)R225 C4H∗2+CH3C2H −→ H2+ prod 1.59× 10−13 Zwier and Allen (1996)R226 −→ C2H2+ prod 2.46× 10−13 Zwier and Allen (1996)R227 −→ C2H3+ prod 8.68× 10−14 Zwier and Allen (1996)R228 −→ C6H2+CH3+H 2.31× 10−13 Zwier and Allen (1996)R229 C4H∗2+C3H6 −→ H2+ prod 1.63× 10−13 Zwier and Allen (1996)R230 −→ CH3+H+ prod 3.76× 10−13 Zwier and Allen (1996)R231 −→ C2H2+ prod 2.29× 10−13 Zwier and Allen (1996)R232 C4H∗2+C4H2 −→ C6H2+C2H2 8.17× 10−13 Zwier and Allen (1996)R233 −→ C8H2+ 2H 2.57× 10−13 Zwier and Allen (1996)R234 −→ C8H2+H2 2.57× 10−13 Zwier and Allen (1996)R235 −→ C8H3+H 1.0× 10−12 Zwier and Allen (1996)R236 C4H5+H2 −→ 1−C4H6+H 6.61× 10−15 T0.5 e(−1864/T) Weissman and Benson (1988R237 C4H5+C2H2 −→ C6H6+H 3.16× 10−17 T1.47 e(−2471/T) Westmorelandet al. (1989)R238 C6H+H2 −→ C6H2+H 1.2× 10−11 e(−998/T) Est. based on R177R239 C6H+CH4 −→ C6H2+CH3 1.2× 10−11 e(−491/T) Est. based on R178R240 C6H+C2H2 −→ C8H2+H 1.1× 10−10 e(28/T) Est. based on R179R241 C6H+C2H6 −→ C6H2+C2H5 3.5× 10−11 e(3/T) Est. based on R181R242 C6H+C4H2 −→ prod 1.1× 10−10 e(28/T) Est. based on R179R243 C6H+C6H2 −→ prod 1.1× 10−10 e(28/T) Est. based on R179R244 C6H+C8H2 −→ prod 1.1× 10−10 e(28/T) Est. based on R179

a M represents any third body, prod represents higher order products.b 3 −1
Two-body rate constants and high-pressure-limiting rate constants for three-body reactions (k∞) are in units of cm s . Low-pressure-limiting rate constants
6 −1

e4

coa

wlt,

rra

s inrel-

e inay be

as-

eval-

for three-body reactions (k0) are in units of cm s .

in dissociating CH4 on Saturn is clear from Table II; the relativJ values for R5–R9 at the top of the atmosphere are withinof the branching ratios adopted for Lyα photolysis.

As with the Jupiter model of Gladstoneet al. (1996), weconsider a fairly complete set of bimolecular and termolelar chemical reactions for hydrocarbons containing fromto four carbon atoms. The complete list of hydrocarbon retions is shown in Table III. As can be seen from Table III,were obliged (because of a lack of laboratory or theoreticaformation) to estimate the rate coefficients for several ofreactions. For the reactions in which measurements existrate coefficients have seldom been investigated at the verytemperatures relevant to Saturn’s stratosphere, and the extlation to such low temperatures is not known. Therefore, launcertainties in the rate coefficients often exist. A system
investigation of all the rate coefficients is beyond the scope
%

u-nec-ein-hethelowapo-getic

this work. We do, however, examine the effects of variationcertain key reaction rates (see Section 4.6). Note that theative rates of certain reactions often play an important roldetermining species abundances, and these relative rates mmore easily determined than the absolute rates.

For the three-body (termolecular) addition reactions, wesume that the rate constants (in cm6 s−1) obey the form

k = k0k∞k0[M ] + k∞

, (1)

wherek0 is the low-pressure three-body limiting value (cm6 s−1),k∞ is the high-pressure limiting value (cm3 s−1), and [M ] is thetotal atmospheric density (cm−3). More accurate formulas havbeen suggested for interpolation between the two limiting
ofuesk0 andk∞ (e.g., Troe 1977, DeMoreet al. 1992); however,

am

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o

it0p

o

.

d

o

e.t

a

eind

a

e

o

s

fe

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al

72a)en-thats

not

rast,re-

sLee

3),on ofb),

neerantsratend

tainionsin

tedost

eranasnd

256 MOSES

sufficient information is often not available to determine the vues of several of the parameters that make up the more cocated expressions. For this reason, and because Gladstoneet al.(1996) have demonstrated that the use of these more compliexpressions has minimal effect on the resulting concentratof the major hydrocarbon compounds in their model, we hchosen to use the simpler expression.

For the important combination reaction CH3+HM−→CH4

(R95), we use the interpolation expression from Eq. (1) alwith the actual rate measurements in Brouardet al. (1989) to de-rive the temperature-dependent low- and high-pressure limrate constants shown in Table III. Our expression fits the 3600 K data of Brouardet al. (1989) reasonably well. The extraolation to colder temperatures is not known, however, andsimply assume no temperature dependence below∼300 K; thisassumption was adopted mainly to avoid an unphysical turnin the rates at low temperatures.

For R99, H+C2H2M−→C2H3, bothk0 andk∞ are uncertain

We use an intermediatek∞ value recommended by Baulchet al.(1994). Fork0, we use the temperature dependence deriveHoyermannet al. (1968), but tie the expression to the rate costant derived by Gordonet al. (1978).

As we will show in later sections, the C2H6/C2H2 ratio inSaturn’s lower stratosphere is very sensitive to two reactiR101 and R102, that hinder the efficiency of C2H2 recycling.Both reactions have been well studied in the laboratory at tperatures higher than is relevant for Saturn’s atmospherereaction R101, we use the expression from Eq. (1) to fitH+C2H3 data of Monkset al. (1995) and Fahret al. (1991)(reactions R100, R101), although we first assume thatk100 re-mains fixed at the value recommended by Baulchet al. (1994),2× 10−11 cm3 s−1. Our expression (see Table III) providesexcellent fit to the total rate (k100 + k101) measured by bothMonkset al. (1995) and Fahret al. (1991). On the other handthe branching ratios we have adopted for R100 and R101not consistent with the branching ratios inferred from expments involving hydrogen atoms reacting with deuterated vradicals (Monkset al. 1995) in which reaction R100 was founto be more important (especially at low temperatures). Becit is not clear that the reaction of H with C2D3 would behavethe same way as H+C2H3, we let reaction R101 dominate ovR100 (despite the possible inconsistency with Monkset al. 1995)to provide for a mechanism for converting C2H2 to C2H6 in Sat-urn’s stratosphere. For the combination reaction of H+C2H4

(R102), we use the low- and high-pressure limiting rate cstants suggested by Baulchet al. (1994) along with Eq. (1) todetermine the rate constant for reaction R102. Our expresresults in rates that are slightly higher than those measurethe laboratory by Lightfoot and Pilling (1987). Again, we prethe higher rates to keep open a pathway for converting C2H2 toC2H6 in Saturn’s lower stratosphere. A similar conclusion wreached by Allenet al. (1992).

Our expression for the limiting rate constants of the com
nation of H with C2H5 (R105) along with Eq. (1) fits the rate
ET AL.

l-pli-

atedonsve

ng

ing0–-we

ver

byn-

ns,

m-Forhe

n

,areri-yl

use

r

n-

iond inr

as

bi-

constant of 1.66× 10−10 cm3 s−1 determined by Sillesenet al.(1993) for 298 K and 100 mbar of H2.

The photochemistry of methylacetylene (CH3C2H) and al-lene (CH2CCH2) has not been well determined in the labortory, and the important production and loss mechanismsnot certain. Wagner and Zellner (1972a) have studied theaction of methylacetylene with H atoms (R110, R111). Thfind two possible reaction channels: (a) H+CH3C2H→CH3–C==CH∗2 (i.e., 1-C3H5) followed by collisional stabilization of1-C3H5 (R111) or (b) H+CH3C2H→CH3–CH==CH∗ (i.e., 2-C3H5) followed by fragmentation into CH3 and C2H2 (R110).The first channel dominates the production of C3H5 in our model;therefore, the 1-methylvinyl radical rather than the allyl radicmay be the dominant form of C3H5 in Saturn’s atmosphere. Wehave chosen the rate constants of Wagner and Zellner (19and Whytocket al. (1976) for R110 and R111, although thlimiting low-pressure rate constant for R111 is not well costrained. Wagner and Zellner (1972a) have also determinedthe reaction of H with 1-C3H5 results in four possible channel(R114–R117). At 326 K and 15 torr, they find thatk114≈ k115

and (k116+ k117)/(k114+ k115)≈ 0.7. The relative importanceof R116 and R117 under their experimental conditions isclear, although they suggest that the decrease in the C3H6 yieldand the corresponding increase in the CH3 yield at low pres-sures indicates that both channels are in operation. In contHanning-Lee and Pilling (1992) find that the high-pressulimiting rate constant for the addition of H with allyl is quitelarge (2.8× 10−10 cm3 s−1), and Baulchet al. (1994) recom-mend that (k114+ + k117,∞)≈ 0.1. Our choice of rate constantfor these series of reactions is more consistent with Hanning-and Pilling (1992) and Baulchet al. (1994) than with Wagnerand Zellner (1972a).

For the reaction of hydrogen atoms with allene (R112, R11the product channels are not certain. Based on the suggestiYunget al. (1984) and the data of Wagner and Zellner (1972the dominant reaction pathway for H+CH2CCH2 at lowpressures might be to convert allene to methylacetyle(R112). The addition pathway (R113) is important at highpressures (Wagner and Zellner 1972b). Our rate constfor R112 and R113 have been chosen so that the totalk112+ k113 matches the low-temperature results of Wagner aZellner (1972b).

We use Eq. (1) in combination with the data of Beckeret al.(1991) at a wide variety of temperatures and pressures to obthe rate constants for R139 and R140. The resulting expressalso compare well with the 100-torr data of Berman and L(1984).

The products of the reactions of CH radicals with unsaturahydrocarbons (R142, R143) are not certain. The reactions mlikely proceed by an addition mechanism, followed by eithcollisional stabilization of the adduct or fragmentation (Bermet al. 1982). For R142, the reaction most likely proceedsdelimited in Table III, although the reaction may not be seco
order at low pressures. For R143, neither the relative importance

ST

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en

o

o

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oc

cfc

-

r

Btra

bo

trie

eho

ionsofed

pa-zero

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heseiso-

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on

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herewer

set


of collisional stabilization nor the possible decomposition pructs of the reaction have been determined. We have triedsidering R143 as both a bimolecular and a termolecular reacwith various products (C3H5, H+C3H4, and C2H2+CH3) andfind no significant difference in the model results, primarily bcause most of the CH is consumed by the fast reactions RR140, and R141. Similarly, other products of the reaction ofradicals with C2H6 (R144) may be possible (e.g., C2H4+CH3,CH4+C2H3, or C3H7).

Although the recombination of CH3 radicals (R158) hasbeen well studied at high temperatures, the rate constatemperatures below∼200 K is less certain. In particular, thlow-pressure-limiting rate constant is not known at temperatrelevant to the stratospheres of the outer planets. We havesen thek158,0 rate constant recommended by MacPhersonet al.(1983). We also test the rate constant suggested by Slagleet al.(1988). Because reaction R158 is crucial to the abundancCH3 and C2H6 in Saturn’s stratosphere, we examine the setivity of the results to this reaction rate in a later section.

For R198 and R199, Tsang and Hampson (1986) recmend k198+ k199,∞= 2.5× 10−11 and k198/k199,∞= 2.5×10−36 T11.25 e(3289/T). The rate constants fork198 andk199,∞ arethen found by simple algebraic manipulation (as in Gladstet al. 1996).

Gladstoneet al. (1996) suggest that C2H2 can be convertedto C4H2 through the reactions C3H2+C2H2→C4H2+ 3CH2

and C3H3+C2H2→C4H2+CH3. They find that these two reactions are responsible for the bulk of the column productioC4H2 in Jupiter’s atmosphere. However, according to the thmodynamic information in their Table I (and other informatiregarding the heats of formation of these reactants and produthe C3H3+C2H2 reaction is endothermic and is unlikely to ocur in the jovian (or saturnian) stratosphere. We have thereomitted this C4H2 formation pathway from our model. The reation of C3H2 with C2H2 to form C4H2 and3CH2 is endothermicif propargylene is the relevant C3H2 isomer in Saturn’s stratosphere but will be exothermic for other forms of C3H2 (e.g., cy-clopropenylidene or vinylidenecarbene; see Kaiseret al. 1996for calculations of the enthalpies of formation of various C3H2

isomers). However, we feel that the most likely product of theaction of acetylene with C3H2 would be to form a C5H4 adduct,and we have included this reaction (R205) in the model.analogy with the reaction of3CH2 with C2H2, we estimate thaR205 is likely to have a large activation energy and a smallcoefficient at 140 K.

Diacetylene absorbs ultraviolet radiation at wavelengths athe threshold for dissociation, and the resulting metastablecited states (C4H∗2) can play a role in diacetylene photochemis(e.g., Glicker and Okabe 1987, Zwier and Allen 1996). Zwand Allen (1996) predict that reactions involving C4H∗2 can dom-inate over free-radical mechanisms in the formation of somthe larger hydrocarbon molecules in Titan’s atmosphere; twe have included these reactions (R218–R235) in our m
to test their possible importance in Saturn’s atmosphere. W
RY IN SATURN’S ATMOSPHERE 257

d-on-ion

-39,H

t at

esho-

ofsi-

m-

ne

ofr-

nts),

-ore-

e-

y

te

veex-yr

ofus,del

find that these reactions are not important for Saturn: reactR218–R235 account for only 4% of the total production rateCn (n≥ 5) hydrocarbons in the model. We have also includsimilar reactions involving metastable excited acetylene (C2H∗2),although we leave a discussion of their importance to a laterper and simply set the reaction rates of R186-R189 equal toin this study.

3.6. Condensation

Condensation and evaporation have been included inmodel. Details of our technique for including condensationgiven in a companion paper (Moseset al. 2000). Water, C4H2

(diacetylene), and C4H10 (both iso- andn-butane) can con-dense in Saturn’s lower stratosphere, and condensation of tmolecules has been included in the model. Benzene or othermers of C6H6, vinylacetylene (C4H4), and triacetylene (C6H2)will also condense, but their contribution to the total condention flux is minor, and condensation will not have much effectthe predicted total column abundance of C4H4, C6H2, or C6H6

vapor. Therefore, condensation of these molecules has notincluded in our model. Our expression for the vapor pressurC4H2 is derived from a generalized least-squares fit to the datKhannaet al. (1990), J. E. Allen, Jr. (personal communicatio1990), and Tanneberger (1933)

log10 P(torr)= 96.26781− 4651.872

T− 31.68595 log10 T,

whereT is in kelvins, and the expression is valid in the regi127< T < 249 K. The expression for C4H10 is appropriate forn-butane and is taken from Ziegler (1959)

log10 P(torr)= 8.446− 1461.2

T

in the region 128< T < 196 K. Note that the condensation temperatures in Saturn’s atmosphere are lower than the minimtemperatures achieved for the measurement of C4H2 and C4H10

vapor pressures in the laboratory; therefore, the extrapolaof the above expressions for both diacetylene and butanthe lower temperatures typical of Saturn’s lower stratosphis fraught with uncertainties.

With our condensation scheme, vapor is assumed to cdense on and evaporate from preexisting condensation nuin Saturn’s atmosphere (i.e., no separate nucleation step iquired). The preexisting stratospheric aerosol concentrationsize are chosen to be roughly consistent with the resultsKarkoschka and Tomasko (1993) and Westet al. (1983). Theparticles are assumed to be present throughout the stratospand troposphere, although their abundance diminishes at lopressures. The particle size is fixed at 0.15µm at all altitudes,and the particle concentration (in per cubic centimeter) isequal to the pressure in mbar atP< 20 mbar (e.g., the particlenumber density atx mbar isx cm−3) and is set to 20 cm−3 at

eP> 20 mbar.

258 MOSES ET AL.

FIG. 2. Photoabsorption rate coefficients (J values) for some of the major hydrocarbon molecules as a function of altitude in our nominal model. TheJ values
for the alkanes (CH4, C2H6, C3H8, and C4H10) are shown in the upper panel, while theJ values for H2, C2H2, C2H4, C3H6, and C4H2 are shown in the lower
loe

s

ed

a

m,

ratedato-

pho-and

,d

dis-s is

panel. Calculations are for single-scattering only.

4. PHOTOCHEMICAL MODEL RESULTS AND DISCUSSION

The photoabsorption rate coefficients (J values) for severamajor atmospheric constituents in our nominal model are shin Fig. 2. The photoabsorption coefficients are not accuratwavelengths greater than∼170 nm due to our neglect of (aRayleigh and aerosol scattering and (b) absorption by aerocloud particles, and tropospheric molecules such as NH3. As aresult, theJ values cannot be regarded as accurate in theposphere. At high altitudes, molecular hydrogen dominatesabsorption of extreme ultraviolet photons with wavelengths lthan∼110 nm. Methane near the homopause region (anlower altitude levels) dominates the absorption of∼110–145 nmwavelength radiation. Acetylene in the stratosphere domin
the absorption of ultraviolet photons with wavelengths grea
wnat

)ols,

tro-thessat

tes

than 145 nm, but C2H6 contributes at wavelengths up to 160 nand C2H4 contributes at wavelengths longer than 160 nm.

The interesting photochemical interactions can be sepainto two distinct regions of the atmosphere: (1) the upper strsphere near the methane homopause (10−5 mbar to∼<10−2 mbar),where methane is photolyzed directly by Ly-α and other veryshort wavelength radiation or where reactions of methanetolysis products dominate radical production (e.g., R103),(2) the middle and lower stratosphere (∼<10−2 mbar to∼10 mbar)where direct photolysis of C2H2 and indirect photosensitizedissociation of CH4 occurs.

The results regarding hydrocarbon photochemistry arecussed below. The photochemistry of oxygen compounddiscussed in a companion paper (Moseset al. 2000), where the
terISO observations of stratospheric H2O and CO2 are used to help


FIG. 3. A schematic diagram illustrating the important reaction pathways for C, C2, C3, and C4 hydrocarbons in our nominal model. The symbol hν corresponds
to a solar ultraviolet photon. Radical species are outlined as ovals and stable molecules as rectangles. Methane photolysis initiates all the hydrocarbon reactions in
gg

y

p

cn

u

rm-.g.,re-tiont.ro-s thatnt inling

or C,theonstheand% is

ed

ble

Saturn’s stratosphere.

constrain the influx of external oxygen to Saturn. For the oxyinfluxes inferred from these observations, the additional oxyphotochemistry has little effect on the abundances of theserved hydrocarbons, and the hydrocarbon photochemistrbe considered separately.

4.1. Hydrocarbon Photochemistry

Figure 3, which emphasizes the importance of methanetolysis in initiating the hydrocarbon photochemistry on Satudescribes the most important pathways for synthesizing theplex hydrocarbons in our nominal model. Photodissociatiomethane results in the formation of the short-lived radicals C3,1CH2, 3CH2, and CH (see Sections 3.5 and 4.2 for a discsion of the importance of the different methane photolysis chnels). Note that unlike the jovian model of Gladstoneet al.(1996), we allow for the direct production of CH3 from methanephotolysis.

Although our adopted photochemical pathways and rate
efficients differ from the Gladstoneet al. (1996) model in many
enen

ob-can

ho-rn,om-of

Hs-

an-

co-

important ways, we agree that the primary methods for foing complex hydrocarbons include CH insertion reactions (eR141, R142, R144) and three-body radical recombinationactions (e.g., R158, R160, R162, R167, R194, R208). Inserreactions involving C2H (e.g., R179, R180) are also importanPhotolysis and cracking by atomic hydrogen are the main pcesses that break carbon–carbon bonds. Exchange reactiondo not produce or destroy carbon–carbon bonds are prevalethe model, as are recycling reactions. Exchange and recycreactions maintain the steady-state abundances of the majC2, C3, and C4 hydrocarbons, and without these reactions,predicted concentrations of many of the major hydrocarbwould be grossly misestimated. Methane itself is recycled instratosphere. Of all the methane destroyed by photolysisother reactions throughout the entire upper atmosphere, 42recycled, 57% is permanently converted to C2 hydrocarbons,0.5% is converted to C3 hydrocarbons, and 0.06% is convertto C4 hydrocarbons.

The mixing ratios of the major radical species and sta
molecules containing one- and two-carbon atoms are shown in

th error

260 MOSES ET AL.

FIG. 4. The mixing ratios of the major C and C2 hydrocarbons in our nominal model as a function of altitude and pressure. The various data points wi
bars represent observational measurements. The circles refer to CH4, the squares to C2H6, the triangles to C2H2, and the star to CH3. The filled symbols refer to
e

tiaedie

is

sin

-eg.

lyIRo

tfie

imnh

heby

hisme

rwer

ane

di-nd

C

our reanalysis of the ISO observations (see Section 5 and Table V), and th

Fig. 4. We use the traditional atmospheric chemistry definiof “mixing ratio” q in this paper (concentration of a particulconstituent at any altitude level divided by the total atmosphdensity at that level) rather than use the convention adopteobservers of referring to the term “mixing ratio” as the specconcentration divided by the H2 concentration. On Saturn, thtwo definitions are equivalent to within∼<4%. Also plotted on thefigure are various data points relevant to the observations lin Table I. The photochemical model reproduces the ISO C2H2

and C2H6 observations within observational uncertainties (Section 5.2); in addition, the model was purposely constrato fit the Smithet al. (1983) Voyager UVS CH4 occultationprofile and the Courtinet al. (1984) Voyager IRIS lower stratospheric CH4 mixing ratio. The non-ISO data points are includfor comparison purposes only, although our models seemerally consistent with the other C2H2 and C2H6 measurementsOne exception is the C2H6 Voyager–IRIS-derived mixing ratiopresented in Courtinet al. (1984). The ISO observations impan ethane abundance that is 2–3 times that observed withPart of the difference is due to different modeling assumptisuch as the fact that our 1-mbar temperatures are 7 K colderthan those adopted by Courtinet al. (1984) and the fact thaCourtin et al. (1984) assumed a constant mixing ratio prowith height; however, some of the difference could represreal latitudinal/seasonal variations.

The altitude profiles for the reaction rates of the mostportant reactions involving the production, loss, and exchaof C and C hydrocarbons are shown in Figs. 5 and 6. T
2
dominant mechanism for synthesizing C2 hydrocarbons in our

open symbols refer to the non-ISO observations listed in Table I.

onrricby

es

ted

eeed

den-

IS.ns

lent

-gee

Saturn model is methyl–methyl recombination (R158). In tupper stratosphere, methyl radicals are produced directlymethane photolysis, and two methyl radicals produced in tmanner combine to form ethane. The following simple scheillustrates this process:

2(CH4hν−→ CH3+ H) R5

2CH3M−→ C2H6 R158

Net : 2CH4 −→ C2H6+ 2H. (2)

Similar schemes involving R6 (CH4hν−→ 1CH2+H2) followed

by R146 (1CH2+H2−→CH3+H) also operate in the uppestratosphere, with the same net result. In the middle and lostratosphere, the production of C2H6 operates with other C2species as catalysts.

The second most important mechanism for forming C2 hy-drocarbons in Saturn’s atmosphere is CH insertion into methto form ethylene. The scheme can be illustrated as

CH4hν−→ CH+ H+ H2 R9

CH+ CH4 −→ C2H4+ H R141

Net : 2CH4 −→ C2H4+ H2+ 2H. (3)

This scheme is effective in the upper atmosphere whererect methane photolysis is important. Reactions R158 aR141 together account for 85% of the total production of2

hydrocarbons in the model. The remaining 15% results

xchange


FIG. 5. The altitude profiles of the reaction rates for the six most important reactions involving production (left panel), loss (middle panel), and e
(right panel) of C compounds in our nominal model. The reactions are listed in order of decreasing column reaction rate in the stratosphere.
-n

t

e

g

e

-C

sx

bon

dataim-

eain-n de-

a

primarily from destruction of C3, C4, and higher order hydrocarbons. Schemes (2) and (3) were identified as being the domischemes for producing C2H4 and C2H6 in Titan’s atmosphere(Laraet al. 1996).

Many of the reaction schemes deemed important in our Samodel differ from those in the Jupiter model of Gladstoneet al.(1996). In virtually all cases, the conflicts are due to our differadopted reaction rates and pathways rather than to intrinsicferences between the two planets. Specific details regardinimportant reaction schemes responsible for the productionloss of the major C2 hydrocarbons in our model are presentin Appendix B.

Of all the C2 species produced from C, C3, C4, and Cn (n≥ 5)hydrocarbons, 52% remains as C2 hydrocarbons, 33% is returned to CH4 and other C compounds, 10% is converted to3

compounds, 5% is converted to C4 compounds, and¿1% isconverted to C compounds. The net column production ra
n
of C2 hydrocarbons (i.e., the rate at which C2 compounds are

ant

urn

ntdif-the

andd

te

produced from C, C3, and C4 species minus the rate at which C2

species are converted to non-C2 species) in our nominal model i9.84× 108 cm−2 s−1. This net production is balanced by the fluof C2H6 (9.82× 108 cm−2 s−1), and, to a lesser extent, by C2H2

(1.2× 106 cm−2 s−1) and C2H4 (1.0× 105 cm−2 s−1) through thelower boundary of the model.

The mixing ratios of the stable species containing three caratoms are shown in Fig. 7. The photochemistry of C3 hydrocar-bons has been less well studied than that of C2 hydrocarbons,and our results are correspondingly less certain. Laboratoryhave been particularly sparse for the chemistry of the twoportant isomers of C3H4, methylacetylene (CH3C2H) and allene(CH2CCH2), making it difficult to unambiguously determine thimportant reaction schemes producing, destroying, and mtaining these molecules. Because methylacetylene has beetected in spectra of Saturn’s atmosphere (Hanelet al. 1981 andde Graauwet al. 1997), the lack of laboratory data becomes
particularly acute problem.

xchange

262 MOSES ET AL.

(right panel) of C2 compounds in our nominal model. The reactions are listed in order of decreasing column reaction rate in the stratosphere.
m

lve

o

uc-ndsyn-

of

f Clossin

i-

The double peak that is typically exhibited with C3 radicalsand most of the stable C3 molecules shown in Fig. 7 results frothe separation of the two different regimes leading to methdissociation. The high-altitude peak is due to H Ly-α and othershort-wavelength photolysis of methane, while the peak inmiddle stratosphere is initiated by the photosensitized catadestruction of CH4 via C2H2 photolysis. Species that do not haeffective chemical loss mechanisms and that are far from bin photochemical equilibrium do not exhibit the double pe(e.g., the alkanes C2H6, C3H8, and C4H10). For these moleculesdiffusion plays a large role in controlling their mixing ratio prfiles.

The profiles for the reaction rates of the most importantactions involving the production, loss, and exchange of C3 hy-drocarbons are shown in Fig. 8. In the upper atmosphere3

compounds are synthesized by CH insertion into Cmolecules.
2
For instance, CH insertion into C2H6 forming C3H6 (R144) is

ane

theyticeing

ak,-

re-

, C

responsible for 10% and CH insertion into C2H2 forming C3H2

(R142) is responsible for 5% of the total primary column prodtion of C3 hydrocarbons in our nominal model. In the middle alower stratosphere, radical–radical combination reactionsthesize C3 compounds. Reaction R160 (CH3+C2H3

M−→C3H6)is responsible for 53% and R162 (CH3+C2H5

M−→C3H8) is re-sponsible for 11% of the total primary column productionC3 hydrocarbons in the model. The photolysis of C4 hydro-carbons contributes another 20% to the total production o3

compounds. Specific details regarding the production andof the major C3 hydrocarbons in our model are presentedAppendix C.

The net column production of C3 hydrocarbons in our nomnal model (5.4× 105 cm−2 s−1) is balanced by diffusion of C3H8

(5.4× 106 cm−2 s−1) and, to a lesser extent, of CH3C2H (8.7×102 cm−2 s−1) and CH2CCH2 (1.7× 102 cm−2 s−1) through the
lower boundary of our model. Of all the C3 species produced


FIG. 7. The mixing ratios of the stable C3 hydrocarbon molecules in our nominal model as a function of altitude and pressure. The data point with associated5

C

-fico

es

io

md

l

s

p

deld asns,canin

l sen-by

erst theper-ingra-

s norbon

l is

themo-andtter

CHlowpho-

seethisf the

d by

error bars is from our analysis of the CH3C2H ISO observations (see Section

from C, C2, C4, and Cn (n≥ 5) hydrocarbons, only∼2.2% re-mains as C3 species, 70% is returned to C and C2 compounds,14% is converted to C4 compounds, and 14% is converted to Cn

compounds.Figure 9 shows the mixing ratio profiles for the stable4

molecules in our nominal model. The important C4 compounddiacetylene (C4H2) is included in Fig. 10, which shows the mixing ratio profiles of the stable polyyne molecules. The profor C6H6 has also been included in Fig. 10 for convenienand C2H2 is included for comparison purposes. The six mimportant reactions that produce, destroy, and exchange C4 com-pounds are shown in Fig. 11. Note that C2H insertion reactions(e.g., R179 and R180) and radical–radical recombination rtions such as R194, R167, R163, R164, and R171 are responfor most of the production of C4 compounds in the model. C2Hinsertion reactions account for 36% and radical–radical reactfor 64% of the total column production rate of C4 compounds.Photolysis is the predominant loss process for C4 compounds.Specific details regarding the production and loss of thejor C4 and heavier hydrocarbons in our model are presenteAppendix D.

The net column production of C4 hydrocarbons in our nominamodel (5.4× 105 cm−2 s−1) is balanced by diffusion of C4H10

through the lower boundary of our model. Of all the C4 speciesproduced from C, C2, C3, and Cn (n≥ 5) hydrocarbons, only∼0.6% remains as C4 species, 55% is returned to C and C3

compounds, 44% is returned to C2 compounds, and 0.3% iconverted to Cn compounds.

Although our nominal model (discussed in detail in the A
pendices) provides a good fit to the ISO and other known o
.2 and Table V).

lee,st

ac-ible

ns

a-in

-

servations, the fit is not unique. Several of the important moinputs are not well constrained, and these inputs are treatefree parameters in the modeling. In the following subsectiowe discuss how a change in some of the input parametersaffect the altitude profiles of the observable hydrocarbonsSaturn’s upper atmosphere. Rather than repeat the generasitivity studies that were presented for Jupiter’s atmosphereGladstoneet al. (1996), we select certain key free parametand demonstrate how specific allowable changes can affechydrocarbon abundances. One sensitivity test that we haveformed but do not discuss in detail below is the effect of adoptthe Hubbardet al. (1997) stratospheric/thermospheric tempeture profile rather than a profile based on Smithet al. (1983). Wefind that changing the upper atmospheric temperatures hameasurable effect on the abundances of the major hydrocamolecules.

Another sensitivity test that we do not discuss in detaithe sensitivity of the results to the assumed CH4 mixing ratioin Saturn’s upper troposphere. Because of uncertainties intemperature structure and cloud properties in Saturn’s atsphere, the methane mixing ratio in the lower stratosphereupper troposphere is uncertain. However, we find that no mawhat assumption we make about the upper-tropospheric4abundance, methane becomes very optically thick just bethe methane homopause, and stratospheric hydrocarbontochemistry is photon-limited rather than methane-limited (Romani and Atreya 1988 for a more detailed discussion ofphenomenon). The total stratospheric column abundances oobservable hydrocarbons are therefore relatively unaffecte
b-changes in our adopted methane abundance.

xchange

264 MOSES ET AL.

(right panel) of C3 compounds in our nominal model. The reactions are listed in order of decreasing column reaction rate in the stratosphere.
ytd

ss

eN

lo

with

.y-

sultsmunt

and

tions,

4.2. Sensitivity to CH4 Branching Ratios

The photolysis of methane at 121.6 nm (from the H Lα solar line) drives much of the hydrocarbon photochemisin Saturn’s upper atmosphere. Unfortunately, as mentioneSection 3.5, the branching ratios of CH4 at Ly-α are not wellknown due to the high reactivity of some of the photolysis proucts and to other experimental difficulties. For a good discusof the current state of knowledge regarding this problem,Smith and Raulin (1999), Brownswordet al. (1997), Hecket al.(1996), and Romani (1996). The channel producing CH3 (R5) isclearly important (Hecket al. 1996, Mordauntet al. 1993), butthe sometimes contradictory data regarding the quantum yiof H, H2, and CH make branching-ratio assignments difficult.single branching-ratio scheme satisfies all the laboratory da

For our nominal model, we have chosen to most closely folthe H and H2 photofragment imaging results of Hecket al. (1996)
(see Section 3.5). Our adopted branching ratios at H Ly-α and
-ryin

d-ionee

ldso

ta.w

the resulting quantum yields for CH3, H2, H, and CH formationare shown in Table IV. Our branching ratios are consistentthe results of Mordauntet al. (1993) and Hecket al. (1996) butmay be inconsistent with other laboratory data (see below)

The H atom quantum yield from methane photolysis at H Lαhas been determined to be8H= 0.47± 0.11 from Brownswordet al. (1997),8H= 1.0+0.6

−0.4 from Mordauntet al. (1993),8H≥0.42 from Laufer and McNesby (1968), and8H≈ 1.16 fromSlanger and Black (1982). The large variation in these reillustrates the difficulties in determining this yield. Our H-atoquantum yield of 0.8 is consistent with the results of Mordaet al. (1993) but not with the results of Brownswordet al.(1997). The molecular hydrogen quantum yield of CH4 pho-tolysis at 123.6 nm was determined to be 0.58 by LauferMcNesby (1968). Because some of the H2 formed in the Lauferand McNesby experiments may be due to secondary reac
the derived H2 quantum yield is considered to be an upper limit,

265

abov

with a

HYDROCARBON PHOTOCHEMISTRY IN SATURN’S ATMOSPHERE

s

ble

9)oos

FIG. 9. The mixing ratios of the stable C4 hydrocarbon molecules (e

and our adopted branching ratios are consistent with the Lauand McNesby results. Rebbert and Ausloos (1972) have demined that the quantum yield of CH formation from methanphotolysis is 0.06± 0.005 at a wavelength of 123.6 nm and i0.23± 0.03 at wavelengths of 104.8–106.7 nm. If we take th

e CH quantum yields at face value,8 at H Ly-α should

oints

(1972) CH quantum yield and the Brownswordet al. (1997) H
CH
FIG. 10. The mixing ratios of acetylene, the stable polyyne molecules, and C6H6 in our nominal model as a function of altitude and pressure. The data p
ssociated error bars are from our analysis of the C2H2 and C4H2 ISO obser
xcept C4H2) in our nominal model as a function of altitude and pressure.

ferter-e

e

lie somewhere between 0.06 and 0.23, with more probavalues near 0.06. Our adopted CH4 photolysis branching ratioslead to a CH quantum yield that falls outside this range.

Branching-ratio schemes similar to Smith and Raulin (199(see Table IV) are more consistent with the Rebbert and Ausl

vations (see Section 5.2 and Table V).

266 MOSES ET AL.

FIG. 11. The altitude profiles of the reaction rates for the six most important reactions involving production (left panel), loss (middle panel), and exchangeed in order of decreasing column reaction rate in the stratosphere.

r

a,

p

h

ye

lt inower

dif-aysts

acts%

he

e

onsatedan-

(right panel) of C4 compounds in our nominal model. The reactions are list

quantum yield but are inconsistent with the H2 photofragmentimaging results regarding the relative importance of R9 and(Hecket al. 1996) and with the derived large H quantum yieldsMordauntet al. (1993) and Slanger and Black (1982). Until whave more information regarding the kinetic energy and intestate distributions of the CH4 photolysis fragments CH, CH2,and H2, the methane photolysis branching ratios will remuncertain (cf. Brownswordet al. 1997). As we will show belowdeterminations of the relative production of CH and3CH2 com-pared with the other photolysis products would be particulauseful in constraining photochemical models.

Because of the fundamental uncertainty in the methanetolysis pathways, we have examined the sensitivity of the resto different suggested schemes. Table IV and Fig. 12 showthe abundances of the major observable hydrocarbons varychanges in the methane photolysis branching ratios at H Lα.The results from our nominal model lie somewhere betw
the two suggested cases of Mordauntet al. (1993), while the
R6ofenal

in

rly

ho-ultsow

with-en

branching ratios suggested by Smith and Raulin (1999) resuabundances of complex hydrocarbons that are measurably lthan our nominal model.

An examination of Fig. 3 reveals the reasons for theseferences. Regardless of the initial photodissociation pathwchosen, CH and CH3 are the most important radical producof methane photolysis. For instance, virtually all the3CH2 pro-duced in our models of the upper atmosphere of Saturn rewith atomic hydrogen to form CH (by reaction R93), while 88of the 1CH2 formed in the upper atmosphere reacts with H2 toproduce CH3 by reaction R146, and the remaining 12% of t1CH2 reacts with H2 to form 3CH2 and eventually CH (by re-action R145 followed by R93). The relative inefficiency of thSmith and Raulin branching ratios in synthesizing C2–C4 hy-drocarbons illustrates the importance of CH insertion reactisuch as R141, R142, and R144 in creating complex unsaturhydrocarbon molecules in Saturn’s stratosphere. The low qu
tum yields of both CH and3CH2 in the Smith and Raulin scheme

267

This choice was m

HYDROCARBON PHOTOCHEMISTRY IN SATURN’S ATMOSPHERE

TABLE IVSensitivity to Methane Photolysis Branching Ratios

Mordauntet al. (1993)Smith and Raulin Our nominal

Model 1 Model 2 (1999) model

Branching ratiosa

R5 CH4−→CH3+H 0.51 0.49 0.41 0.48R6 CH4−→ 1CH2+H2 0.24 0 0.53 0.2R7 CH4−→ 1CH2+ 2H 0 0 0 0R8 CH4−→ 3CH2+ 2H 0.25 0 0 0R9 CH4−→CH+H+H2 0 0.51 0.06 0.32

Quantum yields8CH3 0.51 0.49 0.41 0.488H2 0.24 0.51 0.59 0.528H 1.01 1.0 0.47 0.88CH 0 0.51 0.06 0.32

Column abundancesb

CH3 4.06× 1013 4.28× 1013 3.83× 1013 4.13× 1013

C2H2 8.35× 1017 1.12× 1018 6.67× 1017 9.15× 1017

C2H4 6.82× 1015 1.04× 1016 4.71× 1015 7.80× 1015

C2H6 3.64× 1019 4.16× 1019 3.22× 1019 3.80× 1019

CH3C2H 1.26× 1015 1.97× 1015 8.23× 1014 1.46× 1015

C4H2 1.34× 1014 3.23× 1014 5.88× 1013 1.79× 1014

a Branching ratios at H Lyman-α (121.6 nm).
es
e(

ndit

tytemtli

phh

rpin

)o

ostto-suchrs

yearhy-

other(e.g.,

ex-x.

rtu-

ueshellitehe

heent

un-Themanceon

b The resulting stratospheric column abundanc

limit the production of C2H4, C2H2, and C2H6 by removingthe effectiveness of some of the reaction schemes discussSection 4.1 and Appendix B. For example, schemes (3),(6), (7), (9), (12), or (13) followed by (16), (19), (20), (21), a(22) are inhibited by the lack of CH production with the Smand Raulin branching ratios. On the other hand, Model 2Mordauntet al. (1993) has the largest total CH production rain the stratosphere and the largest abundances of complex hcarbons. If future laboratory experiments demonstrate thabranching ratios of R8+R9 are significantly different from thadopted values in our nominal model, then other “free” paraeters such as the eddy diffusion coefficient or uncertain reacrates will need to be adjusted to keep the model results inwith the ISO observations.

Note that the higher order hydrocarbons such as C4H2 andCH3C2H are particularly sensitive to changes in the CH4 pho-tolysis pathways because they are produced through comreaction schemes involving several hydrocarbons, each of whave been affected by the changes in the branching ratios. Tfore, molecules such as C4H2 and CH3C2H are particularlygood indicators of the important chemical processes in Satuatmosphere. Without the ISO observations of these commolecules, we would find it much more difficult to constrathe hydrocarbon photochemistry.

4.3. Sensitivity to Solar Flux

The solar flux adopted in our nominal model (Section 3.4appropriate for average or lower-than-average solar conditi

otivated by the fact that molecules formed ne

(above 63 mbar) for selected hydrocarbons (in cm−2).

d in4),

hofedro-the

-ionne

lexichere-

n’slex

isns.

the methane homopause region in our nominal model take alm600 years to diffuse down to the 10-mbar level in the lower strasphere and that many of the stable hydrocarbon moleculesas C2H2 and C2H6 have chemical lifetimes greater than 11 yeaat the altitudes at which the observations are sensitive—11-solar-cycle-induced variations in the abundances of stabledrocarbons are thus averaged out over a solar cycle. On thehand, because species that have short chemical lifetimesCH3) or that are concentrated at higher altitudes (e.g., CH3 orC2H4) may be more affected by solar-cycle variations, weamine the sensitivity of the model to changes in the solar flu

The solar maximum flux values we have adopted are vially identical to those used in Gladstoneet al. (1996), exceptthat the 1-AU H Ly-α flux is 5.01× 1011 photons cm−2 s−1. Thesolar minimum flux values are taken from the July 1976 valof Torr and Torr (1985) in the extreme ultraviolet and from t15 August 1985 data of the Solar Mesospheric Explorer sate(R. T. Clancy, personal communication to M. Allen, 1989) in tfar ultraviolet. The 1-AU flux at H Ly-α in our solar-minimummodel is 2.47× 1011 photons cm−2 s−1. Note that the ISO ob-servations were taken during a period of low solar activity. Tresults of model calculations for the different assumed incidsolar flux values are shown in Fig. 13.

The solar flux can have a large effect on the total column abdances of the major hydrocarbons in Saturn’s atmosphere.increase in ultraviolet flux from sunspot minimum to maximuleads to a 50% increase in the stratospheric column abundof CH3. Hydrocarbon molecules containing two or more carb
aratoms are even more affected by the solar flux because they rely

eeer

268 MOSES ET AL.

FIG. 12. Sensitivity of the CH3, C2H6, C2H2, C2H4, CH3C2H, and C4H2 mixing ratios to changes in the CH4 photolysis branching ratios at H Ly-α. The solidcurves represent our nominal model, the dashed curves incorporate branching ratios from Model 2 of Mordauntet al. (1993), and the dotted curves incorporatthe branching ratios suggested by Smith and Raulin (1999). See Table IV for a description of the CH4 photolysis branching ratios for these different models. Thdash-triple-dot curves show the saturation vapor mixing ratios for the different species; note that C4H2 condenses in Saturn’s lower stratosphere while the oth
hydrocarbons shown here do not. The data points with associated error bars mark the mixing ratios derived from our analysis of the ISO observations (see Table V).Changes in the CH4 photolysis pathways have little effect on CH3 and C2H6 but a much greater effect on the unsaturated complex hydrocarbons.
h

bm

ldAc

in acesvitysuch

ia-l

ola-tionolarto

ere

on the photolysis or photosensitized destruction of two or mmethane molecules and the subsequent combination of twmore already depleted molecules or radicals. The stratospcolumn abundances of C2H2, C2H4, C2H6, CH3C2H, and C4H2

are increased respectively by a factor of 2.3, 2.5, 2.1, 2.9,7.7 from sunspot minimum to sunspot maximum. However,cause the solar ultraviolet flux varies over relatively short tiscales compared with either diffusion or the chemical lifetimof many of the hydrocarbon molecules, these results shouinterpreted with caution. The situation is very complicated.though the hydrocarbon molecules are long-lived, the radithat produce these molecules tend to be short-lived. All the abmodels assume a constant solar flux with time—a scenariois very unrealistic. More realistic models of the actual tempo
variation of the solar flux and of the species abundances o
oreo oreric

ande-e

esbel-alsovethatral

many solar cycles will need to be developed before we can gameaningful picture of how the hydrocarbon column abundanchange with solar cycle. On the other hand, simple sensititests such as these demonstrate that complex hydrocarbonsas C4H2 should exhibit strong latitudinal and seasonal vartions in abundance—C4H2 will be more abundant at equatoriaregions than at mid-latitudes due to changes in solar instion (note that polar regions have possible auroral producmechanisms, which can complicate the simple picture of sinsolation variations). Meridional winds, however, will tendwash out latitudinal contrasts.

4.4. Sensitivity to H Influx

Atomic hydrogen is produced in the thermosphere/ionosph
verof Saturn from the dissociation of molecular hydrogen by

inalotted cuixio


FIG. 13. Sensitivity of the CH3, C2H6, C2H2, C2H4, CH3C2H, and C4H2 mixing ratios to changes in the solar flux. The solid curves represent our nommodel, the dashed curves represent a model in which the solar flux was held constant at values typical to conditions at sunspot maximum, and the drvesrepresent a model in which the solar flux was held constant at values typical of sunspot minimum. The data points with associated error bars mark the mng ratiosderived from our analysis of the ISO observations (see Table V). Because the solar flux varies on time scales that are short compared with the diffusionr chemical
lifetimes of many of the observable hydrocarbons, these models may not accurately portray the species abundances at solar cycle minimum and maximum.More
l

oin

tnoe

e

x

tlyn-sthe

sing

Caby

the

the

2 6 3 4

realistic models that incorporate a time-variable solar flux need to be deve

photons, photoelectrons, and, in the auroral regions at leasenergetic charged particles (electrons, protons, or heavy iOnce produced, the atomic hydrogen will diffuse downwardthe stratosphere where it can have a large effect on hydrocaphotochemistry (see Gladstoneet al. 1996). Although we donot extend our model to high enough altitudes to accounthis source of H, the process can be simulated by imposidownward flux of atomic hydrogen at the upper boundary ofmodel. Estimates of the influx of H into Saturn’s stratosphrange from 6× 108 to ∼5× 109 cm−2 s−1 (Waite 1981, Smithet al. 1983, Ben-Jaffelet al. 1995). In our nominal model, thdownward H flux at the upper boundary is assumedbe 1.1× 109 cm−2 s−1, a factor of 3.6 smaller than the H influestimated in the Jupiter model of Gladstoneet al. (1996).

Although Gladstoneet al. (1996) provide a good discussio
of the effects of changing the H influx on the hydrocarbon ph
oped.

t, byns).to

rbon

forg aurre

to

n

tochemistry of Jupiter, our updated reaction list is sufficiendifferent from their model that a repetition of the H influx sesitivity tests of Gladstoneet al. is warranted. Figure 14 showthe effects of varying the assumed H flux by a factor of 4 atupper boundary of our Saturn model. We find that increathe H flux by a factor of 4 leads to a 21% decrease in the C2H2

stratospheric column abundance, a 9% decrease in the2H6

abundance, a 33% decrease in the CH3C2H abundance, and54% decrease in the C4H2 abundance. Decreasing the H fluxa factor of 4 leads to a 9% increase in the C2H2 stratosphericcolumn abundance, a 1% increase in the C2H6 abundance, a 14%increase in the CH3C2H abundance, and a 30% increase inC4H2 abundance.

An increase in the H flux at the upper boundary inhibitsproduction of CH by allowing the reaction H+CH

M−→CH
o-(R95) to compete with methyl–methyl recombination (R158) in

del.

of the

270 MOSES ET AL.

FIG. 14. Sensitivity of the CH3, C2H6, C2H2, C2H4, CH3C2H, and C4H2 mixing ratios to the flux of atomic hydrogen at the upper boundary of the moThe solid curves represent our nominal model (8H= 1.1× 109 cm−2 s−1), the dashed curves represent a model in which8H= 4.4× 109 cm−2 s−1, and the dottedcurves represent a model in which8 = 2.8× 108 cm−2 s−1. The data points with associated error bars mark the mixing ratios derived from our analysis
H
b

ni

-

lr

on-to-sion-handdyturn

a-

r-y

ISO observations (see Table V). Heavier hydrocarbons are more affected

Saturn’s upper stratosphere. However, the decrease in C2H6 isalmost balanced by an increase in the conversion of acetyleethane through reaction schemes such as (16) (see Appendthe ethane abundance is therefore relatively unaffected bychange in hydrogen influx. The major photochemical respoto variations in the atomic hydrogen influx is the changethe rate of conversion of C2H2 to C2H6 (and hence the efficiency of acetylene recycling). Species such as C2H2 (and otherspecies such as C4H2 and CH3C2H for which C2H2 is a “parent”molecule) are strongly affected by the choice of the H influxthe upper boundary.

4.5. Sensitivity to Eddy Diffusion Coefficient

Changes in the eddy diffusion coefficient profile can ahave a dramatic effect on the abundances of the hydroca
molecules in Saturn’s stratosphere. We use the ISO obse
y changes in the H flux at the upper boundary than light hydrocarbons.

e tox B);the

nsein

at

sobon

tions of C2H6 as well as the other hydrocarbons to help cstrain the eddy diffusion coefficient profile in the lower strasphere. Ethane is a particularly good tracer of the eddy diffucoefficient profile because once C2H6 is produced in the upper atmosphere, it is relatively stable, and diffusion rather tchemistry controls its abundance at lower altitudes. The ediffusion coefficient near the methane homopause of Sahas been derived from analyses of the H Ly-α airglow (Waite1981, Atreya 1982, Sandelet al. 1982, Ben-Jaffelet al. 1995),the He 58.4-nm airglow (Sandelet al. 1982, Parkinsonet al.1998), theVoyagerUVS solar and stellar occultation observtions of CH4 absorption (Sandelet al. 1982, Atreya 1982, Smithet al. 1983), and ISO observations of CH4 fluorescence at neainfrared wavelengths (Drossartet al. 1999). Quotes for the edddiffusion coefficient at the methane homopause (Kh) range from
rva-5× 106 cm2 s−1 (Smith et al. 1983) to (4± 1)× 107 cm2 s−1

les.

ffi

r diffudances

a points

FIG. 15. Sensitivity of the CH4, H, CH3, C2H6, C2H2, C2H4, CH3C2H, and C4H2 mixing ratios to changes in the assumed diffusion coefficient profiThe first panel shows the CH4 diffusion coefficients and the eddy diffusion coefficients used in these models. The solid curve depicts theK profile in our nominalmodel, the dashed curve depicts theK profile used in the model of Smithet al. (1983), the dash-triple-dot curve depicts the CH4 molecular diffusion coefficientsin our nominal model, and the dotted curve depicts an alternative model in which the CH4 molecular diffusion coefficients are as described in Gladstoneet al.(1996). For the other panels, the solid curves represent our nominal model, the dashed curves represent a model in which the molecular diffusion coecients arethe same as our nominal model but that have an eddy diffusion diffusion coefficient profile that is constant with altitude atK = 5× 106 cm−2 s−1 (as in Smithet al. 1983). The dotted curves represent a model in which the eddy diffusion coefficient profile is the same as our nominal model but that have moleculasioncoefficients as are described in Gladstoneet al. (1996). Note that changes in the diffusion coefficient profiles can have a dramatic effect on the column abunof hydrocarbons in Saturn’s stratosphere. The CHdata points with associated error bars correspond to UVS occultation data (see Table I). The other dat
4
with associated error bars mark the mixing ratios derived from our analysis of the ISO observations (see Table V).

271

ie

bi,ehutsc

dih

gt

re

e

n

es

ev

atp-odelddy. Tova-ientent

f

t tohaneghlyourhemnthe

notistin-sis

f weingals

, andiffu-

).that

d

eon-t ofaionityomi-ngel isined

ob-

theis in-herfine

u-ing aof

272 MOSES

(Drossartet al. 1999) to (8± 4)× 107 cm2 s−1 (Sandelet al.1982) to 1.7+4

−1× 108 cm2 s−1 (Atreya 1982) toKh∼> 109 cm2 s−1

(Parkinsonet al. 1998)—a huge range in values (see the revof Atreyaet al.1984).

Why are there such large differences in the derived valuesKh? One reason is that different analysis techniques haveused, each with its own sources of uncertainty. Anotherportant reason is that the derivations are model-dependentthe different authors use different assumptions in their modQuotes ofKh are useless without further information about tassumedshapeof the eddy diffusion coefficient profile and abothe adopted expression for the molecular diffusion coefficienSaturn’s H2-dominated atmosphere. Unfortunately, these pieof information (especially the latter) are not always provided

Figure 15 demonstrates that in terms of the amount of methlocated at high altitudes in Saturn’s stratosphere, the valueKh in our nominal model (1.7× 107 cm2 s−1) is actually con-sistent with theKh value of 5× 106 cm2 s−1 reported by Smithet al. (1983). In this case, the shape of theK profile plays adominant role in determining how much methane gets carriehigh altitudes. Smithet al. (1983) have adopted a profile thatconstant with altitude, while we have adopted a profile thata stagnant lower stratosphere withK increasing with altitude.Because the stagnant lower stratosphere inhibits methaneinto the upper atmosphere, a larger eddy diffusion coefficienthe upper stratosphere is required to compensate. The stalower stratosphere is essential for reproducing the observaof the other hydrocarbon molecules in Saturn’s stratosphereFig. 15); however, based on CH4 absorption alone, Smithet al.’schoice was quite reasonable.

By the same token, the height of the methane homopadepends critically on the CH4 molecular diffusion coefficientsadopted in the various models. This information is rarely pvided, and it is not even clear whether theoretical or expmental estimates have been used in the above papers. Figualso illustrates how the hydrocarbon abundances are affeby different assumptions about the molecular diffusion coecients. Using theK profile derived for our nominal model, wallow the CH4 molecular diffusion coefficients to be calculateeither as in Section 3.3 above (i.e., our nominal model) or adescribed in Eq. (6) of Gladstoneet al. (1996). The moleculardiffusion coefficients reported by Gladstoneet al. are a factorof 2–3 times smaller than those adopted here. For the samKprofile, the use of the smaller molecular diffusion coefficieallows methane to be carried to altitudes higher than those innominal model. Methane is not recycled as efficiently at thaltitudes, and the production of higher order hydrocarboncorrespondingly augmented.

For a great discussion of how the molecular abundancesaffected by changes in the slope of the eddy diffusion coecient profile and the value ofK at the methane homopaussee Gladstoneet al. (1996). We will not repeat these sensitiity tests in this paper. Instead, we develop two models that
both consistent with the ISO observations of CH3, C2H2, C2H6,
ET AL.

w

foreenm-andls.etines

.anefor

tosas

flowt innant

ions(see

use

o-ri-

re 15ctedffi-

ds is

etsourseis

areffi-,-are

CH3C2H, C4H2, CO2, and H2O in Saturn’s atmosphere, but thhave very different eddy diffusion coefficient profiles in the uper stratosphere. Figure 16 demonstrates how our nominal m(Model A) compares with a model that has a much smaller ediffusion coefficient at the methane homopause (Model B)keep the Model B CH3 abundance consistent with the obsertions, we have adopted a low-pressure-limiting rate coefficfor CH3–CH3 recombination (reaction R158) that is consistwith a low-temperature extrapolation of the data of Slagleet al.(1988) rather than that of MacPhersonet al. (1983); the rest othe kinetic rate coefficients are identical to Model A.

The main difference between the two models is the heighwhich methane is carried in the upper atmosphere. The methomopause in Model B is located at a pressure level rouan order-of-magnitude greater (lower altitude) than that fornominal model. However, the lower diffusion coefficient in tlower stratosphere in Model B allows the hydrocarbon coluabundances to be comparable to our nominal model. Withexception of the CH3 observations, the ISO observations arevery sensitive to the upper stratosphere, and we cannot dguish between different diffusion coefficient profiles on the baof the ISO observations alone. This situation could change iwere to obtain new laboratory data for the low-pressure-limitrate constant for methyl–methyl recombination; methyl radicare concentrated at high altitudes in Saturn’s stratospherethe CH3 observations could be used to constrain the eddy dsion coefficient near the methane homopause ifk0 for R158 wereprecisely known (see B´ezardet al. 1998 and Section 4.6 below

Fortunately, we have several other pieces of informationhelp constrainKh. As mentioned above, the H Ly-α airglow,the He 58.4-nm airglow, theVoyagerUVS occultation data, anISO observations of methane fluorescence near 3.3µm can beused to constrainKh on Saturn. A discussion of the H and Hairglow data is deferred to Section 5.3. Model B is not csistent with UVS occultation results concerning the heighthe methane homopause (Smithet al. 1983, Festou and Atrey1982) while Model A is. However, because the eddy diffuscoefficients might vary with time due to variable wave activand changes in thermal structure, we cannot say that our nnal model (Model A) is any better than Model B in explainithe ISO observations. On the other hand, our nominal modperhaps more consistent with the temperature profiles obtafrom the analyses of the UVS occultations (Smithet al. 1983,Festou and Atreya 1982) and with the 28 Sgr occultationserved from the Earth (Hubbardet al. 1997). Although thesetemperature profiles differ from each other, both show thatbase of the thermosphere is located at higher altitudes thandicated by the low-altitude loss of methane in Model B. Furttemperature and radiative-balance modeling should help rethis conclusion.

Note that Models A and B both have very low eddy diffsion coefficients at pressures greater than 1 mbar, indicatstagnant lower stratosphere on Saturn. These low valuesK
are necessary to reproduce the high C2H6 abundance inferred

erlal rate

ns


FIG. 16. Sensitivity of the CH4, H, CH3, C2H6, C2H2, C2H4, CH3C2H, and C4H2 mixing ratios to changes in the eddy diffusion coefficient in Saturn’s uppatmosphere. The solid curves represent our nominal model (Model A), and the dashed curves represent a model with aK profile as shown in the top left pane(Model B). The top left panel also shows the CH4 molecular diffusion coefficient profile in these models (dash-triple-dot curve). The adopted photochemiccoefficients are the same for the two models except for the low-pressure limiting rate constant for methyl–methyl recombination,k158,0, which is lower in Model Bthan in Model A. Both models are consistent with the ISO observations of CH3, C2H2, C2H6, CH3C2H, and C4H2 in Saturn’s stratosphere, and other observatio
(e.g., ultraviolet occultations) are needed to constrain the eddy diffusion coefficient in the upper stratosphere. The data points are as marked in Fig. 15.

-

n

n

9

uhthh

us

a

a

rlessten-m-

entster-

haturern’srity

e ex-rob-

nt

ncealagleywithgn’shere

274 MOSES

from ISO observations. Although the lower stratospheric C2H6

abundance is sensitive to the entireK profile, we find that onlywhenK ∼< 3× 104 cm−2 s−1 at P∼> 1 mbar are we able to reproduce the ISO observations.

4.6. Sensitivity to Key Reaction Rates

Many of the reaction rate coefficients in our model havebeen measured at the low temperatures (∼140 K) relevant toSaturn’s stratosphere, and the corresponding uncertainties ireaction rates can be large. We now examine the consequencaltering the rate coefficients for four key reactions: R158, RR100, and R101. Although many of the other reactions listedTable III are important for determining the steady-state abdances of the observable hydrocarbons in Saturn’s atmospthese four reactions are particularly critical in determiningrelative abundance of ethane and acetylene in the stratospDiscrepancies in the observed C2H2/C2H6 ratio compared withthat predicted from photochemical models have hindered ourderstanding of the photochemistry of all the outer planets (Allen et al. 1992, Romani 1996, Gladstoneet al. 1996, Romaniet al. 1993). Our good agreement between the predictedobserved C2H2/C2H6 ratio on Saturn is due in large part to ouchoice of rate coefficients for reactions R158, R95, R101,R102.

Reaction R158 (2 CH3M−→C2H6) is responsible for virtu-

ally all the C2H6 formation in our nominal model and is the

ded in

between the extrapolation of MacPhersonet al. (1983) and the

FIG. 17. Experimentally and/or theoretically derived temperature dependence of the low-pressure-limiting rate constant (k0) for methyl–methyl recombination(reaction R158). The solid curve represents the expression of MacPhersonet al. (1983), the dashed curve represents the expression given in Slagleet al. (1988),and the dotted curve represents the expression given in Walteret al. (1990). All the curves have been extrapolated to temperatures lower than that recommen
the original references. The data points are from the experiments of MacPheet
ET AL.

ot

thees of5,inn-ere,eere.

n-ee

ndrnd

dominant loss mechanism for CH3. Due to its importance inhydrocarbon combustion studies and as a prototype barriereaction, methyl–methyl recombination has been studied exsively both experimentally and theoretically, but mostly at teperatures above 300 K. Recently, Walteret al. (1990) have pro-vided measurements at 200 K; unfortunately, their experimwere not conducted at a low enough pressure to truly demine the behavior of the reaction in the falloff regime so twe do not have good information concerning the low-presslimit of the rate constant at temperatures relevant to Satuatmosphere. In addition, the expressions given in the majoof the higher temperature studies do not provide reasonabltrapolations to low temperatures. Figure 17 illustrates this plem.

In Fig. 17, we plot the low-pressure limiting rate constak0 from several recent studies (MacPhersonet al. 1983, Slagleet al. 1988, Walteret al. 1990). The MacPhersonet al. and Slagleet al. expressions are valid down to 296 K, and the Walteret al.expression is valid down to 200 K. Note, however, the divergeof the expressions at temperatures below 200 K. The actuk0

is unlikely to decrease at low temperatures (as in the Slet al. and Walteret al. extrapolations), but it is also unlikelto increase dramatically with decreasing temperature asthe MacPhersonet al. extrapolation. The low-pressure-limitinrate coefficient at∼140 K, the temperature relevant to Saturupper atmosphere, is not certain but probably lies somew

rsonal. (1983). Note thatk0 for reaction R158 is very uncertain at low temperatures.


FIG. 18. Sensitivity of the CH3 mixing ratio to changes in the low-pressure-limiting rate constant (k0) for reaction R158 (2CH3M−→C2H6). The solid curve

represents our nominal model in which the MacPhersonet al. (1983) expression is used, the dashed curve represents a model in whichk158,0 is assumed to be−25 6 −1
1.0× 10 cm s at temperatures below 200 K, and the dotted curve represents a model in which the Slagleet al. (1988) expression is used. The data point
s

ti

hd

r

lln

C.er

turn.etoand-picalte

leadwith1990)co-

stoinalffects

se--o re-ere,

re-

ances

with associated error bars marks the mixing ratios derived from our analy

extrapolations of Slagleet al. (1988) and Walteret al. (1990).To test the sensitivity of the model to this rate constant, we hvariedk0 between the extremes show in Fig. 17. The resuleffect on the abundance of CH3 in Saturn’s atmosphere is showin Fig. 18. Methyl radicals are the only species significanaffected by the change in the rate constant; the stratospC2H6 column abundance varies by only 14% between mothat use the MacPhersonet al. expression and the Slagleet al.expression, and the effects on the other observable hydrocaare even less apparent.

To be consistent with the ISO observations of CH3 in Saturn’sstratosphere given the eddy diffusion coefficient and otherrameters adopted in our nominal model, we find that we neeadopt ak0 for reaction R158 that is very high, as in MacPherset al. (1983) (see also the discussion in B´ezardet al. 1998).The Slagleet al. expression in combination with theK profileand other assumed chemistry in our nominal model leadsCH3 stratospheric column abundance that is a factor of∼4 toohigh to be consistent with the CH3 ISO observations of B´ezardet al. (1998). The Slagleet al. (1988) expression might stibe consistent with the CH3 observations for other assumptioabout the eddy diffusion coefficient profile (e.g., see Sectionand Fig. 16), the solar flux, the chemical rate constants, the4photolysis pathways, or the lower stratospheric CH4 abundanceNew measurements ofk0 for R158 at low temperatures are esstial for us to achieve a better understanding of the hydrocaphotochemistry on the outer planets.

The combination reaction CH3+HM−→CH4 (R95) repre-

sents an important chemical loss for methyl radicals and co

is of the ISO observations (see Table V).

avengntlyericels

bons

pa-d toon

to a

s4.5H

n-bon

petes with R158 in the middle and lower stratosphere of SaOur adopted expressions fork0 andk∞ are consistent with thdata of Brouardet al. (1989) and indeed provide a better fitthe Brouardet al. data than the expressions given in CobosTroe (1990) or Baulchet al. (1992). Again, the rate for this reaction has not been measured at the low temperatures tyin Saturn’s stratosphere. We assume thatk0 remains constanat 2.52× 10−29 cm6 s−1 at temperatures below 300 K. Somother experimentally or theoretically derived expressionsto low-temperature extrapolations that continue to increasedecreasing temperatures. For instance, Cobos and Troe (suggest the following expression for the R95 reaction rateefficient in a bath gas of He:k0= 1.78× 10−24 T−1.8 cm6 s−1.Their suggested rate coefficient with C2H6 as the bath gas ik0= 8.63× 10−24 T−1.8 cm6 s−1. Both these expressions leada reaction rate for R95 that is higher than that in our nommodel at saturnian temperatures. Figure 19 illustrates the eof using the Cobos and Troe expressions fork95,0 rather than theexpression given in Table III.

An increase in the rate ofk95,0 leads to an overall decreain the CH3 abundance. In addition, CH3 radicals are more efficiently converted to CH4 rather than to C2H6, reducing the stratospheric ethane concentration. The faster rate for R95 alsduces the H-atom abundance in the lower stratosphreducing the effectiveness of scheme (16) that converts C2H2 toC2H6 in Saturn’s lower stratosphere (see Appendix B). Thefore, the C2H2 abundance increases, and the C2H6 abundancedecreases even further. The increase in the acetylene abund
m-also causes an increase in the C4H2 and CH3C2H abundances.

R95obos

276 MOSES ET AL.

FIG. 19. Sensitivity of the CH3, C2H6, C2H2, C2H4, CH3C2H, and C4H2 mixing ratios to changes in the low-pressure-limiting rate constant for reaction(H+C3

M−→CH4). The solid curves represent our nominal model in whichk95,0= 2.52× 10−29 cm6 s−1, the dashed curve represents a model in which the C
and Troe (1990) expression for a bath gas of He is used, and the dotted curve represents a model in which the Cobos and Troe (1990) expression for a bath gasof
ra

or

0tla

et

xo

o

e,

f theges

forwithratehedrate

d toen

C2H6 is used. The data points with associated error bars mark the mixing

Photochemical models for the outer planets would benefit frnew measurements of the low- and high-pressure-limitingcoefficients of R95 at temperatures below 200 K.

The reaction of atomic hydrogen with vinyl radicals (R1and R101) is important in controlling the abundances ofunsaturated hydrocarbons in Saturn’s stratosphere. The rerates of R100 and R101 help determine the effectiveness of C2H2

and C2H4 recycling via reaction schemes (10), (11), and (1compared with removal of these molecules via reaction sch(16). The higher the relative rate of R101 than that of R100,more efficient the conversion of C2H2 into C2H6, and the largerthe C2H6/C2H2 ratio in the lower stratosphere. Although our epressions for the rate coefficients of R100 and R101 fit the treaction rate coefficient (k100+ k101) measured by Fahret al.(1991) and Monkset al. (1995), our relative rates of the tw
reactions do not match the branching ratios inferred from e
tios derived from our analysis of the ISO observations (see Table V).

mate

0hetive

5)mehe

-tal

periments involving the reaction of atomic hydrogen with C2D3

(Monks et al. 1995). At 213 K and 1 torr of background HMonkset al. (1995) find that H+C2D3−→C2D2+HD occurs76% of the time while the addition mechanism occurs 24% otime. We now examine the sensitivity of our results to chanin the rate coefficients of R100 and R101.

Figure 20 illustrates the effect of using branching ratiosR100 and R101 that are consistent with the reaction of HC2D3. The solid curves represent our nominal model, withcoefficients described in Table III and Section 3.5. The dascurves represent a model with temperature-independentcoefficients that, in combination with Eq. (1), are designefit the 213-K experimental and theoretical information givin Monks et al. (1995). In this model (Model C),k100= 7.6×10−11 cm3 s−1, k101,0 = 7.17× 10−28 cm6 s−1, andk101,∞ =
x-9.5× 10−11 cm3 s−1. The dotted curves represent a model with

ext), and


FIG. 20. Sensitivity of the CH3, C2H6, C2H2, C2H4, CH3C2H, and C4H2 mixing ratios to changes in the rate constants for reaction R100 (H+ C2H3 −→C2H2+H2) and R101 (H+C2H3

M−→C2H4). The solid curves represent our nominal model (see Table III), the dashed curves represent Model C (see t
the dotted curves represent Model D (see text). The relative rates of R100 and R101 help determine the effectiveness of C2H2 recycling versus its conversion toC2H6. The data points with associated error bars mark the mixing ratios derived from our analysis of the ISO observations (see Table V).
to

sua1

ns

ucedel.bar,

tiveorp-

temperature-sensitive reaction rate coefficients designedthe 213- and 298-K data of Monkset al. (1995). In this model(Model D), k100= 7.6× 10−11 cm3 s−1, k101,0= 5.76× 10−24

T−1.3 cm6 s−1, andk101,∞= 4.93× 10−17 T2.64 cm3 s−1.At 0.1 mbar in our nominal model,k100/k101∼< 1, whilek100/

k101À 1 in Models C and D at the same pressure level. Aresult, acetylene recycling through reaction scheme (15) is mmore efficient in Models C and D than it is in our nominmodel. On the other hand, the increased importance of Rrelative to R101 for Models C and D inhibits C2H4 recyclingthrough schemes (10) and (11). Therefore, Models C and D ha higher stratospheric column abundance of C2H2 and a lowercolumn abundance of C2H4 than our nominal model. The columabundances of CH3C2H and C4H2, whose formation depend
on C2H2, are also increased. Due to the decreased efficien
fit

achl00

ave

of reaction scheme (16), the ethane/acetylene ratio is redin Models C and D compared with that in our nominal modFrom the standpoint of the column abundance above 10 mthe C2H6/C2H2 ratio is∼25 in our nominal model and∼14 inModels C and D.

5. COMPARISONS WITH OBSERVATIONS

5.1. Generation of Synthetic Spectra

Synthetic spectra were generated from a line-by-line radiatransfer program which includes the collision-induced abstion from H2–He and molecular opacity from PH3, NH3, CH4,CH3D, C2H6, C2H2, C4H2, CH3C2H, and CH3. Test calculations
cyalso included absorption from compounds not detected in the

ed

s

lA

s

ooa

sd

it

le

ee.cs

Othe

is

sion

ofr).

ntse

n inericthat

are

tis-

nty

278 MOSES

ISO spectra (C2H4, C3H8, CH2CCH2); these calculations werused to derive upper limits on molecular abundances anconstrain the photochemical modeling. Line parameters wextracted from the Geisa 96 line compilation (Jacquinet-Huset al. 1998), except for CH3 (Bezardet al. 1998), CH2CCH2

(M. Dang-Nhu, private communication to B. B´ezard, 1990),and C4H2 (for which we generated a line list for theν8 andthe 2ν8− ν8 bands using molecular constants from Ari´e andJohns (1992) and infrared band strengths from Koopset al.(1984)).

The temperature profile used in the radiative transfer calations is that adopted for the photochemical model (seependix A and Fig. 1). In the range 0.4–500 mbar, the profileconstrained by the same ISO observations that are used to tephotochemicalmodel outputs. The H2–He continuum longwardof 13µm and in microwindows between 9 and 11µm providesinformation on tropospheric levels (0.1–0.5 bar). The pressrange 1–5 mbar in the stratosphere is probed by the S(0) andH2 quadrupolar lines at 17 and 28µm, while emission from the7.7-µm CH4 band originates from the 0.4-mbar region (see Apendix A and Fig. A1; see also Lellouchet al. 1998, Bezardet al. 1998). Because temperature information is derived frthe same ISO measurements that serve to analyze the hydrbon emission bands, calibration uncertainties in the abundretrievals are minimized. We adopted the PH3 and NH3 mixingratio profiles used by de Graauwet al. (1997). Spectral radiancewere calculated for disk-averaged conditions and convertefluxes taking into account the fraction of Saturn’s disk includin the SWS aperture.

5.2. Comparisons with ISO Spectra

Figures 21–26 show a comparison of ISO observations wsynthetic spectra generated from the nominal photochemmodel distributions. Figure 27 shows the contribution functioassociated with the various emission features. This functgiving the relative contribution of a given pressure level toobserved emission, is defined as

−∫ ν0+1ν

ν0−1νBν(T)

d[2E3(τ )]

d ln P

f (ν − ν0)

Iν0

dν,

whereBν(T) is the Planck function at temperatureT , τ is theoptical depth at pressure levelP, E3 is the exponential integraof third order,f is the spectral convolution function defined ovthe interval [−1ν,+1ν], and Iν0 is the convolved radiance awavenumberν0.

The observed C2H6 band is well reproduced by the mod(Fig. 21), indicating that the model abundance is correct at lin the region probed by the emission (∼0.05–5 mbar, Fig. 27)On the other hand, the continuum level shows a slope thatnot be reproduced with the calculations and which might refrom calibration problems at this edge of the SW band. The c
tribution function at the band center (12.161µm) peaks around
ET AL.

toereon

cu-p-ist the

ureS(1)

p-

mcar-nce

toed

ithicalnson,he

rt

last

an-ult

on-

FIG. 21. Comparison of the C2H6 ν9 emission band as observed by ISon 15 June 1996 (SW band; AOT 01) with a synthetic spectrum based onnominal photochemical model.

the 0.5-mbar pressure level where the nominal mixing ratio9× 10−6.

The model also succeeds in reproducing the various emisfeatures associated with the C2H2 band in the region 13–14.5µm(Fig. 22). Emission from the main peak, due to the Q branchthe fundamentalν5 band, originates mainly from the 1.4-mbaregion (0.25–6 mbar at half maximum contribution) (Fig. 27A small secondary contribution arises around 0.6µbar fromthe Doppler cores of the lines. The two Q-branch componefrom the hot bandν5+ ν4− ν4 are also clearly detected (notthat the energy level of theν4 mode is 613 cm−1). Because theycorrespond to transitions with higher energy levels, emissiothis hot band originates from a higher and warmer atmosphregion centered at 0.3 mbar, i.e., 1.5 scale height abovecorresponding to the fundamentalν5 band. The relative inten-sities of the features from the hot and fundamental bandsthen sensitive to the mixing ratio gradient in the region∼0.3–1.4 mbar. The nominal C2H2 photochemical profile has abouthe right slope as it reproduces nicely the whole set of emsion features. A distribution in which the mixing ratio gradie(−d ln q/d ln P) is lowered by half would yield unacceptabl
weak hot band features, while an increase by 50% of this ratio

ominal


FIG. 22. Comparison of the C2H2 ν5 band as observed by ISO on 6 December 1996 (LW band; AOT 06) with a synthetic spectrum using the n
photochemical model. The spectra distinctly show emission features from the fundamental bandν5 as well as from the Q branches (l’= 0 and l’= 2) of the hot
,

un

n

t

)O.r-ow-eo-an

bandν5+ ν4− ν4.

must also be rejected (Fig. 23). By best fitting of the ISO datainfer a C2H2 mixing ratio of (2.7± 0.3)×10−7 at 1.4 mbar and1.2+0.8−0.5× 10−6 at 0.3 mbar, where the error bars only acco

for the uncertainty in the relative strengths of the fundameand hot band features.

Emission from the other detected species (C4H2, CH3C2H,CH3) is optically thin so that it is proportional to the column desity. No vertically resolved information is thus accessible andcontribution functions peak in the stratosphere approximawhere the absorber number density is maximum: 0.2µbar for
CH3, 0.5 mbar for C4H2 and CH3C2H. As illustrated in Fig. 24,
we

nttal

-theely

the model overestimates the C4H2 emission (and abundanceby 45%. The C4H2 column density rescaled to match the ISemission is 1.2× 1014 molecules cm−2 above the 10-mbar levelTo a lesser extent, the CH3C2H emission seems to be also oveestimated, by some 25%. The observed spectrum suffers hever from instrumental fringing which particularly affects thCH3C2H band, and the relative intensity of the various compnents is not well reproduced. Because of this limitation, we conly derive that the best fitting CH3C2H column abundance is0.8± 0.15 times that in the nominal model, i.e., (1.1± 0.2)× 1015

molecules cm−2 (above the 10-mbar level).

line),caled tog

280 MOSES ET AL.

FIG. 23. Comparison of the C2H2 ν5 band observed by ISO (dots) with calculations assuming different vertical profiles of the C2H2 abundance. Our nominalcase (solid line) is shown along with (a) a C2H2 profile that has a mixing-ratio gradient that is half that of our nominal model in the microbar region (dottedand (b) a C2H2 profile that has a mixing-ratio gradient that is 1.5 times that of our nominal model in the same region (dashed line). All profiles are resreproduce the fundamentalν5 peak intensity. The mismatch between observations and calculations around 13.73µm is not understood. It could be due to a missin
opacity in the C2H2 line compilation or to an unknown stratospheric emitter.
e

ie

hens; the“Ob-el

l re-ingrmi-

The CH3 nominal distribution allows us to reproduce themission observed in theν2 band at 16.50µm by Bezardet al.(1998) (Fig. 25). The corresponding column density is 4× 1013

molecules cm−2, within the range derived by B´ezardet al. TheCH3 column density is a sensitive parameter of the low-tempeture rate assumed for R158, as discussed in Section 4.6.

To obtain improved values for the column densities impl
from our reanalysis of the ISO observations, we have multipliednations bear systematic errors due to the spectroscopic data and
TABLE VConstraints from ISO Spectra

Column density at 10 mbar(molecules cm−2) Mixing ratio

Molecule Model prediction Observed Model prediction Observed

C2H6 0.5 mb: 9× 10−6 9± 2.5×10−6

C2H2 0.3 mb: 1.0× 10−6 1.2+0.9−0.6× 10−6

1.4 mb: 2.8× 10−7 2.7± 0.8× 10−7

C2H4 7× 1015 <4× 1015 0.5 mb: 9× 10−9 <5× 10−9

CH3C2H 1.4× 1015 1.1± 0.3× 1015 0.5 mb: 2.3× 10−9 1.8± 0.5× 10−9

CH2CCH2 2× 1014 <3× 1015 0.5 mb: 4× 10−10 <6× 10−9

C3H8 1.3× 1017 <1.5× 1017 0.5 mb: 6× 10−8 <7× 10−8

C4H2 1.8× 1014 1.2± 0.3× 1014 0.5 mb: 3× 10−10 2.1± 0.5× 10−10

C6H2 3× 1012 <3× 1013 0.5 mb: 6× 10−12 <6× 10−11

CH3 4× 1013 4+2−1.5× 1013 0.2 µb: 1× 10−7 1+0.5

−0.4× 10−7

ra-

d

the photochemical model profiles by a constant factor (wnecessary) to more closely match the ISO emission featureresulting species column abundances are listed under theserved” column in Table V. (Note, however, that the “Modprediction” columns are not adjusted, but are exact modesults.) Also provided in the table are the corresponding mixratios at the peak of the contribution functions. These dete

S

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s

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ver

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FIG. 24. Same as described in the legend to Fig. 22 but for theν9 band ofCH3C2H and theν8 band of C4H2. Residual instrumental fringing is presentthe data and may explain the discrepancy between the observed and mstructures near 15.8µm.

radiative transfer model, and to uncertainties from the ISOibration. Uncertainties due to spectroscopic data can bemated to 10% for the best known bands (C2H2 and C2H6),20% for CH3C2H and C4H2 which have hot bands incompletly accounted for in the line compilations, and 30% for C3(Wormhoudt and McCurdy 1989). As discussed in B´ezardet al.(1998), the CH3 determination also suffers from uncertaintiesthe temperature profile in the microbar region, where no inpendent constraints from ISO observations are available. Aditional±30% uncertainty was accordingly assigned by B´ezardet al. on the CH3 abundance. Theabsoluteflux calibration ofthe ISO–SWS data is precise to about 20%. The precision orelativecalibration, i.e., between the SW (λ<12.5µm) and LW(λ>12.5 µm) bands, or between two successive observatis much higher, on the order of 5%. The temperature profilthe regions probed by the observed compounds (except CH3) isconstrained by ISO observations of the S(0) and S(1) lineH2 (in the LW range) and in theν4 band of CH4 (in the SWrange). The influence of temperature errors in the abundretrievals is thus only due to uncertainties in the relative cbration of independent ISO measurements. We conservatestimate that this uncertainty leads to a 10% uncertainty in
calculated emission intensity. This turns into a±10% error bar

ndeled

al-sti-

-

inde-ad-

the

onsin

of

nceli-

velythe

for the abundances of the optically thin species (CH3C2H, C4H2)and±25% for C2H6 and C2H2. Error bars in Table V corresponto the combination of the various error bars discussed abov

Other hydrocarbons included in the photochemical mohave strong rovibrational bands in the range covered by thespectrum. These include theν21 band of C3H8 at 13.4µm, theν7 band of C2H4 at 10.5µm, the ν10 band of CH2CCH2 at11.9µm, and theν11 band of C6H2 at 16.1µm. Their nondetec-tion by ISO can be used to set upper limits on their abundanceSaturn’s stratosphere (see Table V). Figure 26 shows the reof the ethylene band with two calculations including no C2H4

and the nominal profile from the photochemical model. Coparison with the ISO spectrum shows that this profile produtoo much emission byat leasta factor of 2. We derive an uppelimit for the C2H4 column density of 4× 1015 molecules cm−2

at the 10-mbar level. A similar analysis for the 13.4-µm band ofpropane leads to an upper limit of 1.5× 1017 molecules cm−2,similar to the column abundance in the nominal model. Theper limit we derived for allene (CH2CCH2) is 3× 1015 moleculescm−2 at 10 mbar, which is 15 times larger than the abundapredicted by the model. A line compilation for theν11 band ofC6H2 is not yet available. The structure of this band is howevery similar to that of theν8 band of C4H2 nearby (15.92µm).As both bands are optically thin in Saturn’s atmosphere, thetensity ratio of the emission features produced in the two bais simply equal to the ratio of the two band strengths multipliby the ratio of the two column abundances. We estimate thawould have detected any feature at 16.1µm amounting to 20%of the C4H2 feature. Using a band strength of 312 cm−2 atm−1

(Delpechet al. 1994), we derive an upper limit for the C6H2

column density of 3× 1013 molecules cm−2. This is an order ofmagnitude larger than predicted by the photochemical mod

5.3. H and He Emission

Observations of ultraviolet dayglow emissions from atomhydrogen at 121.6 nm and neutral helium at 58.4 nm can pvide useful constraints on the upper atmospheric structure ogiant planets (e.g., Carlson and Judge 1971, Judgeet al. 1980,McConnellet al. 1981, McGrath and Clarke 1992, Vervacket al.1995, Parkinsonet al. 1998). On Saturn, these lines are primarexcited by resonance scattering of sunlight in the thermosph(Saturn’s auroral emissions are generally very much weakerJupiter’s, so that dayglow emissions dominate the planetarynal), and their observed brightness relative to one anotherstrong function of the eddy diffusion coefficient in the regionthe homopause. The primary absorbers for the hydrogen Lα

line at 121.6 nm and the helium line at 58.4 nm are CH4 and H2,respectively (CH4 and H also absorb strongly at 58.4 nm, bare much less abundant than H2). Since in the 58.4-nm radiativetransfer problem the scatterer (He) is heavier than the abso(H2), while in the Ly-α radiative transfer problem the absorb(CH4) is heavier than the scatterer (H), the brightnesses oftwo emissions respond in opposite ways to any change in
turbulence level of the upper atmosphere. Thus, an increase in

inal

282 MOSES ET AL.

FIG. 25. Comparison of the CH3 ν2 band as observed by ISO on 30 December 1997 (B´ezardet al. 1998) with a synthetic spectrum generated with the nomphotochemical model. The calculated continuum originating from the troposphere is about 4% lower than that observed, within the calibration uncertainty. Residual
instrumental fringing is present in the data.
to

e

s

ageAU

-onsel

the eddy diffusion coefficient near the homopause will resulthe brightening of the He I 58.4-nm line and the dimmingLy-α, and vice versa.

The column abundance of H atoms in the nominal modelmosphere is 1.0× 1017 cm−2. The column of H atoms above thτ = 1 level for CH4 absorption of Ly-α in the nominal model at-mosphere is considerably less than the total column, only ab3.7× 1016 cm−2. Using the partial frequency redistribution reonance line radiative transfer program of Gladstone (1988),
obtain a disk-average
re-

iffusion coefficient at the
(µ=µ0= 0.5) Ly-α brightness of 941 R
TABLE VIObserved and Modeled Ly-α and He 58.4-nm Brightnesses

Item ILyα (R) I584 (R) H Column (cm−2)a

Voyagerobservationsb 4500, 3000 3.1± 0.4, 4.2± 0.5 —IUE observationsc 1100± 357 — —EUVE observationsd — <2 —

Modelse

Model A (nominal) 941 0.40 1.04× 1017

Model B (low KH ) 1446 0.06 1.01× 1017

Low H flux (2.8× 108 cm−2 s−1) 689 0.40 7.08× 1016

High H flux (4.4× 109 cm−2 s−1) 1485 0.40 2.25× 1017

a Total column density of H in the model atmosphere.b Range of subsolar brightnesses as determined byVoyager 1and2 measurements (Yelleet al.

1987, Parkinsonet al. 1998).c Mean and standard deviation of 29 disk Ly-α brightnesses determinations from IUE measu

ments during 1980–1990 (McGrath and Clarke 1992).d Upper limit from a November 1997 observation (Gladstone, in preparation, 1999).

atmospheres in which the eddy d

e All model runs assumeµ=µ0= 0.5, approp

inf

at-

out-we

with the nominal model atmosphere (assuming a solar Ly-α fluxat 1 AU of 3.2× 1011 photons cm−2 s−1). The correspondingcalculation for the He 58.4-nm line results in a disk-averbrightness of 0.4 R (assuming a solar He 58.4-nm flux at 1of 2.0× 109 photons cm−2 s−1).

Table VI compares IUE, EUVE, andVoyagerUVS bright-ness measurements of the HI Ly-α and He I 58.4-nm dayglow on Saturn with the results of radiative transfer calculatifor the nominal model (Model A) and for a few other mod

riate for disk-average comparisons.

ed

tiu-

n

rt

c

i

R at

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sE

edndd byntsla-,e of-

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FIG. 26. The ISO SW spectrum is compared with synthetic spectracluding no C2H4 (thick line) and the nominal photochemical profile (thin linbetween 10.2 and 11µm. Emission from theν7 ethylene band is not detectein the ISO spectrum. Absorption features are mostly due to PH3 with somecontribution from NH3 opacity (de Graauwet al. 1997).

homopause or the downward flux of atomic hydrogen atupper boundary of the model has been varied. The nommodel Ly-α and 58.4-nm brightnesses of 941 and 0.4 R are msmaller than that observed by theVoyagerUVS, but are reasonably consistent with the IUE and EUVE results. As discussedMcGrath and Clarke (1992), reconciling the large discrepabetween the IUE andVoyagerobservations of Ly-α requires ei-ther (1) an improbably large extinction by interplanetary hydgen or (2) a relative calibration problem between the two detors. Similarly, Parkinsonet al. (1998) find that theVoyager58.4-nm brightnesses are very difficult to simulate with realistic moels, given that the solar flux at 58.4 nm is currently thought toabout half that assumed in previous studies. Only by increing the He abundance to near-solar values (i.e.,∼13% ratherthan∼3%) can Parkinsonet al. reproduce theVoyager58.4-nmobservations. Our models have a similar problem, and onlytaking helium to be well mixed throughout the nominal modatmosphere (effectively assuming an infinite eddy diffusionefficient) does our predicted He I disk-averaged brightness re4.2 R. Increasing the tropospheric He mixing ratio to 10% w
our nominal eddy diffusion coefficient profile does not resol

in-)

henalch

bycy

o-ec-

d-beas-

byelo-achth

the problem; the predicted brightness then becomes 0.7258.4 nm.

Changing the eddy diffusion coefficient profile or the assuminflux of atomic hydrogen can alter the predicted emissiontensity. Lowering the eddy diffusion coefficient, as in Model Bresults in about a 50% increase in the Ly-α brightness, buta factor of 7 drop in the 58.4-nm brightness. Preliminary rsults from EUVE observations of G. R. Gladstone (unpublishmanuscript, 1999) indicate an upper limit to Saturn’s 58.4-nbrightness of<2 R. By varying the H flux assumed to be incident from the upper boundary of the model, we can raiselower the model Ly-α brightness considerably. Flux variationof this type could explain some of the large variability in the IUmeasurements of Saturn’s disk-averaged Ly-α brightness (e.g.,McGrath and Clarke 1992).

In summary, the Ly-α and He I 58.4-nm brightnesses expectfor our nominal model atmosphere are in accord with IUE aEUVE expectations, but are much less than those measuretheVoyagerUVS experiments. Based on all the other constraimet by the nominal model, we feel that the most likely expnation for theVoyagerdiscrepancy is in the UVS calibrationand that the nominal model provides a reasonable estimatSaturn’s actual H I 121.6-nm and He I 58.4-nm dayglow brightnesses.

6. IMPLICATIONS FOR STRATOSPHERICAEROSOL FORMATION

Diacetylene (C4H2), butane (C4H10), and water (H2O) willcondense in Saturn’s lower stratosphere and are expected totribute to the observed stratospheric haze layers. Other spesuch as C4H4, C6H2, and C6H6 can also condense, but their contribution to the total aerosol mass is minor (see Appendix D). Tfluxes of butane, water, and diacetylene vapor through theof their condensation levels in our nominal model are 2× 10−16,

3× 10−17, and 2× 10−18 g cm−2 s−1, respectively. However,chemical production of diacetylene occurs within the condesation region and contributes greatly to the total aerosol mIn fact, C4H2 condensation has some interesting effects onchemistry of acetylene and the other hydrocarbons. Whenacetylene molecules are sequestered in the condensed pacetylene recycling is reduced (see reactions R45, R46, oaction scheme (52) in Appendix D), causing a reduction intotal stratospheric column abundance of C2H2. A correspond-ing reduction in the column abundance of C2H6 via reactionscheme (16) in Appendix B is also evident. CondensationC4H10 also has some effects on the photochemistry. When cdensation is included in the models, more of the stratosphcarbon is bound up in the condensed phase, altering someportant gas-phase chemical production and loss schemes.carbon is not liberated until C4H2 and C4H10 evaporate again inthe upper troposphere.

We have neglected PH3 and NH3 photochemistry in our Saturn
vemodel, and P2H4 is a potentially dominant aerosol constituent,

s exhibitegions

284 MOSES ET AL.

FIG. 27. Contribution functions (see text for definition) calculated at the peaks of the emission bands from the detected hydrocarbons. The functionapeak at 100 mbar or deeper in the troposphere due to the H2–He collision-induced opacity. Stratospheric emission from the hydrocarbons originates from r
centered at 0.5 mbar (CH , CH C H, and CH ), 1.4 mbar (CH /ν ), 0.3 mbar (CH /ν + ν − ν ), and 0.2µbar (CH ).
e

p

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loseresideforthan

2 6 3 2 4 2 2 2 5

especially in the upper troposphere (Kaye and Strobel 19Westet al. 1986). Its importance in the lower stratosphere is lcertain because unlessK ≈ 106 in the lower stratosphere/uppetroposphere, the models of Kaye and Strobel (1984) indicthat the PH3 abundance and the corresponding P2H4 productionrate drop off sharply below the tropopause. However, duethe potentially large production rate of P2H4 and our improvedunderstanding of Saturn’s composition and structure, thesetochemical calculations deserve to be examined anew.

Because the condensed phases are treated as separatein the model, we can easily keep track of the total amouncondensed material at all atmospheric levels. In our nommodel, the total column abundance of condensed materiroughly 2× 10−5 g cm−2, with condensed C4H2 making up thebulk of the mass. For an assumed density of 1.0 g cm−3, this col-umn mass corresponds to an aerosol column density of 5.5× 109

particles cm−2 for an assumed particle radius of 0.1µm or 1.6×109 particles cm−2 for an assumed particle radius of 0.15µm.Because we only consider diffusion in our model and doconsider particle sedimentation or other aerosol dynamicalcesses, this column abundance will be an overestimate.instance, at the tropopause in our nominal model, the sedimtation time scale for 0.15µm particles is 11 years while thdiffusion time scale is 500 years.

From theVoyager 2photopolarimeter observations at 264 a750 nm and a variety of phase angles, Westet al. (1983) havedetermined that the stratospheric haze layer in the equatoria
gion of Saturn (3–10◦ latitude) is located (centered) in the regio
2 2 5 4 4 3

84,ssrate

to

ho-

peciesof

nall is

otro-Foren-

d

l re-

30 to 70 mbar, with a total column density of 9× 108 cm−2 forparticles of an assumed radius of 0.1µm. A diffuse haze fitstheir data better than a thin layer, but the aerosol mixing rmust diminish above 10 mbar. Similarly, ground-based Teclipse observations of Smithet al. (1981) indicate that Saturnstratospheric aerosol extends from 10 to 85 mbar at−23◦ lati-tude, with a derived optical depth that is consistent with the hmodel of Westet al. (1983). Using Hubble Space Telescope oservations, Karkoschka and Tomasko (1993) favor stratosphaze models in which the particle radius is∼0.15µm, and theparticles are distributed uniformly in the top 10 km–am of g(above∼80 mbar in our model).

Our model of C4H2, C4H10, and H2O condensation at middland lower latitudes (i.e., nonauroral regions) on Saturn segenerally consistent with the above stratospheric haze ovations. Although the column aerosol abundance in our mis somewhat larger than the abundance derived by Westet al.(1983), our neglect of sedimentation causes the column adance above the base of our aerosol region to be as mua factor of 10 higher than it would be if sedimentation weincluded. Our haze layer extends from roughly 1 to 300 maerosol production is greatest at the tropopause (63 mbar).imentation would cause the particles to fall more quickly frothe upper levels, causing the particles to be concentrated cto the tropopause. Evaporation constrains the hazes to rabove∼300 mbar in our model. Note that this base levelour aerosol layer is at lower altitudes (higher pressures)
nis generally assumed in observational analyses. It is unlikely

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that the hazes could be removed rapidly from the regiontween the tropopause and∼300 mbar unless condensationa species such as P2H4 effectively increases the particsize—sedimentation of 0.1–0.15µm particles is simply not efficient enough. Detailed models coupling photochemistryaerosol microphysics would be needed before we could mmore definite conclusions regarding stratospheric aerosolperties.

7. IMPLICATIONS FOR CH3C2H AND C4H2 CHEMISTRY

From the sensitivity tests discussed above, it is clear thaacetylene (C4H2) and methylacetylene (CH3C2H) are excellenttracers of important chemical processes and physical condiin Saturn’s upper atmosphere, and one of the main goals oinvestigation has been to determine the important productionloss schemes for these molecules. Our dominant methylalene chemical production schemes are different from thosetermined by Gladstoneet al. (1996) or Laraet al. (1996). Forinstance, unlike the Titan model of Laraet al. (1996), we findthat reaction R154 (3CH2+C2H2

M−→CH3C2H) should not bean important route to methylacetylene production on SaturnTitan). According to the work of B¨ohlandet al. (1986), the reaction of 3CH2 with acetylene most likely possesses a large avation energy, which would prevent these species from reacrapidly at the low temperatures typical of Saturn’s (or Titanstratosphere.

In our model, CH3C2H is not synthesized directly from C anC2 compounds but is produced by exchange reactions involother C3 species. These precursor C3 species are formed by phtolysis of C4 compounds (e.g., reaction schemes (28), (29),(30) in Appendix C), by radical–radical recombination reacti(e.g., reaction scheme (31)), and by CH insertion reactions (reaction schemes (26) and (27)). Reactions schemes invoCH insertion account for only 4% of the total CH3C2H produc-tion rate in our model (compared with∼50% in the Gladstoneet al. (1996) Jupiter model). Nearly 45% of the total prodtion rate of CH3C2H in our nominal model results from reation schemes that convert allene (CH2CCH2) to CH3C2H (seeR112, and reaction schemes (34) and (36)). Methylacetyis destroyed by photolysis or by addition with atomic hydgen to form C3H5. Because both the photolysis products aC3H5 can react to reform CH3C2H, methylacetylene can be rcycled on Saturn and is more stable in the stratosphere thchemical loss rate would indicate. To truly determine the imptant reaction pathways for methylacetylene in outer planeatmospheres, we need to obtain better information concerthe rate coefficients for reactions R107–R117, R163–R167R206–R208.

Our chemical production mechanisms for diacetylene (C4H2)are also different from those of Gladstoneet al. (1996). Specifi-cally, we have omitted two reactions that Gladstoneet al. foundto be the dominant pathways for synthesizing CH on Jupiter:
4 2
(a) C3H2+C2H2−→C4H2+ 3CH2 and (b) C3H3+C2H2−→


be-f

ndakero-

di-

ionsourandety-de-

(or

ti-ting’s)

ing-ndns.g.,

ving

c--

eneo-nd-n itsor-arying

and

C4H2+CH3. The latter reaction is endothermic, and the formreaction, while exothermic for the most stable C3H2 isomer, isunlikely to proceed without a substantial energy barrier. Ththe primary pathway for synthesizing diacetylene in our Samodel becomes R179 (C2H+C2H2−→C4H2+H), where theC2H is produced directly or indirectly from acetylene photoly(see reaction schemes (43) and (44) in Appendix D). Diacetyis removed by photolysis, by reaction with atomic hydrogen,by condensation.

8. SUMMARY AND CONCLUSIONS

8.1. Model Strengths and Weaknesses

We use a one-dimensional diurnally averaged chemical kics/diffusion/radiative transfer model to investigate the detaihydrocarbon photochemistry on Saturn. The model containupdated and expanded subset of the hydrocarbon reactionsin the Gladstoneet al. (1996) survey of jovian photochemistrThe model results are compared with ISO observations toidentify the important chemical schemes that control the abdances of the observed constituents CH3, C2H2, C2H6, CH3C2H,C4H2, H2O, and CO2 in Saturn’s stratosphere. The presencethe oxygen compounds in Saturn’s upper atmosphere sugan external source of oxygen to the planet (e.g., Feuchtgret al. 1997), and our models include a micrometeoritic abtion source or a ring-derived vapor source; details of the oxyphotochemistry and implications for the supply of extraplatary material to Saturn are discussed in a separate paper (Met al. 2000). Few published models devoted to Saturn phchemistry currently exist. By combining improved and updaphotochemical models with the many new constraints impoby the ISO observations, we have considerably enhanced ouderstanding of the photochemistry of the atmospheres of Sand the other outer planets.

The ISO observations are a valuable resource for consting photochemistry and diffusion on Saturn because (1) thmultaneous detection of several molecular species eliminpotential problems due to temporal variability or different anysis techniques, (2) the wide wavelength coverage helpsstrain temperatures to more uniquely determine molecular adances, and (3) the observations of the complex hydrocarCH3C2H and C4H2, which are particularly sensitive to chemicand physical conditions, help identify the important chempathways operating in Saturn’s stratosphere. Synthetic spcreated from our nominal model reproduce the emission feaof most of the molecules observed by ISO to within the∼20%uncertainty inherent in the ISO spectra. Such an unprecedematch between model and data contrasts sharply with theation for Jupiter, where even the abundances of the simpl2

hydrocarbons are difficult to reconcile with observations (eAllen et al. 1992, Gladstoneet al. 1996, Romani 1996). Ousuccess for Saturn demonstrates both the power of the ISOservations and our increasing knowledge of important reac
rate coefficients and chemical schemes. However, our nominal

t

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nsere.

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286 MOSES

Saturn photochemical model predicts slightly more C4H2 thanis observed and too much C2H4 compared with the upper limiimposed by the ISO spectra, suggesting that problems withadopted reaction rates, reaction pathways, photolysis crosstions, or diffusion coefficients still exist.

The problem with C2H4 is particularly disconcerting in thaC2H2 and C2H4 are closely linked. By changing certain reactirate coefficients within prescribed uncertainty limits, we canduce the C2H4 abundance in the model but not without affectithe C2H2 abundance. We find that a change in any single retion rate coefficient cannot resolve the problem; however,have not explored all possible combinations of changes. Nmeasurements of low-temperature cross sections for C2H4 areneeded and might shed light on this issue. The close matchtween the observed and modeled abundances of C2H2, CH3C2H,and C4H2 suggests that the pathways leading from C2H2 toCH3C2H and C4H2 are well represented.

Diacetylene (C4H2) and methylacetylene (CH3C2H) are goodindicators of important chemical processes and physical cotions in Saturn’s upper atmosphere, and the ISO observaof these molecules substantially aid our understanding ofdrocarbon photochemistry on the outer planets. Our chemschemes for the production and loss of these molecules dconsiderably from previous researchers. For further detailsthe photochemistry of these molecules, see Appendices CD and Section 7.

8.2. Hydrocarbon Haze Formation

Diacetylene, butane, and water will condense in Satulower stratosphere and are likely constituents of the obsestratospheric haze layers. The photochemical model predicconcerning the haze location and column abundance seemsistent with the haze observations of Smithet al. (1981), Westet al. (1983), and Karkoschka and Tomasko (1993), althofurther aerosol microphysical modeling using initial conditiofrom the photochemical model results is warranted. Satu“stratospheric” haze probably extends from∼10 mbar throughthe tropopause down to∼300 mbar. Evaporation and other aersol loss processes might not be effective at pressures less∼300 mbar.

8.3. Implications for Cassini

Comparisons of our photochemical models with futureCassiniobservations, particularly from the Composite Infrared Sptrometer (CIRS) and Ultraviolet Imaging Spectrograph (UVIinstruments, should contribute valuable information conceing chemical processes in Saturn’s upper atmosphere (seeand Herrell 1996, Kundeet al. 1996). The CIRS and UVIS instruments will complement each other well. Together, they wprobe Saturn’s entire upper atmosphere from above the methomopause to below the tropopause, acquiring a relatively cplete survey of the most abundant stratospheric constituents
providing important tests for photochemical models. The UV
ET AL.

oursec-

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be-

di-onsy-

icalfferonand

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onscon-

ghsn’s

-than

c-)

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illanem-and

instrument will probe Saturn’s vertical structure and compsition through occultations of the Sun and stars and throcenter-to-limb and multiple phase-angle observations. Thecultations will provide vertical profiles with∼5 km resolutiondown past the methane homopause, contributing informaabout temperatures, vertical mixing, and composition. Thiscultation capability will provide unique information that cannbe obtained from ground-based observations.

The CIRS instrument will also provide global informatioon gas composition, temperatures, and cloud/haze propecompositional and/or temperature variations will be mappedtracked over several orbits (Kundeet al. 1996). The spectral resolution (0.5–20 cm−1) is higher than that of theVoyagerIRISinstrument, allowing the CIRS instrument to probe an extenaltitude range with sharper contribution functions. As with tISO telescope, the simultaneous multiwavelength observatshould allow temperatures and hence concentrations to beconstrained. Unlike the ISO observations, the CIRS observatwill be spatially resolved, and latitudinal or other global varitions can be examined. Our photochemical models can be ushelp with the planning of some of the observational sequenfor the Cassinimission, and with the analysis of theCassinidata. Comparisons of our model results withCassiniobserva-tions will lead to better constraints on the physical conditioand dominant photochemical pathways in Saturn’s atmosph

8.4. Future Work

Our Saturn model can be applied to photochemical studiethe other outer planets, where differences in temperature, sflux, atmospheric mixing, and methane abundance cause dences in stratospheric composition. Such a comparative petology approach will help determine whether the same bchemical cycles operate in all the outer-planetary atmosphor whether any unique processes exist. Although an influx ofogenic oxygen-bearing material does not affect the backgrohydrocarbon photochemistry on Saturn, this result might chafor Jupiter (where H2O would condense at lower altitudes) oUranus (where eddy mixing is greatly reduced). Further moding is warranted.

APPENDIX A

Atmospheric Structure

The vertical temperature profile that we have adopted for the photochemmodel is shown in Fig. 1. The thermospheric profile was taken from the Uoccultation analysis of Smithet al. (1983). The stratospheric profile is a smoothversion of the best fit 28 Sgr occultation results of Hubbardet al. (1997). Theprofile for the troposphere and lower stratosphere (at pressures∼>1 mbar) is amodification of theVoyager 2ingress radio science (RSS) occultation profi(Lindal et al. 1985, Lindal 1992). Our profile is cooler than the RSS profilethe region∼1–6 mbar (by as much as 8 K; see Fig. 1) to better reproduceISO observations of the methaneν4 band emission in the spectral region 7–8µmand the S(0) and S(1) lines of H2 at 17.0 and 28.2µm. Our profile is warmerthan the RSS data at pressures∼>6 mbar to better fit the ISO observations of thH2–He continuum longward of 13µm (de Graauwet al. 1997, Bezardet al.
IS1998).

therum.(1) li


FIG. A1. Observed and synthetic spectra of the CH4 ν4 band and of the S(0) and S(1) H2 lines that are used for constraining the temperature profile inlower stratosphere (0.4–5 mbar). The top left panel shows the observed spectrum in the CH4 ν4 region, and the bottom left panel shows the synthetic spectFor the panels on the right, the solid lines represents the ISO spectra and the dashed lines the synthetic spectra. The ISO spectra of the S(0) and Snes havebeen rescaled by factors of 1.12 and 0.94 respectively to reproduce the calculated H–He tropospheric continuum. The zero level in the ISO data of the CHband
2 4
is uncertain by about 20 Jy due to dark current subtraction. Reflected sunlight from the rings or the planet is not included in the CH4 spectrum calculation.

hgc

in

ela

larecleroli

e car-

avity

,ator,foncularme

-

theive ainlloff

Figure A1 shows the spectra of the S(0) and S(1) hydrogen lines and of tν4

methane band. The S(0) and S(1) emission lines probe respectively the re5 and 1.5 mbar, with contribution functions extending over 3 pressure sheights (full width at half maximum). The calculated line-to-continuum ratioapproximately 10% too low for the S(0) line and 15% too high for the S(1) lWe regard this overall agreement as satisfactory. The emission in the CH4 bandat 7.7µm is sensitive to temperature inhomogeneities present over the obsdisk. The synthetic spectrum in Fig. A1 is the average of two spectra calcuwith “cold” and “warm” temperature profiles, departing by−6 and+6 K at0.5 mbar from our nominal profile. The Gaussian perturbations, having awidth at half maximum of 6.5 scale heights, are intended to simulate thelatitudinal gradients that exist on Saturn (B´ezardet al. 1984, Bezard and Gautie1985). The intensity of the CH4 Q branch is correctly reproduced by the modOn the other hand, the relative intensity of the P and R branches is not prefitted for unknown reasons. The observed spectrum exhibits a “continuum”higher than that in the calculations. This difference could originate partly fincorrect subtraction of the dark current in the ISO spectrum or from sunreflected by the rings and the planet not included in the model.

To solve the hydrostatic equilibrium equation, we need to obtain informatabout the gravitational acceleration and the mean molecular mass variation

eionsaleise.

rvedted

fullrge

l.iselyvelm

ght

altitude. We assume that the zonal winds observed in the troposphere arried through to higher altitudes (see Hubbardet al. 1997 for a discussion of theevidence for this assumption). Information on the planetary shape and grparameters (e.g.,J2 and J4) are taken from Lindalet al. (1985) and Campbelland Anderson (1989). Zonal winds provided by Nicholsonet al. (1995) areused; for our nominal model at 30◦N latitude, the zonal wind is roughly zeroand the radius at the 1-bar level is assumed to be 58,521 km. At the equwe assume a zonal wind speed of 440 m s−1 and a 1-bar planetary radius o60,268 km (see Lindalet al. 1985, Nicholson and Porco 1988, and Nicholset al. 1995). At the base of the model atmosphere, we assume that the molehydrogen mixing ratio is 0.963 and the He mixing ratio is 0.0325 by volu(Conrathet al. 1984) while the methane mixing ratio is 4.5× 10−3 (Courtinet al. 1984). Therefore, the mean molecular massµ at the base of the atmosphere is∼2.1435 amu. We further assume thatµ changes with altitude dueto the effects of molecular diffusion and chemistry. By iterating betweenphotochemical model results and hydrostatic equilibrium models, we dermean-molecular-mass profile that is represented by a gradual reductionµfrom 2.1435 amu at 5 bar to 2.136 amu at 1 mbar, followed by a steeper fa

−6
ionwithto 2.008 amu at 10 mbar, and an even more precipitous falloff at higheraltitudes.

S

ssd

hc)rn

yoCn

fo

ativee rateloss% of

CHrate

nt

lving

r-

at

es

ig. 4)thane

mes (7)high

enough to accommodate three-body reactions. There are no major primary pro-

288 MOSE

APPENDIX B

Reaction Schemes for Major C2 Molecules

Reaction scheme (3) from Section 4.1 (CH insertion into methane) iprimary mechanism for producing C2H4 in our Saturn model. Ethylene is alsynthesized in the upper stratosphere through ethane photolysis (R18 anand through the primary production schemes:


H+ CH −→ C+ H2 R91

C+ H2M−→ 3CH2 R137

H+ 3CH2 −→ CH+ H2 R93

CH+ CH4 −→ C2H4 + H R141

Net: 2CH4 −→ C2H4 + 2H2, (4)

and

CH4hν−→ 1CH2 + H2 R6

1CH2 + H2 −→ 3CH2 + H2 R145

H+ 3CH2 −→ CH+ H2 R93

CH+ CH4 −→ C2H4 + H R141

Net: 2CH4 −→ C2H4 + 2H2, (5)

and

CH4hν−→ 3CH2 + 2H R8

H+ 3CH2 −→ CH+ H2 R93

CH+ CH4 −→ C2H4 + H R141

Net: 2CH4 −→ C2H4 + H2 + 2H. (6)

Scheme (3) is responsible for 55% of the primary (nonrecycling) column protion rate of C2H4 in the stratosphere, scheme (4) is responsible for 10%, sc(5) is responsible for 6%, and scheme (6) is responsible for 3%. In all, reainvolving R141 are responsible for 80% of the C2H4 primary (nonrecyclingproduction rate, while ethane photolysis contributes another 10%, primaactions involving R101 contribute another 7%, and photolysis and crackiC4 and C3 hydrocarbons contribute 3%. Contrary to Gladstoneet al. (1996), wefind that reaction R150 (3CH2+CH3−→C2H4+H) is not a major pathwafor forming ethylene in our model because we consider the methane photchannel leading to3CH2 (R8) to be less important than those producingor CH3 (R9 and R5), and both CH3 and3CH2 are removed by more efficiemeans.

Ethylene is lost from the upper stratosphere of Saturn by photolysis toacetylene (R15 and R14) and by three-body addition with atomic hydr(R102). The addition mechanism causes the conversion of C2H4 to either C2H6

or CH4:

H+ C2H4M−→ C2H5 R102

H+ C2H5 −→ 2 CH3 R103

2CH3M−→ C2H6 R158

Net: C2H4 + 2H −→ C2H6, (7)

ET AL.

theoR19)

duc-emetions

y re-g of

lysisH

t

ormgen

or

H+ C2H4M−→ C2H5 R102

H+ C2H5 −→ 2 CH3 R103

2(H+ CH3M−→ CH4) R95

Net: C2H4 + 4H −→ 2CH4. (8)

The relative efficiencies of the two schemes (7) and (8) depend on the relrate coefficients of R158 and R95 at low temperatures and pressures. Thescoefficients are not well known. In our nominal model, R158 dominates theof CH3 radicals in the upper stratosphere (e.g., R158 is responsible for 98the CH3 loss at optical depth unity for H Ly-α), while R95 dominates in the lowerstratosphere and upper troposphere (e.g., R95 is responsible for 87% of the3

loss at the tropopause). From the standpoint of the total column-integratedin the stratosphere, R158 is responsible for∼63% of the CH3 loss, while R95 isresponsible for∼30% of the CH3 loss. Scheme (7) is therefore more importain permanently removing C2H4 from Saturn’s stratosphere.

Ethylene is also converted to acetylene and ethane through reactions invoCH:

2(CH4hν−→ CH+ H+ H2) R9

2(CH + C2H4 −→ C2H2 + CH3) R143

2CH3M−→ C2H6 R158

Net: 2CH4 + 2C2H4 −→ 2C2H2 + C2H6 + 2H+ 2H2. (9)

Ethylene photolysis does not necessarily result in the permanent loss of C2H4

from the stratosphere; some of the acetylene produced from C2H4 photolysisends up recycling ethylene through the two pathways

C2H4hν−→ C2H2 + 2H R15

H+ C2H2M−→ C2H3 R99

H+ C2H3M−→ C2H4 R101

Net: Nothing (10)

and

C2H4hν−→ C2H2 + H2 R14

H+ C2H2M−→ C2H3 R99

H+ C2H3M−→ C2H4 R101

Net: 2H−→ H2. (11)

Primary (nonrecycling) C2H4 production schemes that utilize R141 (CH insetion into methane) account for only 33.5% of the total C2H4 column productionrate of 1.1× 109 cm−2 s−1 in our model; other primary production schemes thinvolve photolysis and cracking of C2, C3, and C4 hydrocarbons account foranother 8.4% of the total C2H4 column production rate, and recycling schemsuch as (10) and (11) make up the bulk of the remainder (∼58%). The C2H4

recycling schemes in our model differ from those in Gladstoneet al. (1996) inthat we do not consider the reaction C2H3+H2 −→ C2H4+H (R190) to beimportant at low temperatures (see Fahret al. 1995, Romani 1996).

Ethylene is concentrated at high altitudes in Saturn’s stratosphere (see Fbecause the main production mechanism (scheme (3)) occurs near the mehomopause and because the major permanent loss processes (e.g., scheand (8)) do not become efficient until the atmospheric densities become

duction schemes for ethylene in the middle and lower stratosphere, and chemical

o

n

e

at

ere

re

errt

1)

nt

ult-o


loss is relatively efficient in this region; thus, the ethylene mixing ratio dropsrapidly at lower altitudes.

Acetylene is produced in the stratosphere primarily by ethylene photoly(R14 and R15), with a smaller contribution from ethane photolysis (R20). Thimportant primary schemes in the upper stratosphere are

CH4hν−→ CH+ H + H2 R9

CH+ CH4 −→ C2H4 + H R141

C2H4hν−→ C2H2 + 2H R15

Net: 2CH4 −→ C2H2 + H2 + 4H, (12)

and

CH4hν−→ CH+ H + H2 R9

CH+ CH4 −→ C2H4 + H R141

C2H4hν−→ C2H2 + H2 R14

Net: 2CH4 −→ C2H2 + 2H2 + 2H, (13)

or

2(CH4hν−→ CH3 + H) R5

2CH3M−→ C2H6 R158

C2H6hν−→ C2H2 + 2H2 R20

Net: 2CH4 −→ C2H2 + 2H2 + 2H. (14)

Direct C2H4 and C2H6 photolysis account for only 15% of the total columproduction rate of C2H2 in our nominal model, and indirect nonrecycling reactions involving R100 account for another∼2%. Roughly 80% of the total C2H2

column production rate is due to recycling schemes (see below). Schemesand (13) were also recognized as being the dominant schemes for the produof C2H2 in Titan’s atmosphere (Laraet al. 1996).

In the upper stratosphere, near the methane photolysis region, C2H2 is pro-duced much more readily than it is destroyed, so that a net production of C2H2

occurs in the region 3× 10−5 to 3× 10−3 mbar. At these high altitudes, CH in-sertion into acetylene (R142) forming C3H2 is an important “permanent” C2H2

destruction process, but it and the other major loss mechanisms (e.g., R99,R11) cannot keep pace with C2H2 formation. At higher pressures, loss processbegin to dominate as C2H2 becomes optically thick and is photolyzed and a

three-body reactions such as R99 (H+C2H2M−→C2H3) become more effective.

At 8× 10−3 mbar, three-body addition with hydrogen (R99) becomesefficient loss mechanism for acetylene. The vinyl radicals that are formed inprocess act to either recycle the C2H2 (∼90% of the time at this altitude),

H+ C2H2M−→ C2H3 R99

H+ C2H3 −→ C2H2 + H2 R100

Net: 2H−→ H2, (15)

or to permanently remove the acetylene through reactions that convert the ac


ff

sisree

-

(12)ction

R10,ss

nhis

lene to ethane,

H+ C2H2M−→ C2H3 R99

H+ C2H3M−→ C2H4 R101

H+ C2H4M−→ C2H5 R102

H+ C2H5 −→ 2CH3 R103

2 CH3M−→ C2H6 R158

Net: C2H2 + 4H −→ C2H6. (16)

This latter scheme is effective throughout the middle and lower stratosphand, unlike the model of Gladstoneet al. (1996), is the dominant mechanism forconverting C2H2 to C2H6 in Saturn’s stratosphere. The hydrogen atoms that arequired for this process to operate are produced from CH4 and C2H4 photolysiswithin (and just above) this altitude region; H atoms also flow in from highaltitude levels due to H2 photolysis. Schemes (12) and (16) combine to convetwo methane molecules into one C2H6 and one H2 molecule.

From 3× 10−2 mbar to the upper troposphere, photolysis (R10 and R1dominates the destruction of C2H2. As with the Jupiter model of Gladstoneet al. (1996) and the Titan model of Yunget al. (1984), we find that the acetylenephotolysis products frequently react with molecular hydrogen in two importaprocesses that act to catalytically destroy H2 in the middle and lower stratosphereof Saturn:

C2H2hν−→ C2H+ H R10

C2H+ H2 −→ C2H2 + H R177

Net: H2 −→ 2H, (17)

and

C2H2hν−→ C2 + H2 R11

C2 + H2 −→ C2H+ H R175

C2H+ H2 −→ C2H2 + H R177

Net: H2 −→ 2H. (18)

Acetylene photolysis products can also catalytically destroy methane, resing in the production of C2H6 in the middle and lower stratosphere (see alsYunget al. 1984, Toublancet al. 1995, and Gladstoneet al. 1996):

2(C2H2hν−→ C2 + H2) R11

2(C2 + CH4 −→ C2H+ CH3) R176

2(C2H+ H2 −→ C2H2 + H) R177

2CH3M−→ C2H6 R158

Net: 2CH4 −→ C2H6 + 2H, (19)

and

2(C2H2hν−→ C2H+ H) R10

2(C2H+ CH4 −→ C2H2 + CH3) R178

2 CH3M−→ C2H6 R158

ety- Net: 2CH4 −→ C2H6 + 2H, (20)

S

c

f

a

t

p

o

tt

5p

tao

f

ne:

t keepne

s not

rere—tal

tmo-ted. Ins

290 MOSE

and

C2H2hν−→ C2 + H2 R11

C2 + CH4 −→ C2H+ CH3 R176

C2H+ CH4 −→ C2H2 + CH3 R178

2 CH3M−→ C2H6 R158

Net: 2CH4 −→ C2H6 + H2, (21)

and

2(C2H2hν−→ C2 + H2) R11

2(C2 + H2 −→ C2H+ H) R175

2(C2H+ CH4 −→ C2H2 + CH3) R178

2 CH3M−→ C2H6 R158

Net: 2CH4 −→ C2H6 + 2H. (22)

All of these catalytic schemes allow acetylene to be very efficiently recyin the middle and lower stratosphere of Saturn. Despite ineffective primproduction mechanisms in this region (mostly cracking and photolysis o3

and C4 hydrocarbons), the recycling mechanisms prevent the C2H2 mixing ratiofrom dropping off as rapidly with altitude as C2H4 (see Fig. 4).

Ethane is formed in the upper stratosphere by methane photolysis via rescheme (2) or schemes such as

CH4hν−→ 1CH2 + H2 R6

CH4hν−→ CH3 + H R5

1CH2 + H2 −→ CH3 + H R146

2CH3M−→ C2 H6 R158

Net: 2 CH4 −→ C2H6 + 2H. (23)

Throughout the stratosphere, C2H4 and C2H2 are converted to C2H6 throughschemes involving R102 and R103 (e.g., schemes (7) and (16)). In the mand lower stratosphere, photosensitized destruction of methane through careactions involving C2H2 are important mechanisms for producing ethane (eschemes (19), (20), (21), and (22)). Each of these mechanisms (direc4

photolysis, C2H2 and C2H4 conversion, and photosensitized CH4 dissociation)account for roughly one-third (25, 31, and 39%, respectively) of the totalduction of ethane in the stratosphere in our nominal model. Schemes that inphotolysis and cracking of C3 and C4 hydrocarbons contribute∼5% to the totalstratospheric production of C2H6. Photosensitized destruction is more imptant and C2H4 and C2H2 conversion less important than in the Gladstoneet al.(1996) Jupiter model.

The production of C2H6 by CH3–CH3 recombination (R158) competes wiH–CH3 recombination (R95) in the stratosphere. Reaction R158 accoun∼63% of the CH3 loss in the stratosphere, R95 accounts for∼30%, other radical–radical combination reactions (e.g., R159, R160, R162, R167) account forand CH3 photolysis accounts for 1.5%. Reaction R158 dominates in the ustratosphere, but R95 takes over at pressures>5 mbar.

Chemical loss processes are not effective at removing ethane from Sastratosphere. As a result, C2H6 has a long lifetime and is the most abundof all the disequilibrium hydrocarbons. Ethane builds up in the stagnant lstratosphere (see Fig. 4), and its removal is largely controlled by diffusionthe troposphere, where it will eventually be thermally decomposed to re
CH4. Photosensitized destruction of C2H6 via C2H2 photolysis (R181) operatesin the middle and lower stratosphere; however, the products tend to rec
and

ET AL.

ledaryC

ction

iddletalytic.g.,CH

ro-volve

r-

hs for

.6%,per

urn’sntwerintoorm

C2H6:


C2H+ C2H6 −→ C2H2 + C2H5 R181

H+ C2H5 −→ 2CH3 R103

2CH3M−→ C2H6 R158

Net: Nothing. (24)

At pressures>15 mbar, photosensitized destruction of C2H2 followed by propane(C3H8) formation is the only noticeable chemical loss mechanism for etha

2(C2H2hν−→ C2H+ H) R10

C2H+ C2H6 −→ C2H2 + C2H5 R181

C2H+ CH4 −→ C2H2 + CH3 R178

CH3 + C2H5M−→ C3H8 R162

Net: CH4 + C2H6 −→ C3H8 + 2H. (25)

Insertion of CH into C2H6 to form propylene (C3H6) by reaction R144 is animportant loss mechanism in the upper stratosphere, but its rate cannopace with the primary production of C2H6. Throughout the stratosphere, ethais lost by photolysis to produce C2H4, C2H2, CH4, or CH3; however, methane toa large extent shields ethane from ultraviolet radiation, so that photolysis ian effective removal process. Therefore, a large net production of C2H6 existsfrom 3× 10−5 to 10 mbar in our nominal model. Diffusion into the tropospheis the most effective method for removing ethane from Saturn’s stratosphediffusion of C2H6 through the lower boundary accounts for 78% of the tocolumn production of ethane in our nominal model.

APPENDIX C

Reaction Schemes for Major C3 Molecules

Methylacetylene and allene are formed numerous ways in Saturn’s asphere (see Fig. 3), and the reaction schemes can become quite convoluthe upper atmosphere, CH3C2H and CH2CCH2 are formed by primary processeinvolving CH insertion:


CH+ C2H2 −→ C3H2 + H R142

H+ C3H2M−→ C3H3 R107

H+ C3H3M−→ CH3C2H (CH2CCH2) R108 (R109)

Net: CH4 + C2H2 −→ CH3C2H (CH2CCH2)+ H2, (26)

and


CH+ C2H6 −→ C3H6 + H R144

C3H6hν−→ C3H5 + H R34

H+ C3H5 −→ CH3C2H (CH2CCH2)+ H2 R114 (R115)

Net: CH4 + C2H6 −→ CH3C2H (CH2CCH2)+ 2H2 + 2H. (27)

In the middle and lower stratosphere, primary production of CH3C2H andCH2CCH2 is accomplished through radical–radical combination reactions

yclethrough photolysis of C4 compounds. The most important schemes (in

S

i

e

ei

l

nfor

We

ions

n itssms

a-

ne

’sThe


decreasing order of importance) are

1-C4H6hν−→ C3H3 + CH3 R51


Net: 1-C4H6 + H −→ CH3C2H (CH2CCH2)+ CH3, (28)

and

C4H8hν−→ C3H5 + CH3 R69


Net: C4H8 + H −→ CH3C2H (CH2CCH2)+ CH3 + H2, (29)

and

1,3-C4H6hν−→ C3H3 + CH3 R65


Net: 1,3-C4H6 + H −→ CH3C2H (CH2CCH2)+ CH3, (30)

and


H+ C2H2M−→ C2H3 R99

CH3 + C2H3M−→ C3H6 R160

C3H6hν−→ C3H5 + H R34


Net: CH4 + C2H2 −→ CH3C2H (CH2CCH2)+ H2. (31)

Schemes involving CH insertion reactions such as schemes (26) andaccount for 4% of the total column production of methylacetylene in our nommodel (compared with∼50% for the Jupiter model of Gladstoneet al. 1996).Schemes involving radical–radical recombination reactions such as schemaccount for∼8% of the total CH3C2H production rate, and photolysis of C4

compounds accounts for∼9% of the total production rate. The bulk of thCH3C2H production rate in our nominal model results from secondary reactinvolving conversion of allene into methylacetylene or by recycling reactioFor example, roughly one-third of CH3C2H production rate in our nominamodel results from recycling schemes such as

CH3C2Hhν−→ C3H3 + H R26

H+ C3H3M−→ CH3C2H R108

Net: Nothing (32)

and

H+ CH3C2HM−→ C3H5 R111

H+ C3H5 −→ CH3C2H+ H2 R114

Net: 2H−→ H2. (33)

Nearly 45% of the total CH3C2H production rate is due to reaction schemes thconvert CH2CCH2 to CH3C2H, such as reaction R112 (H+CH2CCH2−→

mn


(27)nal

(31)

onsns.

at

CH3C2H+H) or

CH2CCH2hν−→ C3H3 + H R29


Net: CH2CCH2 −→ CH3C2H, (34)

or

CH2CCH2hν−→ C3H2 + H2 R30

H+ C3H2M−→ C3H3 R107


Net: CH2CCH2 + 2H −→ CH3C2H+ H2. (35)

Note that unlike the Titan model of Laraet al. (1996), reaction R154 (3CH2+C2H2

M−→CH3C2H) is not an effective way of producing methylacetylene iour Saturn model. Although our adopted low-pressure limiting rate constantthis reaction is larger than that used by Laraet al., our high-pressure limitingrate constant (at 140 K) is many orders of magnitude smaller than theirs.base our estimate of this rate coefficient on the experimental work of B¨ohlandet al. (1986), who found a large activation energy for the reaction of3CH2 withacetylene.

The dominant loss mechanisms for methylacetylene are photolysis reactor addition with atomic hydrogen to form C3H5 (R111). However, both thephotolysis products and C3H5 can act to recycle the CH3C2H so that methy-lacetylene, like acetylene, survives in the stratosphere much longer thaphotolysis rate would indicate. The predominant “permanent” loss mechanifor methylacetylene involve photolysis and the subsequent formation of C3H3

radicals (e.g., R26 or R27 followed by R107) followed by three-body combintion reactions with CH3 to form 1-C4H6 and 1,2-C4H6 (R163 and R164) or byC3H3–C3H3 recombination reactions that produce C6H6 (R208). Note that theC6H6 produced by this process (at least initially) is not in the form of benze(Alkemade and Homann 1989, Morteret al. 1994).

Propylene (C3H6) is synthesized primarily by reaction R160 in Saturnstratosphere (R160 is responsible for 65% of the total column production).dominant reaction scheme in the upper stratosphere is


H+ C2H2M−→ C2H3 R99

CH3 + C2H3M−→ C3H6 R160

Net: CH4 + C2H2 −→ C3H6, (36)

and the dominant scheme in the middle and lower stratosphere is


C2H+ CH4 −→ C2H2 + CH3 R178

H+ C2H2M−→ C2H3 R99

CH3 + C2H3M−→ C3H6 R160

Net: CH4 + C2H2 −→ C3H6. (37)

CH insertion into ethane (R144) accounts for another 14% of the colu
production rate (compared with∼33% in the Gladstoneet al. model), and C3H8

S

o

6

.hle

C

o

ing

to-ate.

he

re

ts

292 MOSE

photolysis for another 10% through schemes such as


H+ C2H4M−→ C2H5 R102

CH3 + C2H5M−→ C3H8 R162

C3H8hν−→ C3H6 + H2 R40

Net: CH4 + C2H4 −→ C3H6 + H2. (38)

Unlike the situation for methylacetylene, recycling reactions account for∼8% of the total column production rate of C3H6. The dominant recyclingscheme is

C3H6hν−→ C3H5 + H R34

H+ C3H5M−→ C3H6 R117

Net: Nothing, (39)

although photolysis reaction R38 followed by the combination reaction R1also an important recycling scheme.

Propylene is one of the first and most important C3 compounds formed frommethane photolysis, and as such is a stepping stone to synthesizing manyother C3 and C4 hydrocarbons in Saturn’s atmosphere (see Figs. 3 and 8)major loss process for C3H6 is photolysis, which can act either to recycle tC3H6 or to produce other C3 compounds such as methylacetylene and al(see above) and eventually higher order hydrocarbons such as variousof C4H6, C4H8, C4H10, and C6H6. Addition of H with C3H6 (R120) is alsoan important loss mechanism and results in the production of a C3H7 radical,which either recycles the propylene or eventually helps form the alkanes3H8

and C4H10.Propane (C3H8) is the most abundant C3 compound formed in Saturn’s strat

sphere (see Fig. 7). Reaction R162 (CH3+C2H5M−→ C3H8) is responsible for

97% of the column production rate of propane in our nominal model. Inupper atmosphere, the following scheme dominates:


H+ C2H4M−→ C2H5 R102

CH3 + C2H5M−→ C3H8 R162

Net: CH4 + C2H4 −→ C3H8. (40)

In the middle atmosphere, schemes such as the following are important:


C2H+ CH4 −→ C2H2 + CH3 R178

H+ C2H4M−→ C2H5 R102

CH3 + C2H5M−→ C3H8 R162

Net: CH4 + C2H2 −→ C3H8 (41)

and

2(C2H2hν−→ C2H+ H) R10

C2H+ CH4 −→ C2H2 + CH3 R178

C2H+ C2H6 −→ C2H2 + C2H5 R181

CH3 + C2H5M−→ C3H8 R162

Net: CH4 + C2H6 −→ C3H8 + 2H. (42)

ET AL.

nly

0 is

of theTheene

forms

-

the

Photolysis is the only important chemical loss mechanism for destroyC3H8 in Saturn’s stratosphere. However, as with the other alkanes, C3H8 is par-tially shielded by methane (and ethane) from ultraviolet radiation, and pholysis is not a sufficient means for balancing the chemical production rA total of 81% of the column loss rate of C3H8 in the model is accountedfor by photolysis, while diffusion through the lower boundary makes up tother 19%.

APPENDIX D

Reaction Schemes for Major C4 and Heavier Molecules

Diacetylene is produced primarily by R179 through schemes such as


C2H+ C2H2 −→ C4H2 + H R179

Net: 2C2H2 −→ C4H2 + H, (43)

and

C2H2hν−→ C2 + H2 R11

C2 + CH4 −→ C2H+ CH3 R176

C2H+ C2H2 −→ C4H2 + H R179

Net: CH4 + 2C2H2 −→ C4H2 + CH3 + H2 + H. (44)

Reaction R179 accounts for half of the total production of C4H2 in the strato-sphere. Other primary production schemes include


C2H+ C2H4 −→ C4H4 + H R180

C4H4hν−→ C4H2 + H2 R48

Net: C2H2 + C2H4 −→ C4H2 + H2 + 2H, (45)

and schemes that involve photolysis of other C4 hydrocarbons,

1-C4H6hν−→ C4H4 + 2H R50

C4H4hν−→ C4H2 + H2 R48

Net: 1-C4H6 −→ C4H2 + H2 + 2H, (46)

or similar schemes involving 1,2-C4H6 and 1,3-C4H6. Scheme (46) is not trulya primary C4H2 production scheme because much of the 1-C4H6 in our model isproduced from C4H2 via R128, R129, and R130. Schemes involving R179 aresponsible for the bulk of the primary (nonrecycling) production of C4H2 inthe model, with scheme (45) accounting for∼1%, and schemes involving C4H6

isomers accounting for another∼4% of the total primary production of C4H2.Note that as on Saturn, scheme (43) was found to be the dominant primary C4H2

production scheme on Titan (Laraet al. 1996).Roughly 45% of the total C4H2 production rate in the stratosphere resul

from catalytic recycling schemes such as

H+ C4H2M−→ C4H3 R126

H+ C4H3 −→ C4H2 + H2 R127a

Net: 2H−→ H2, (47)

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HYDROCARBON PHOTOCHEM

and


H+ C4H3 −→ C4H2 + H2 R211

Net: H2 −→ 2H, (48)

and

H+ C4H2M−→ C4H3 R126

H+ C4H3M−→ C4H4 R128

C4H4hν−→ C4H2 + H2 R48

Net: 2H−→ H2, (49)

and


C4H+ CH4 −→ C4H2 + CH3 R212

Net: CH4 −→ CH3 + H. (50)

Note the similarity of these catalytic schemes to those involving C2H2 in themiddle and lower stratosphere. These effective recycling mechanisms allacetylene to remain relatively abundant in Saturn’s stratosphere (see FiUnlike the model of Gladstoneet al. (1996), we do not believe that reactionsvolving C3H2 or C3H3 with acetylene will be important pathways for producC4H2 in outer-planetary atmospheres.

Although C4H2 readily absorbs ultraviolet radiation at wavelengths lonthan the dissociation threshold, resulting in the production of reactive metaexcited states (C4H∗2), we find that these metastable states will be quiquenched via collisions with H2 (R219, 69% of the time) or will spontaneousdecay back to ground-state C4H2 (R218, 30% of the time). Therefore, unlithe situation on Titan (Zwier and Allen 1996), we find that C4H∗2 is neitheran important participant in diacetylene chemistry nor an important routeformation of higher order hydrocarbons in Saturn’s atmosphere. There amain reasons for the differences between the Titan and Saturn modelinregard to C4H∗2 reactions. First of all, hydrocarbon mixing ratios are greateTitan than on Saturn, implying that production of Cn (n ≥ 5) hydrocarbons imore efficient and quenching of C4H∗2 is less efficient on Titan than on SatuSecondly (and more importantly), C4H2 condenses in the lower stratosphereour Saturn model, and the production of both C4H and C4H∗2 is inhibited inthe condensation region, thus reducing the total production rate of highder hydrocarbons. Diacetylene also condenses on Titan, but condensatinot included in the model of Zwier and Allen (1996). Although productioCn hydrocarbons via C4H∗2 pathways accounts for only 4% of the total coluproduction rate of higher order hydrocarbons in our nominal Saturn moderelative percentage becomes a maximum of 21% just above the condenregion.

Diacetylene is lost from the stratosphere by reaction with atomic H (Rby photolysis (R44, R45, R46), and by condensation. The following reaschemes are important “permanent” chemical loss mechanisms:

H+ C4H2M−→ C4H3 R126

H+ C4H3M−→ C4H4 R128

H+ C4H4M−→ C4H5 R129

H+ C4H5M−→ 1-C4H6 R131

Net: C4H2 + 4H −→ 1-C4H6, (51)

STRY IN SATURN’S ATMOSPHERE 293

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26),tion

and

H+ C4H2M−→ C4H3 R126

H+ C4H3 −→ 2C2H2 R127

Net: C4H2 + 2H −→ 2C2H2, (52)

and

H+ C4H2M−→ C4H3 R126

H+ C4H3M−→ C4H4 R128

H+ C4H4M−→ C4H5 R129

C4H5 + H2M−→ 1-C4H6 + H R236

Net: C4H2 + H2 + 2H −→ 1-C4H6. (53)

Schemes such as these account for a little over half of the total column lossof C4H2 (not including condensation). The remaining chemical loss rate resfrom catalytic recycling reactions such as (47), (48), (49), and (50).

Condensation accounts for most of the loss rate of C4H2 between∼4 mbar andthe tropopause. The onset of condensation is clearly evident in Fig. 10, wthe mixing ratio drops precipitously near 6 mbar. Condensation in the lostratosphere is balanced by evaporation in the troposphere so that condendoes not represent a net loss of carbon in the model atmosphere. This rwill hold true for Jupiter, Uranus, and Neptune, but not for Titan, whose surftemperature is too low to allow reevaporation of the condensates.

The formation of higher order polyynes (e.g., C6H2 and C8H2) is initiatedby reaction schemes similar to scheme (44). For further details on polyphotochemistry, see Gladstoneet al. (1996). Higher order polyyne chemistryadds an extremely minor perturbation to chemistry of the major C, C2, C3, andC4 hydrocarbons (including the chemistry of C4H2) in our model and could beeasily ignored—condensation of C4H2 on Saturn greatly inhibits the productionof C6H2 and C8H2. Polyynes heavier than C4H2 will most likely condense inthe lower stratosphere of Saturn (their vapor pressures are not well known)their contribution to the total mass of the stratospheric haze layers at equator mid-latitudes will be small. If C6H2 and C8H2 have vapor pressures similato C4H2, then C6H2 will condense at∼15 mbar and C8H2 at 20 mbar in Saturn’sstratosphere; the condensation fluxes of C6H2 and C8H2 will be, respectively,∼10−21 and 2× 10−23 g cm−2 s−1.

Vinylacetylene (C4H4) is produced primarily by diacetylene photochemistr(68% of the time; for example, see reaction scheme (49)). Photolysis of C4H6

isomers (either directly by R50, R57, or R64 or indirectly by photolysis to C4H5

followed by R130) accounts for 26% of the total production of C4H4, and∼5%of the column production rate of C4H4 is due to the scheme


C2H+ C2H4 −→ C4H4 + H R180

Net: C2H2 + C2H4 −→ C4H4 + 2H. (54)

Vinylacetylene is lost by photolysis to form C4H2 or C2H2, or it combines withatomic hydrogen to form C4H5 and eventually 1-C4H6. Recycling reactions arenot as effective for C4H4 as they are for some of the other stable C4 hydrocarbons;thus, the mixing ratio of vinylacetylene is quite low in the lower stratosphere (Fig. 9). Vinylacetylene becomes supersaturated in the lower stratosphereC4H4 condensation (which has not been included in our nominal model) whave a minor contribution to the total loss rate. Vinylacetylene would condeat∼30 mbar in our model, and the condensation flux would be∼3× 10−23 gcm−2 s−1.

As with the Jupiter study of Gladstoneet al. (1996), we follow the photo-chemistry of three separate C4H6 isomers, 1-C4H6 (ethylacetylene), 1,2-C4H6

(methylallene), and 1,3-C4H6 (bivinyl). Radical–radical reactions such as R16
R164, and R194 are responsible for the bulk of the primary production of C4H6

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294 MOSES

in the model (see Gladstoneet al. 1996). Photolysis of C4H8 (R68) provides anadditional source of 1,3-C4H6. Ethylacetylene is the most abundant of the vaous C4H6 isomers because it is more efficiently recycled and because theforms can be converted to 1-C4H6 through reaction schemes in which C4H4 andC4H5 are intermediates (see Fig. 3). Photodissociation, primarily to C3H3+H,is responsible for the loss of all the C4H6 isomers.

We do not distinguish between various isomers of butene (C4H8) in ourmodel. In our nominal model, reaction R167 is responsible for 72% of the tbutene production, while photolysis of butane (R75) is responsible for ano23%, and reaction R199 contributes 5% to the total production rate. Buis lost from the stratosphere by photodissociation (R68–R74), with R69 bdominant.

Butane (eithern-C4H10 or iso-C4H10) is produced almost entirely (94% othe time) by reaction R171, with a 6% contribution from R202. Butane ismost abundant of the stable C4 hydrocarbons despite the fact that it has onethe lowest initial production rates of any of these molecules (only 1,2-C4H6 islower). This behavior results from the fact that photolysis is the only chemloss mechanism, and C4H10 is partially shielded from ultraviolet radiation bCH4 and the other alkanes. Butane can also condense in the lower stratosof Saturn (at∼25 mbar), and condensation accounts for roughly 12% of the tloss of C4H10 from the stratosphere.

Recombination of C3H3 radicals (R208) leads to the production of C6H6 invarious isomeric forms. The predominant scheme is

2(CH2CCH2hν−→ C3H3 + H) R29

2C3H3M−→ C6H6 R208

Net: 2CH2CCH2 −→ C6H6 + 2H, (55)

although there are many other pathways to C3H3 formation. Reaction R208 is responsible for∼100% of the C6H6 production in our nominal model. Laboratorinvestigations of C3H3–C3H3 recombination indicate that the C6H6 is producedas the 1,5-hexadiyne isomer roughly 65% of the time and as 1,2-hexad5-yne roughly 30% of the time (A. Fahr, personal communication, 1997;also Alkemade and Homann 1989). These isomers can either be deactivatestabilized following the reaction, or they might rearrange to form benzen1,3-hexadien-5-yne. The dominant loss process for any of these isomers wbe photodissociation, but their photochemistry has not been investigatedthe photolysis products and yields are not certain. For this investigationhave assumed that 1,5-hexadiyne is the most abundant C6H6 isomer formed andthat the photolysis products are as indicated in Table II. However, due to aof laboratory data on absorption cross sections for this isomer, we havebenzene absorption coefficients instead. Our choices for the cross sectionphotolysis pathways may have some effect on the CH3C2H abundances derivedfrom the model, and this whole problem deserves further investigation.

If the C6H6 in our model were all in the form of benzene, then we would expbenzene to condense in our model at∼10 mbar. The predicted condensation fluof benzene would then be∼4× 10−20 g cm−2 s−1. Benzene has been tentativeidentified in ISO spectra of Saturn (e.g., B´ezard 1998). The inferred columabundance above 5 mbar (7× 1013 cm−2) is a factor of∼3 higher than ourmodel predictions of the C6H6 abundance.

ACKNOWLEDGMENTS

The Caltech/JPL KINETICS code was developed jointly by Y. L. Yung aone of the authors (M.A.), with assistance from many people over the yearsthank Y. L. Yung, A. Fahr, P. Romani, J.-L. Ollivier, L.-M. Lara, and an anonmous referee for useful advice and suggestions, Y.-T. Lee for tracking dsome of the references to the photoabsorption cross sections, and R. Wproviding low-temperature C2H2 cross sections in advance of publication. Thwork was supported by NASA Contract NAG5-6915 and by the Lunar and Pltary Institute, which is operated by the Universities Space Research Assocunder NASA Contract NASW-4574. This paper represents LPI Contribu
982.
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Note added in proof. Inclusion of multiple Rayleigh scattering to the modecauses a marked decrease in theJ values for the C2–C4 hydrocarbons at pres-sures greater than a few millibars (as compared to the single-scatteringJ valuespresented in Fig. 4). However, the stratospheric mixing ratio profiles for mof the complex hydrocarbons are not greatly affected by the inclusion of mtiple scattering. Exceptions are the alkanes C3H8 and C4H10, which exhibit arespective 45% and 60% increase in the stratospheric column abundancemultiple scattering is included. The mixing ratios of many of the hydrocarbo(except C2H6) in the multiple-scattering model begin to depart from thosethe single-scattering model at higher (tropospheric) pressures; thus, muscattering cannot be ignored for tropospheric modeling (see also Ollivieret al.1999).

REFERENCES

Alkemade, U., and K. H. Homann 1989. Formation of C6H6 isomers by recom-bination of propynyl in the system sodium vapour/propynylhalide.Z. Phys.Chem. Neue Folge161, 19–34.

Allara, D. L., and R. Shaw 1980. A compilation of kinetic parameters for tthermal degradation ofn-alkane molecules.J. Phys. Chem. Ref. Data9, 523–559.

Allen, M., Y. L. Yung, and G. R. Gladstone 1992. The relative abundanceethane to acetylene in the jovian stratosphere.Icarus100, 527–533.

Allen, M., Y. L. Yung, and J. W. Waters 1981. Vertical transport and phochemistry in the terrestrial mesosphere and lower thermosphere (50–120J. Geophys. Res.86, 3617–3627.

Ari e, E., and J. W. C. Johns 1992. The bending energy levels of C4H2. J. Molec.Spectrosc.155, 195–204.

Atreya, S. K. 1982. Eddy mixing coefficient on Saturn.Planet. Space Sci.30,849–854.

Atreya, S. K. 1986.Atmospheres and Ionospheres of the Outer Planets and TSatellites. Springer-Verlag, Berlin.

Atreya, S. K., and P. N. Romani 1985. Photochemistry and clouds of JupSaturn, and Uranus. InRecent Advances in Planetary Meteorology(G. E.Hunt, Ed.), pp. 17–68. Cambridge Univ. Press, Cambridge.

Atreya, S. K., J. H. Waite, Jr., T. M. Donahue, A. F. Nagy, and J. C. McConn1984. Theory, measurements, and models of the upper atmosphere andsphere of Saturn. InSaturn(T. Gehrels and M. S. Matthews, Eds.), pp. 239277. Univ. of Arizona Press, Tucson.

Balko, B. A., J. Zhang, and Y. T. Lee 1992. Photodissociation of ethylene193 nm.J. Chem. Phys.97, 935–942.

Baulch, D. L., C. J. Cobos, R. A. Cox, C. Esser, P. Frank, T. Just, J. A. KM. J. Pilling, J. Troe, R. W. Walker, and J. Warnatz 1992. Evaluated kinedata for combustion modeling.J. Phys. Chem. Ref. Data21, 411–734.

Baulch, D. L., C. J. Cobos, R. A. Cox, P. Frank, G. Hayman, T. Just, J.Kerr, T. Murells, M. J. Pilling, J. Troe, R. W. Walker, and J. Warnatz 199Evaluated kinetic data for combustion modeling.J. Phys. Chem. Ref. Data23, 847–1033. [Errata,J. Phys. Chem. Ref. Data24, 1609–1630.]

Becker, K. H., R. Kurtenbach, and P. Wiesen 1991. Temperature and psure dependence of the reaction CH+H2. J. Phys. Chem.95, 2390–2394.

Ben-Jaffel, L., R. Prang´e, B. R. Sandel, R. V. Yelle, C. Emerich, D. Feng, anD. T. Hall 1995. New analysis of the Voyager UVS H Lyman-α emission ofSaturn.Icarus113, 91–102.

Benilan, Y., D. Andrieux, and P. Bruston 1995. Mid-UV acetylene cross-sectirevisited: Sample contamination risk in source data.Geophys. Res. Lett.81,897–900.

Berman, M. R., and M. C. Lin 1983. Kinetics and mechanisms of the reactiof CH with CH4, C2H6 andn-C4H10. Chem. Phys.82, 435–442.

Berman, M. R., and M. C. Lin 1984. Kinetics and mechanisms of the reacti
of CH and CD with H2, and D2. J. Chem. Phys.81, 5743–5750.

ub

hh

o

rn

e

iotu

s

io

nn

ag

n

pn

t

i

-

9

sK

li

o

ort-

.h,che,S

ed

. J.

g.y,

ties

ber,ne on98,

ousand

A.ddy

m-

tion

ith

ption

ons

mi-

ionpper

sky,iantion

gi-

dical

and


Berman, M. R., J. W. Fleming, A. B. Harvey, and M. C. Lin 1982. Temperatdependence of the reactions of CH radicals with unsaturated hydrocarChem. Phys.73, 27–33.

Bezard, B. 1998. Detection of new hydrocarbons on the giant planets.Bull. Am.Astron. Soc.30, 1059.

Bezard, B., and D. Gautier 1985. A seasonal climate model of the atmospof the giant planets at the Voyager encounter time. I. Saturn’s stratospIcarus61, 296–310.

Bezard, B., H. Feuchtgruber, J. I. Moses, and T. Encrenaz 1998. Detectimethyl radicals (CH3) on Saturn.Astron. Astrophys.334, L41–L44.

Bezard, B., D. Gautier, and B. Conrath 1984. A seasonal model of the satuupper troposphere: Comparison with Voyager infrared measurements.Icarus60, 274–288.

Bishop, J., P. N. Romani, and S. K. Atreya 1998. Voyager 2 ultraviolet sptrometer solar occultations at Neptune—Photochemical modeling of the 1165 nm lightcurves.Planet. Space Sci.46, 1–20.

Bohland, T., S. Dóbe, F. Temps, and H. G. Wagner 1985a. Kinetics of reactbetween CH2 (X 3B1)-radicals and saturated hydrocarbons in the temperarange 296 K≤ T ≤ 707 K.Ber. Bunsenges. Phys. Chem.89, 1110–1116.

Bohland, T., F. Temps, and H. G. Wagner 1985b. The contributions of intersycrossing and reaction in the removal of CH2 (a1A1) by hydrocarbons studiedwith the LMR.Ber. Bunsenges. Phys. Chem.89, 1013–1018.

Bohland, T., F. Temps, and H. G. Wagner 1986. Kinetics of the reactof CH2 (X 3B1)-radicals with C2H2 and C4H2 in the temperature range296 K≤ T ≤ 700 K.Symp. Int. Combust. Proc.21, 841–850.

Boullart, W., and J. Peeters 1992. Product distributions of the C2H2+O andHCCO+H reactions. Rate constant of CH2 (X 3B1)+H. J. Phys. Chem.96,9810–9816.

Brachhold, H., U. Alkemade, and K. H. Homann 1988. Reactions of ethyradicals with alkynes in the system sodium vapour/ethynylbromide/alkyBer. Bunsenges. Phys. Chem.92, 916–924.

Braun, W., A. M. Bass, and M. Pilling 1970. Flash photolysis of ketenediazomethane: The production and reaction kinetics of triplet and sinmethylene.J. Chem. Phys.52, 5131–5143.

Brouard, M., M. T. MacPherson, and M. J. Pilling 1989. Experimental aRRKM modeling study of the CH3+H and CH3+D reactions.J. Phys.Chem.93, 4047–4059.

Brownsword, R. A., M. Hillenkamp, T. Laurent, R. K. Vasta, H.-R. Volpand J. Wolfrum 1997. Quantum yield fof H atom formation in the methadissociation after photoexcitation at the Lyman-α (121.6 nm) wavelength.Chem. Phys. Lett.266, 259–266.

Campbell, J. K., and J. D. Anderson 1989. Gravity field of the saturnian sysfrom PioneerandVoyagertracking data.Astron. J.97, 1485–1495.

Carlson, R. W., and D. L. Judge 1971. The extreme ultraviolet dayglow of JupPlanet. Space Sci.19, 327–343.

Chapman, S., and T. G. Cowling 1970.The Mathematical Theory of NonUniform Gases. Cambridge Univ. Press, Cambridge.

Chen, F., D. L. Judge, C. Y. R. Wu, J. Caldwell, H. P. White, and R. Wagener 1High-resolution low-temperature photoabsorption cross sections of C2H2,PH3, AsH3, and GeH4, with application to Saturn’s atmosphere.J. Geophys.Res.96, 17,519–17,527.

Cobos, C. J., and J. Troe 1990. The dissociation-recombination syCH4+M⇀↽CH3+H+M: Reevaluated experiments from 300 to 3000Z. Phys. Chem. Neue Folge167, 129–149.

Conrath, B. J., D. Gautier, R. A. Hanel, and J. S. Hornstein 1984. The heabundance of Saturn fromVoyagermeasurements.Astrophys. J.282, 807–815.

Courtin, R., D. Gautier, A. Marten, B. Bézard, and R. Hanel 1984. The compsition of Saturn’s atmosphere at northern temperate latitudes fromVoyagerIRIS spectra: NH3, PH3, C2H2, C2H6, CH3D, CH4, and the saturnian D/H
isotopic ratio.Astrophys. J.287, 899–916.

reons.

eresere.

n of

ian

c-25–

nre

tem

ns

yle.

ndlet

d

,e

em

ter.

91.

tem.

um

-

de Graauw, T., and 61 colleagues 1996. Observing with the ISO ShWavelength Spectrometer.Astron. Astrophys.315, L49–L52.

de Graauw, T., H. Feuchtgruber, B. Bézard, P. Drossart, T. Encrenaz, D. ABeintema, M. Griffin, A. Heras, M. Kessler, K. Leech, E. LelloucP. Morris, P. R. Roelfsema, M. Roos-Serote, A. Salama, B. VandenbussE. A. Valentijn, G. R. Davis, and D. A. Naylor 1997. First results of ISO-SWobservations of Saturn: Detection of CO2, CH3C2H, C4H2 and troposphericH2O. Astron. Astrophys.321, L13–L16.

Delpech, C., J. C. Guillemin, P. Paillous, M. Khlifi, and F. Raulin 1994. Infrarspectra of triacetylene in the 4000–200 cm−1 region: Absolute band intensityand implications for the atmosphere of Titan.Spectrochim. Acta A50, 1095–1100.

DeMore, W. B., S. P. Sander, D. M. Golden, R. F. Hampson, M. J. Kurylo, CHoward, A. R. Ravishankara, C. E. Kolb, and M. J. Molina 1992.Chem-ical Kinetics and Photochemical Data for Use in Stratospheric ModelinEvaluation Number 10.JPL Publication 92-20, Jet Propulsion LaboratorPasadena.

Dobrijevic, M., and J. P. Parisot 1998. Effect of chemical-kinetics uncertainon hydrocarbon production in the stratosphere of Neptune.Planet. Space Sci.46, 491–505.

Drossart, P., T. Fouchet, J. Crovisier, E. Lellouch, T. Encrenaz, H. Feuchtgruand J.-P. Champion 1999. Fluorescence in the 3 micron bands of methaJupiter and Saturn. InProc. of the Universe as seen by ISO, Oct. 20–23, 19Unesco, Paris, France.

Duran, R. P., V. T. Amorebieta, and A. J. Colussi 1988. Is the homogenethermal dimerization of acetylene a free-radical chain reaction? Kineticthermochemical analysis.J. Phys. Chem.92, 636–640.

Edgington, S. G., S. K. Atreya, L. M. Trafton, J. J. Caldwell, R. F. Beebe, A.Simon, R. A. West, and C. Barnet 1997. Phosphine mixing ratios and emixing coefficients in the troposphere of Saturn.Bull. Am. Astron. Soc.29,992.

Fahr, A., and A. H. Laufer 1995. Deuterium-isotope effect in vinyl radical cobination disproportionation reactions.J. Phys. Chem.99, 262–264.

Fahr, A., and A. Nayak 1994. Temperature dependent ultraviolet absorpcross sections of 1,3-butadiene and butadiyne.Chem. Phys.189, 725–731.

Fahr, A., and S. E. Stein 1988. Reactions of vinyl and phenyl radicals wethyne, ethene and benzene.Symp. Int. Combust. Proc.22, 1023–1029.

Fahr, A., P. Hassanzadeh, B. Laszlo, and R. E. Huie 1997. Ultraviolet absorcross sections of propargyl (C3H3) radicals in the 230–300 nm region.Chem.Phys.215, 59–66.

Fahr, A., A. Laufer, R. Klein, and W. Braun 1991. Reaction rate determinatiof vinyl radical reactions with vinyl, methyl, and hydrogen atoms.J. Phys.Chem.95, 3218–3224.

Fahr, A., P. S. Monks, L. J. Stief, and A. H. Laufer 1995. Experimental deternation of the rate constant for the reaction of C2H3 with H2 and implicationsfor the partitioning of hydrocarbons in atmospheres of the outer planets.Icarus116, 415–422.

Festou, M. C., and S. K. Atreya 1982. Voyager ultraviolet stellar occultatmeasurements of the composition and thermal profiles of the saturnian uatmosphere.Geophys. Res. Lett.9, 1147–1150.

Festou, M. C., S. K. Atreya, T. M. Donahue, B. R. Sandel, D. E. Shemanand A. L. Broadfoot 1981. Composition and thermal profiles of the jovupper atmosphere determined by the Voyager ultraviolet stellar occultaexperiment.J. Geophys. Res.86, 5715–5725.

Feuchtgruber, H., E. Lellouch, T. de Graauw, B. Bézard, T. Encrenaz, andM. Griffin 1997. External supply of oxygen to the atmospheres of theant planets.Nature389, 159–162.

Garland, L. J., and K. D. Bayes 1990. Rate constants for some radical-racross-reactions and the geometric mean rule.J. Phys. Chem.94, 4941–4945.

Gladstone, G. R. 1988. UV resonance line dayglow emissions on Earth
Jupiter.J. Geophys. Res.93, 14,623–14,630.

ui

taei

c

e

s

c

d

Wis

lt

b

.

s

6

9

a

-

eria,rr´

lmsure-

ua-

tol-

nd

ves-ms

Hers

l

-A

nso the

par-tem

ce of

tion

re of

andation

m-

94.

ec-

urn

E.r the

,

296 MOSES

Gladstone, G. R., M. Allen, and Y. L. Yung 1996. Hydrocarbon photochemisin the upper atmosphere of Jupiter.Icarus119, 1–52.

Glicker, S., and H. Okabe 1987. Photochemistry of diacetylene.J. Phys. Chem.91, 437–440.

Gordon, E. B., B. I. Ivanov, A. P. Perminov, and V. E. Balalaev 1978. A measment of formation rates and lifetimes of intermediate complexes in reverschemical reactions involving hydrocarbon atoms.Chem. Phys.35, 79–89.

Grew, K. E., and T. L. Ibbs 1952.Thermal Diffusion in Gases. Cambridge Univ.Press, Cambridge.

Haider, N., and D. Husain 1993. The collisional behaviour of ground satomic carbon, C(2p2 (3Pj ), with ethylene and acetylene investigated by timresolved atomic resonance absorption spectroscopy in the vacuum ultravJ. Photochem. Photobiol. A70, 119–124.

Hanel, R., B. Conrath, F. M. Flasar, V. Kunde, W. Maguire, J. Pearl, J. PirragR. Samuelson, L. Herath, M. Allison, D. Cruikshank, D. Gautier, P. GierasL. Horn, R. Koppany, and C. Ponnamperuma 1981. Infrared observationthe saturnian system from Voyager 1.Science212, 192–200.

Hanning-Lee, M. A., and M. J. Pilling 1992. Kinetics of the reaction betweH atoms and allyl radicals.Int. J. Chem. Kinet.24, 271–278.

Harding, L. B., R. Guadagnini, and G. C. Schatz 1993. Theoretical studiethe reactions H+CH−→C+H2 and C+H2−→CH2 usingab initioglobalground-state potential surface for CH2. J. Phys. Chem.97, 5472–5481.

Heck, A. J. R., R. N. Zare, and D. W. Chandler 1996. Photofragment imagof methane.J. Chem. Phys.104, 4019–4030.

Hill, H. 1996. Propane and propane accessories.Arlen Herald21, 1–2.

Homann, K. H., and C. Wellmann 1983. Kinetics and mechanism of hydrobon formation in the system C2H2/O/H at temperatures up to 1300 K.Ber.Bunsenges. Phys. Chem.87, 609–616.

Hoyermann, K., H. G. Wagner, and J. Wolfrum 1968. Die geschwindigkeitreaktion von atomarem wasserstoff mit azetylen.Ber. Bunsenges. Phys. Chem72, 1004–1008.

Hubbard, W. B., C. C. Porco, D. M. Hunten, G. H. Rieke, M. J. Rieke, D.McCarthy, V. Haemmerle, J. Haller, B. McLeod, L. A. Lebofsky, R. MarcialJ. B. Holberg, R. Landau, L. Carrasco, J. Elias, M. W. Buie, E. W. DunhaS. E. Persson, T. Boroson, S. West, R. G. French, J. Harrington, J. L. EW. J. Forrest, J. L. Pipher, R. J. Stover, A. Brahic, and I. Grenier 1997. Strucof Saturn’s mesosphere from the 28 Sgr occultations.Icarus130, 404–425.

Husain, D., and A. N. Young 1975. Kinetic investigation of ground state caratoms C(23Pj ). J. Chem. Soc. Faraday Trans. 271, 525–531.

Jacquinet-Husson, N., N. A. Scott, A. Chedin, B. Bonnet, A. Barbe, V.Tyuterev, J. P. Champion, M. Winnewisser, L. R. Brown, R. Gamache, VGolovko, and A. A. Chursin 1998. The Geisa system in 1996: Towards anerational tool for the second generation vertical sounders radiance simulaJ. Quant. Radiat. Transfer59, 511–527.

Jaffe, L. D., and L. M. Herrell 1996. Cassini/Huygens science instrumentsProc. SPIE, Cassini/Huygens: A Mission to the Saturnian Systems(L. Horn,Ed.), Vol. 2803, pp. 84–106. Bellingham, WA.

Judge, D. L., F.-M. Wu, and R. W. Carlson 1980. Ultraviolet photometer obvations of the saturnian system.Science207, 431–434.

Kaiser, R. I., C. Ochsenfeld, M. Head-Gordon, Y. T. Lee, and A. G. Suits 199combined experimental and theoretical study on the formation of intersteC3H isomers.Science274, 1508–1511.

Karkoschka, E., and M. G. Tomasko 1992. Saturn’s upper troposphere 11989.Icarus97, 161–181.

Karkoschka, E., and M. G. Tomasko 1993. Saturn’s upper atmospheric hobserved by the Hubble Space Telescope.Icarus106, 428–441.

Kaye, J. A., and D. F. Strobel 1984. Phosphine photochemistry in the atmospof Saturn.Icarus59, 314–335.

Kerola, D. X., H. P. Larson, and M. G. Tomasko 1997. Analysis of the near
spectrum of Saturn: A comprehensive radiative transfer model of its midand upper troposphere.Icarus127, 190–212.
tion

ET AL.

try

re-ble

te-

olet.

lia,h,

s of

n

of

ing

ar-

er.

.,m,liot,ure

on

G.F.

op-tion.

. In

er-

. Allar

86–

zes

here

IRdle

Kessler, M. F., J. A. Steinz, M. E. Anderegg, J. Clavel, G. Drechsel, P. EstJ. Faelker, J. R. Riedinger, A. Robson, B. G. Taylor, and S. Ximenz de Fean1996. The Infrared Space Observatory (ISO) mission.Astron. Astrophys.315,L27–L30.

Khanna, R. K., J. E. Allen, Jr., C. M. Masterson, and G. Zhao 1990. Thin-fiinfrared spectroscopic method for low-temperature vapor pressure meaments.J. Phys. Chem.94, 440–442.

Killen, R. M. 1988. Longitudinal variations in the saturnian atmosphere. I. Eqtorial region.Icarus73, 227–247.

Kinsman, A. C., and J. M. Roscoe 1994. A kinetic analysis of the phoysis of mixtures of acetone and propylene.Int. J. Chem. Kinet.26, 191–200.

Koops, T., T. Visser, and W. M. A. Smit 1984. The harmonic force field aabsolute infrared intensities of diacetylene.J. Molec. Struct.125, 179–196.

Kunde, V., and 50 colleagues 1996. Cassini infrared Fourier spectroscopic intigation. InProc. SPIE Cassini/Huygens: A Mission to the Saturnian Syste(L. Horn, Ed.), Vol. 2803, pp. 162–177. Bellingham, WA.

Langford, A. O., H. Petek, and C. B. Moore 1983. Collisional removal of C2

(1A1): Absolute rate constants for atomic and molecular collisional partnat 295 K.J. Chem. Phys.78, 6650–6659.

Lara, L. M., E. Lellouch, J. J. L´opez-Moreno, and R. Rodrigo 1996. Verticadistribution of Titan’s atmospheric neutral constituents.J. Geophys. Res.113,2–26.

Laufer, A. H., and J. R. McNesby 1968. Photolysis of methane at 1236a:

Quantum yield of hydrogen formation.J. Chem. Phys.49, 2272–2278.

Laufer, A. H., E. P. Gardner, T. L. Kwok, and Y. L. Yung 1983. Computatioand estimates of rate coefficients for hydrocarbon reactions of interest tatmospheres of the outer Solar System.Icarus56, 560–567.

Lellouch, E., H. Feuchtgruber, T. de Graauw, T. Encrenaz, B. B´ezard, M. Griffin,and G. Davis 1998. D/H ratio and oxygen source: A Jupiter–Saturn comison. In Int. Symp.: The Jovian System after Galileo, the Saturnian Sysbefore Cassini-Huygens, Nantes, France, pp. 21–22.

Lightfoot, P. D., and M. J. Pilling 1987. Temperature and pressure dependenthe rate constant for the addition of H to C2H4. J. Phys. Chem.91, 3373–3379.

Lindal, G. F. 1992. The atmosphere of Neptune: An analysis of radio occultadata acquired with Voyager 2.Astron. J.103, 967–982.

Lindal, G. F., D. N. Sweetnam, and V. R. Eshleman 1985. The atmospheSaturn: An analysis of the Voyager radio occultation measurements.Astron. J.90, 1136–1146.

MacPherson, M. T., M. J. Pilling, and M. J. C. Smith 1983. The pressuretemperature dependence of the rate constant for methyl radical recombinover the temperature range 296–577 K.Chem. Phys. Lett.94, 430–433.

Malkin, J. 1992.Photophysical and Photochemical Properties of Aromatic Copounds. CRC Press, Boca Raton, FL.

Mallard, W. G., F. Westley, J. T. Herron, R. F. Hampson, and D. H Frizzell 19NIST Chemical Kinetics Database: Version 6.0. National Inst. Standards andTech., Gaithersberg, MD.

Marrero, T. R., and E. A. Mason 1972. Gaseous diffusion coefficients.J. Phys.Chem. Ref. Data1, 3–118.

McConnell, J. C., B. R. Sandel, and A. L. Broadfoot 1981. Voyager U.V. sptrometer observations of He 584 A

adayglow at Jupiter.Planet. Space Sci.29,

283–292.

McGrath, M. A., and J. T. Clarke 1992. HI Lyman alpha emission from Sat(1980–1990).J. Geophys. Res.97, 13,691–13,703.

Monks, P. S., F. L. Nesbitt, W. A. Payne, M. Scanlon, L. J. Stief, and D.Shallcross 1995. Absolute rate constant and product branching ratios foreaction between H and C2H3 at T= 213 and 298 K.J. Phys. Chem.99,17,151–17,159.

Mordaunt, D. H., I. R. Lambert, G. P. Morley, M. N. R. Ashford, R. N. Dixonand C. M. Western 1993. Primary product channels in the photodissocia
of methane at 121.6 nm.J. Chem. Phys.98, 2054–2065.

IS

..

e

,

af

t

s

d

ras

l

0

aJ

o

n

z

is

s-

d

9,

tn

e

-tion.

ter

rn’s

nm.

ktion

rgy

nm.

ions

py.

re-

81.itan.

.the

ing

h-

ere.

nia

8,

iter,

e of

at-and

1:

P.

Trafton, L. 1985. Long-term changes in Saturn’s troposphere.Icarus63, 374–

HYDROCARBON PHOTOCHEM

Morter, C. L., S. K. Farhat, J. D. Adamson, G. P. Glass, and R. F. Curl 1994constant measurements of the recombination reaction C3H3+C3H3. J. PhysChem.98, 7029–7035.

Moses, J. I. 1991.I. Phase Transformations and the Spectral ReflectancSolid Sulfur: Possible Metastable Sulfur Allotropes on Io’s Surface. II. Ptochemistry and Aerosol Formation in Neptune’s Atmosphere.Ph.D. thesisCalifornia Institute of Technology, Pasadena.

Moses, J. I., E. Lellouch, B. Bézard, G. R. Gladstone, H. Feuchtgruber,M. Allen 2000. Photochemistry of Saturn’s atmosphere. II. Effects oinflux of external oxygen.Icarus, in press.

Moses, J. I., K. Rages, and J. B. Pollack 1995. An analysis of Neptune’s sspheric haze using high-phase-angle Voyager images.Icarus113, 232–266.

Munk, J., P. Pagsberg, E. Ratajczak, and A. Sillesen 1986. Spectroscopicof i-C3H7 and i-C3H7O2 radicals.Chem. Phys. Lett.132, 417–421.

Nava, D. F., M. B. Mitchell, and L. J. Stief 1986. The reaction H+C4H2: Ab-solute rate constant measurement and implication for atmospheric moof Titan.J. Geophys. Res.91, 4585–4589.

Nicholson, P. D., and C. Porco 1988. A new constraint on Saturn’s zonal gharmonics from Voyager observations of an eccentric ringlet.J. Geophys. Re93, 10,209–10,224.

Nicholson, P. D., C. A. McGhee, and R. G. French 1995. Saturn’s centrafrom the 3 July 1989 occultation of 28 Sgr.Icarus113, 57–83.

Noll, K. S., and H. P. Larson 1990. The spectrum of Saturn from 1992230 cm−1: Abundances of AsH3, CH3D, CO, GeH4, NH3, and PH3. Icarus89, 168–189.

Noll, K. S., R. F. Knacke, A. T. Tokunaga, J. H. Lacy, S. Beck, and E. Ser1986. The abundances of ethane and acetylene in the atmospheres ofand Saturn.Icarus65, 257–263.

Ollivier, J. L., M. Dobrijevic, and J. P. Parisot 1999. New photochemical mof Saturn’s atmosphere.Planet. Space Sci., in press.

Opansky, B. J., and S. R. Leone 1996a. Low-temperature rate coefficieC2H with CH4 and CD4 from 154 to 359 K.J. Phys. Chem.100, 4888–4892.

Opansky, B. J., and S. R. Leone 1996b. Rate coefficients of C2H with C2H4,C2H6, and H2 from 150 to 359 K.J. Phys. Chem.100, 19,904–19,910.

Pantos, E., J. Philis, and A. Bolovinos 1978. The extinction coefficient of benvapor in the region 4.6 to 36 eV.J. Molec. Spectrosc.72, 36–43.

Parkinson, C. D., E. Griffioen, J. C. McConnell, G. R. Gladstone, and BSandel 1998. He 584 A

adayglow at Saturn: A reassessment.Icarus133, 210–

220.

Payne, W. A., and L. J. Stief 1972. Hydrogen formation in the photolyspropyne at 1236 Aa. J. Chem. Phys.56, 3333–3336.

Pedersen, J. O. P., B. J. Opansky, and S. R. Leone 1993. Laboratoryof low-temperature reactions of C2H with C2H2 and implications for atmospheric models of Titan.J. Phys. Chem.97, 6822–6829.

Pitts, W. M., L. Pasternack, and J. R. McDonald 1982. Temperature depenof the C2 (X 16+g ) reaction with H2 and C2 (X 16+g anda 35u equilibratedstates) with O2. Chem. Phys.68, 417–422.

Rabinowitz, M. J., J. W. Sutherland, P. M. Patterson, and R. B. Klemm 1Direct rate constant measurements for H+CH4−→CH3+H2, 897–1729 Kusing the flash photolysis-shock tube technique.J. Phys. Chem.95, 674–681.

Rebbert, R. E., and P. Ausloos 1972. Photolysis of methane: Quantum yieC(1D) and CH.J. Photochem.1, 171–176.

Romani, P. N. 1996. Recent rate constant and product measurements ofactions C2H3+H2 and C2H3+H—Importance for photochemical modeliof hydrocarbons on Jupiter.Icarus122, 233–241.

Romani, P. N., and S. K. Atreya 1988. Methane photochemistry and hazduction on Neptune.Icarus74, 424–445.

Romani, P. N., and S. K. Atreya 1989. Stratospheric aerosols from CH4 photo-
chemistry on Neptune.Geophys. Res. Lett.16, 941–944.

Rate

ofho-

ndan

rato-

tudies

eling

vity.

flash

to

bynupiter

del

ts of

ene

. R.

of

tudies

ence

91.

lds of

he re-g

pro-

Romani, P. N., J. Bishop, B. Bézard, and S. Atreya 1993. Methane photochemistry on Neptune: Ethane and acetylene mixing ratios and haze producIcarus106, 442–463.

Sada, P. V., G. H. McCabe, G. L. Bjoraker, D. E. Jennings, and D. C. Reu1996.13C-ethane in the atmospheres of Jupiter and Saturn.Astrophys. J.472,903–907.

Sandel, B. R., J. C. McConnell, and D. F. Strobel 1982. Eddy diffusion at Satuhomopause.Geophys. Res. Lett.9, 1077–1080.

Satyapal, S., and R. Bersohn 1991. Photodissociation of acetylene at 193.3J. Phys. Chem.95, 8004–8006.

Schwanebeck, W., and J. Warnatz 1975. Reaktionen des butadiins I. Die reamit wasserstoffatomen.Ber. Bunsenges. Phys. Chem.79, 530–535.

Segall, J., Y. Wen, R. Lavi, R. Singer, and G. Wittig 1991. Translational enedistribution from C2H2+ hν (193.3 nm)−→C2H+H. J. Phys. Chem.95,8078–8081.

Seki, K., and H. Okabe 1992. Photodissociation of methylacetylene at 193J. Phys. Chem.96, 3345–3349.

Sillesen, A., E. Ratajczak, and P. Pagsberg 1993. Kinetics of the reactH+C2H4−→C2H5, H+C2H5−→ 2CH3 and CH3+C2H5−→ productsstudied by pulse radiolysis combined with infrared diode laser spectroscoChem. Phys. Lett.201, 171–177.

Slagle, I. R., D. Gutman, J. W. Davies, and M. J. Pilling 1988. Study of thecombination reaction CH3+CH3 −→ C2H6. 1. Experiment.J. Phys. Chem.92, 2455–2462.

Slanger, T. G., and G. Black 1982. Photodissociative channels at 1216 Aa

forH2O, NH3, and CH4. J. Chem. Phys.77, 2432–2437.

Smith, D. W., R. W. Shorthill, P. E. Johnson, E. Budding, and A. S. Asaad 19Measurement of stratospheric aerosol on Saturn using an eclipse of TIcarus46, 424–428.

Smith, G. R., D. E. Shemansky, J. B. Holberg, A. L. Broadfoot, B. RSandel, and J. C. McConnell 1983. Saturn’s upper atmosphere fromVoyager 2 EUV solar and stellar occultations.J. Geophys. Res.88, 8667–8678.

Smith, N., and F. Raulin 1999. Modeling of methane photolysis in the reducatmospheres of the outer Solar System.J. Geophys. Res.104, 1873–1876.

Smith, P. L., K. Yoshino, W. H. Parkinson, K. Ito, and G. Stark 1991. Higresolution, VUV (147-201 nm) photoabsorption cross sections for C2H2 at195 and 295 K.J. Geophys. Res.96, 17,529–17,533.

Strobel, D. F. 1969. The photochemistry of methane in the jovian atmosphJ. Atmos. Sci.26, 906–911.

Strobel, D. F. 1975. Aeronomy of the major planets: Photochemistry of ammoand hydrocarbons.Rev. Geophys. Space Phys.13, 372–382.

Strobel, D. F. 1978. Aeronomy of Saturn and Titan. InThe Saturn System(D. Hunten and D. Morrison, Eds.), pp. 185–194. NASA Conf. Publ. 206Washington, DC.

Strobel, D. F. 1983. Photochemistry of the reducing atmospheres of JupSaturn, and Titan.Int. Rev. Phys. Chem.3, 145–176.

Summers, M. E., and D. F. Strobel 1989. Photochemistry of the atmospherUranus.Astrophys. J.346, 495–508.

Tanneberger, H. 1933. Einige Bemerkungen über die Dampf-druck-Kurve desDiacetylens (Butadiins).Berichte D. Chem. Ges.66, 484–486.

Tomasko, M. G., and L. R. Doose 1984. Polarimetry and photometry of Surn from Pioneer 11: Observations and constraints on the distributionproperties of cloud and aerosol particles.Icarus63, 1–34.

Torr, M. R., and D. G. Torr 1985. Ionization frequencies for solar cycle 2Revised.J. Geophys. Res.90, 6675–6678.

Toublanc, D., J. P. Parisot, J. Brillet, D. Gautier, F. Raulin, and C.McKay 1995. Photochemical modeling of Titan’s atmosphere.Icarus 113,2–26.

405.

re

ar

ar

ar

sa

.

ni

ni

f

net

re

and

89.

micange.

ing

f the

he

delileo.

ere

andNote

298 MOSES

Troe, J. 1977. Theory of thermal unimolecular reactions at low psures. I. Solutions of the master equation.J. Chem. Phys.66, 4745–4757.

Tsang, W. 1988. Chemical kinetic data base for combustion chemistry. PPropane.J. Phys. Chem. Ref. Data17, 887–951.

Tsang, W. 1990. Chemical kinetic data base for combustion chemistry. PIsobutane.J. Phys. Chem. Ref. Data19, 1–68.

Tsang, W. 1991. Chemical kinetic data base for combustion chemistry. PPropane.J. Phys. Chem. Ref. Data20, 221–274.

Tsang, W., and R. F. Hampson 1986. Chemical kinetic data base for combuchemistry. Part 1. Methane and related compounds.J. Phys. Chem. Ref. Dat15, 1087–1279.

Vervack, R. J., Jr., B. R. Sandel, G. R. Gladstone, J. C. McConnell, and CParkinson 1995. Jupiter’s He 584 A

adayglow: New results.Icarus114, 163–

173.

Wagner, H. G., and R. Zellner 1972a. Reaktionen von wasserstoffatomaungesattigten C3-kohlenwasserstoffen. II. Die reaktion von H-atomen mmethylacetylen.Ber. Bunsenges. Physik. Chem.76, 518–525.

Wagner, H. G., and R. Zellner 1972b. Reaktionen von wasserstoffatomaungesattigten C3-kohlenwasserstoffen. III. Die reaktion von H–atomen mallen.Ber. Bunsenges. Physik. Chem.76, 667–672.

Waite, J. H. Jr., 1981.The Ionosphere of Saturn.Ph.D. thesis, University oMichigan, Ann Arbor.

Walter, D., H.-H. Grotheer, J. W. Davies, M. J. Pilling, and A. F. Wag1990. Experimental and theoretical study of the recombination reacCH3+CH3−→C2H6. Symp. Int. Combust. Proc.23, 107–114.

Weissman, M. A., and S. W. Bensen 1988. Rate parameters for thetions of C2H3 and C4H5 with H2 and C2H2. J. Phys. Chem.92, 4080–4084.

West, R. A., M. Sato, H. Hart, A. L. Lane, C. W. Hord, K. E. SimmonsL. W. Esposito, D. L. Coffeen, and R. B. Pomphrey 1983. Photomet

s to

ET AL.

s-

t 3.

t 4.

t 5.

tion

D.

mitt

mitt

rion

ac-

,

and polarimetry of Saturn at 2640 and 7500 Aa. J. Geophys. Res.88,

8679–8697.

West, R. A., D. F. Strobel, and M. G. Tomasko 1986. Clouds, aerosols,photochemistry in the jovian atmosphere.Icarus65, 161–217.

Westmoreland, P. R., A. M. Dean, J. B. Howard, and J. P. Longwell 19Forming benzene in flames by chemically activated isomerization.J. Phys.Chem.93, 8171–8180.

Whytock, D. A., W. A. Payne, and L. J. Stief 1976. Rate of the reaction of atohydrogen with propyne over and extended pressure and temperature rJ. Chem. Phys.65, 191–196.

World Meteorological Organization (WMO) 1985.Atmospheric Ozone 1985.World Meteorological Organization Global Ozone Research and MonitorProject, Report 16, World Meteorological Organization, Geneva.

Wormhoudt, J., and K. E. McCurdy 1989. A measurement of the strength oν2 band of CH3. Chem. Phys. Lett.156, 47–50.

Wu, C. H., and R. D. Kern 1987. Shock-tube study of allene pyrolysis.J. Phys.Chem.91, 6291–6296.

Yelle, R. V., J. C. McConnell, B. R. Sandel, and A. L. Broadfoot 1987. Tdependence of electroglow on the solar flux.J. Geophys. Res.92, 15,110–15,124.

Yelle, R. V., L. A. Young, R. J. Vervack, Jr., R. Young, L. Pfister, and B. R. San1996. The structure of Jupiter’s upper atmosphere: Predictions for GalJ. Geophys. Res.101, 2149–2161.

Yung, Y. L., M. Allen, and J. P. Pinto 1984. Photochemistry of the atmosphof Titan: Comparison between model and observations.Astrophys. J. Suppl.Ser.55, 465–506.

Ziegler, W. T. 1959. The vapor pressures of some hydrocarbons in the liquidsolid state at low temperatures. National Bureau of Standards Technical4.

Zwier, T. S., and M. Allen 1996. Metastable diacetylene reactions as route
ry large hydrocarbons in Titan’s atmosphere.Icarus123, 578–583.

photochemistry of saturn’s atmosphere · the photochemistry of oxygen compounds is discussed in a...

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