Icarus 197 (2008) 247–259

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The temperature of the Venus mesosphere from O2 (a1Δg) airglow observations

Jeremy Bailey a,∗, V.S. Meadows b, S. Chamberlain a, D. Crisp c

a Physics Department, Macquarie University, NSW 2109, Australiab Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195, USAc Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 11 December 2007Revised 30 March 2008Available online 9 May 2008

Keywords:Venus, atmosphereSpectroscopyAtmospheres, structure

We have used near-infrared spectroscopic observations of the Venus nightside taken with the InfraredImager and Spectrograph 2 (IRIS2) on the Anglo–Australian Telescope to derive temperature maps forthe Venus mesosphere at an altitude of ∼95 km. The temperatures are derived from the distributionof rotational line intensities in the O2 (a1Δg ) airglow band at 1.27 μm. To obtain reliable temperaturesat the relatively low spectral resolution of IRIS2, we have developed a forward modeling approach tohandle the blending of individual O2 lines and the telluric absorption in the same O2 band. The techniqueprovides temperature retrievals with accuracy comparable to, or better than that of previous high-spectralresolution determinations. The resulting temperature maps show spatial temperature structure that variesfrom night to night, as does the intensity distribution. Intensity weighted mean temperatures range fromabout 181 to 196 K. The temperatures are typically 15–30 K higher than those expected from the VenusInternational Reference Atmosphere (VIRA) profile. The temperatures fall in regions of low O2 emissionrate to values closer to the VIRA levels. Our temperatures are similar to, but slightly lower than thoseobtained from stellar occultation measurements with SPICAV on Venus Express. We suggest that we areseeing a region of locally enhanced temperature caused by compressional heating in the downwelling gasaround the antisolar point.

© 2008 Elsevier Inc. All rights reserved.

1. Introduction

A prominent feature of the Venus nightside spectrum is theintense emission in the a1Δg − X3Σ−

g airglow band of molec-ular oxygen at a wavelength of 1.27 μm. This emission featurewas discovered on both the nightside and dayside of Venus byConnes et al. (1979). Subsequent observations of the nightside air-glow emission have shown that the spatial structure is complexand varies dramatically from night to night (Allen et al., 1992;Crisp et al., 1996). The airglow emission is thought to be the resultof oxygen atoms formed by photodissociation of CO2 on the day-side, which are then carried by the upper atmosphere circulationto the nightside where they can descend to higher density regions.Here the O atoms can recombine through three body or catalyticreactions to produce O2 molecules in excited states, which thenemit the airglow photons (Yung and DeMore, 1982). The O2 air-glow is thus an important probe of the dynamics and chemistry ofthe mesosphere and upper atmosphere (Bougher et al., 2006).

The location of the O2 airglow emission is constrained by therequirement that the density must be high enough for the threebody reactions needed to produce the excited O2, but not so high

* Corresponding author. Fax: +61 2 9850 8115.E-mail address: jbailey@els.mq.edu.au (J. Bailey).

0019-1035/$ – see front matter © 2008 Elsevier Inc. All rights reserved.doi:10.1016/j.icarus.2008.04.007

that collisional quenching by CO2 will dominate over radiative de-cay. These constraints resulted in chemical model predictions thatthe emission should occur at altitudes of ∼90–100 km (Yung andDeMore, 1982). Recently VIRTIS on Venus Express has been used toobserve the O2 airglow in limb viewing geometry (Drossart et al.,2007) showing that the emission peaks at 96 ± 1 km with littleemission above 100 km.

The O2 airglow can be used to provide measurements of thetemperature in the upper mesosphere. Connes et al. (1979) andCrisp et al. (1996) used high resolution Fourier Transform Spec-trometer (FTS) observations to derive temperatures of 185±15 and186 ± 6 K respectively, by fitting a rotational temperature to therelative strengths of individual lines within the band. These tem-peratures are average temperatures obtained over relatively largesingle apertures. In principle however, the same technique can beused to derive spatially resolved temperature maps. In this pa-per we report the results of spatially resolved spectroscopy of theVenus nightside obtained with the Anglo–Australian Telescope, anduse the spectra to derive O2 rotational temperature maps.

2. Observations

Observations of the Venus nightside were obtained over the pe-riods 2004 July 9–14 and 2005 December 9–11 using the InfraredImager and Spectrograph 2 (IRIS2; Tinney et al., 2004) on the

248 J. Bailey et al. / Icarus 197 (2008) 247–259

3.9 m Anglo–Australian Telescope located at Siding Spring Observa-tory, New South Wales, Australia. IRIS2 is a long slit (∼7.5 arcmin)spectrometer using sapphire grisms and a 1024 × 1024 pixelHAWAII1 HgCdTe detector array. The long slit allows coverage ofthe full width of the Venus disk (∼40 arcsec diameter) with sub-stantial sky regions on either side. Our spectra cover a wavelengthrange of 1.09–1.33 μm, with a spectral resolving power (λ/Δλ) of∼2400. To obtain disk resolved spectra of Venus the telescope (andhence the spectrograph slit) was scanned across the disk of Venuswhile data were recorded. The time series mode of IRIS2 was usedto obtain a continuous series of 5 s exposure times with 0.6 s deadtime. The scan parameters were chosen so that in each exposurethe telescope scanned through 0.45 arcsec, an angular distanceequal to one pixel along the slit. The typical time for a scan wasabout 7.5 min, so a number of scans could normally be recordedon each night. Observations were restricted to times when the sunwas below the horizon and Venus was at a higher elevation thanthe telescope limit of 20◦ . This provided typical visibility durationsof 1 to 1.5 h in the morning (July 2004) or evening (December2005) twilights. Table 1 lists the observations used.

To minimize scattered light from the sunlit crescent, the scanwas performed along the direction of the solar vector from theantisolar direction, and the slit was set perpendicular to this di-

Table 1Journal of observations

Date (UT) Angulardiameter(arcsec)

Illuminatedphase(%)

Radialvelocity(km s−1)

Number ofscans

2004 July 9 40.2 22.6 11.3 22004 July 12 38.4 25.2 11.7 42004 July 14 37.2 27.0 11.9 42005 December 9 41.0 25.9 −11.1 62005 December 10 41.7 25.1 −10.9 42005 December 11 42.4 24.2 −10.8 6

rection. This ensures that half the disk was scanned without thesunlit crescent being on the slit. Each scan provides a 3D spectral“cube” with 0.45 arcsec spatial pixels and 1024 spectral pixels.

2.1. Data reduction

The data were processed by flat field correction using exposuresof a quartz lamp, and wavelength calibration using a Xenon lamp.Sky subtraction was performed by subtracting sky frames at thebeginning and end of the scan, together with a further residualsubtraction of sky spectra from regions of the slit on either side ofVenus.

Fig. 1. Example IRIS2 spectrum of the Venus nightside. This spectrum is taken from the data cube on July 12 and is from a region near the peak of O2 airglow emission. Theupper panel covers the wavelength range from 1.13 to 1.33 μm and shows the thermal emission windows near 1.18, 1.28 and 1.31 μm, as well as the O2 airglow emissionband around 1.27 μm. The lower panel is an expanded view of the region covered by the airglow emission. The spectra are in the form used for the model fits. The spectrahave been wavelength calibrated, but no intensity calibration or telluric correction has been applied (as this is included in the forward model). The intensity units are rawdetector units.

Temperature of the Venus mesosphere 249

Fig. 2. The top panel shows the model emission spectrum in the O2 a1Δg − X3Σ−g (0–0) band. Seven of the nine branches are indicated. The QR and QP branches cover the

same crowded region as the QQ branch. The lower panel shows the transmission spectrum for the Earth’s atmosphere over the same spectral range derived from the VSTARmodel. The Earth atmosphere absorbs in the same O2 band responsible for the airglow emission. The airglow is visible because of the Doppler shift of Venus. Some CO2 andH2O lines are also present in this region.

After these standard calibrations, the spectra still show substan-tial contamination from scattered light from the sunlit crescent.We removed this by obtaining a pure “scattered crescent” spec-trum from a region of the slit off the disk of Venus, but close tothe sunlit crescent. A scaled version of this spectrum was thensubtracted from each spatial pixel of the cube, with the scalingfactor based on the intensity in a region of the spectrum near 1.24–1.25 μm where Venus has no nightside emission. An example of areduced spectrum is given in Fig. 1.

3. Data analysis by forward modeling

Previous measurements of the O2 rotational temperature (Con-nes et al., 1979; Crisp et al., 1996) have been based on high-resolution Fourier Transform spectra. At these high resolutions, itis relatively straightforward to measure the intensities of individualrotational lines within the band. The rotational level populations,and hence the line intensities, are determined by the Boltzmanndistribution, with an exponential dependence on energy. Thus alogarithmic plot of scaled intensity against upper state energy hasa slope that gives the rotational temperature.

This approach cannot be used with our data since the spectralresolving power of IRIS2 is too low to cleanly separate the spec-tral lines within the O2 (a1Δg ) band. Telluric absorption providesan additional problem. The Earth’s atmosphere absorbs in the sameO2 band responsible for the airglow emission. The Venus airglow isonly clearly visible because the relative velocity of Venus providesa Doppler shift of 10 to 12 km s−1 at typical observation times.This shifts the airglow emission away from the cores of the tel-luric absorption lines. However, significant telluric absorption dueto O2 is still present. At low resolution, it is not possible to correctthis telluric absorption by the standard astronomical technique of

dividing by a standard star spectrum, since this does not properlyaccount for the complex unresolved structure in both the absorp-tion and emission. In fact, simulations show that severe errors canbe introduced by using such a technique where the same spectralfeature is present in both the planet and Earth spectrum (Baileyet al., 2007).

Instead, the approach used here is a forward modeling ap-proach. We start from a high-resolution simulation of the O2 emis-sion spectrum, correct it for the Doppler shift of Venus, and thenapply a high-resolution model of the Earth atmosphere transmis-sion. The simulated spectrum is then convolved to the observedresolution and compared with the observed spectrum.

3.1. The emission spectrum

The a1Δg − X3Σ−g (0–0) transition is forbidden under the nor-

mal electric dipole selection rules, and occurs as a magnetic dipoletransition. The ground state occurs in only odd rotational levels(N = 1, 3, 5, etc.) and these are split into 3 spin states with to-tal angular momentum quantum number J = N + 1, N and N − 1.With the selection rule for J (Δ J = 0,±1) this leads to nine dis-tinct transition types that can be labeled ΔNΔ J (N) where Δ J andΔN are represented by the letters O, P, Q, R, S for −2, −1, 0, 1, 2,respectively (Herzberg and Herzberg, 1947). The resulting spectrumis shown in Fig. 2.

Our model for the emission spectrum used the PGOPHER pro-gram (Western, C., PGOPHER, a Program for Simulating RotationalStructure, University of Bristol, http://pgopher.chm.bris.ac.uk) andinput data for the transition provided by the School of Chemistry,University of Bristol (A. Orr-Ewing, private communication). Thelower state parameters are from Rouille et al. (1992) and the up-

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Fig. 3. Example of a model fit to a standard star spectrum (BS 996) using our VSTAR model for the Earth atmosphere transmission. The upper panel is the observed IRIS2spectrum of the GV star. The atmospheric transmission model is shown at full and observed (R ∼ 2400) resolving power. The model spectrum is obtained by applying thistransmission curve to a model solar spectrum. The fit residuals (lower panel) are considerably better than 1% over the full wavelength range.

per state parameters were determined by fitting cavity ringdownspectra of the a1Δg − X3Σ−

g absorption (Newman et al., 1999).The PGOPHER program provided the emission line intensities,

upper state energies and wavenumbers. Calculations were carriedout for a reference temperature of Tref = 200 K. If Sref is the in-tensity of a line at the reference temperature, then the relativeintensity (S) for any other rotational temperature (T ) can be cal-culated by applying the appropriate Boltzmann factor.

S = Srefe−hcΔE/kT

e−hcΔE/kTref,

where ΔE is the upper state energy measured relative to the bandorigin (i.e. the rotational part of the upper state energy).

The line intensities were then used to construct an emissionspectrum using a Gaussian line profile with a Doppler width ap-propriate to the rotational temperature (pressure broadening isnegligible at the altitude where the airglow occurs).

3.2. Telluric absorption

A high resolution Earth atmosphere transmission spectrum wascalculated using the radiative transfer model VSTAR (Bailey, 2006).The model used a 50 level version of a standard mid-latitude at-mosphere, with the lower levels adjusted to provide surface pres-sure, temperature and altitude appropriate to the Siding Spring

site. Molecular absorption was calculated using a line-by-line ap-proach based on line parameters in the HITRAN 2004 database(Rothman et al., 2005). Absorption lines due to CO2, H2O, O2, CH4,CO, O3 and N2O were included in the model, though only O2, H2Oand CO2 are significant at the wavelengths of the airglow band. Inaddition to line absorption, it was also necessary to include col-lision induced continuum absorption associated with the 1.27 μmO2 band. Laboratory measurements of this absorption have beenfitted with an empirical model composed of two components: onevarying linearly, and one varying quadratically with oxygen par-tial pressure (Smith and Newnham, 1999, 2000). This model wasshown to give a good fit to Earth atmosphere transmission ob-servations (Smith et al., 2001) and is incorporated in our VSTARmodel.

To test the telluric model and fix the atmosphere state at thetime of observation, the model was used to fit observations ofG type standard stars obtained during the Venus observing runs.A model solar spectrum (Kurucz, R., 1998. The solar irradiance bycomputation, http://kurucz.harvard.edu/papers/irradiance) is usedto represent the spectrum of the G type star. The VSTAR modelfor Earth atmosphere transmission at the appropriate zenith an-gle was then applied, and convolved to the observed resolution forcomparison with the IRIS2 spectrum. A third order polynomial wasused to model changes in instrument response across the band.

Temperature of the Venus mesosphere 251

Fig. 4. Modeled transmission of the Venus atmosphere from the 70 km level upwards. The full transmission spectrum is shown in the upper panel. The absorption lines aredue to CO2. The middle panel shows the transmission at all wavelengths where there is significant O2 emission (as shown in the lower panel).

With this model it was possible to fit the observed standard starspectra to better than 1% over the 1.2 to 1.3 μm wavelength range.The only atmospheric parameter adjusted to obtain such a fit wasthe tropospheric water vapor mixing ratio. An example of such afit is shown in Fig. 3.

The same atmospheric model was then used to recalculate theatmospheric transmission using a zenith angle appropriate to theVenus observations. An example of the atmospheric transmissionspectrum for a Venus observation at 63◦ zenith angle is shown inthe lower panel of Fig. 2.

3.3. Absorption in the Venus atmosphere

While our model includes absorption in the Earth’s atmosphere,the model assumes that the relative rotational line intensities areunaffected by any absorption in the Venus atmosphere. Crisp et al.(1996) considered the possibility of absorption by O2 in the Venusatmosphere and showed that it was negligible, a conclusion thatis supported by the failure to detect O2 A-band absorption in thespectrum of Venus (Trauger and Lunine, 1983; Mills, 1999).

However, there are also absorption lines of CO2 in this region ofthe spectrum and though they are relatively weak, we need to con-sider the possibility that CO2 lines might align with some of the O2rotational lines and distort the relative intensities. We have there-fore used VSTAR to calculate transmission spectra for the Venusatmosphere above 95 km, and above 70 km. The models usedthe Venus International Reference Atmosphere (VIRA) temperatureprofile (Seiff et al., 1985) and CO2 line data from the Carbon Diox-ide Spectroscopic Databank (CDSD-Venus; Tashkun et al., 2003),Since the O2 airglow is known to arise at about 95 km altitude(Drossart et al., 2007), the 95 km case applies to the direct viewof the airglow emission. We find no significant CO2 absorption atthis altitude.

A significant part of the O2 emission is, however, seen in theform of photons scattered off the cloud tops. The 70 km transmis-

sion spectrum applies to the part of the atmosphere that will betraversed in this case. Significant CO2 absorption is present herewith transmissions of ∼75% at the cores of the strongest lines. Inthe real case, the absorption will be even stronger, since the lightwill pass through most of this atmospheric region twice and gen-erally at inclined paths to the vertical case modeled here. However,Fig. 4 shows that the transmission at the wavelengths where thereis significant O2 emission is always close to 100%. Thus, none ofthe CO2 absorption lines is sufficiently close in wavelength to anO2 line to have any effect on rotational line strengths and henceon the resulting temperature. We are therefore justified in ignoringthe Venus atmosphere transmission in our model.

3.4. Thermal emission

The Venus nightside shows, in addition to the airglow emission,a series of thermal windows, in which emission from hot regionsof the surface and lower atmosphere can be seen. One of thesewindows, known as the 1.28 μm window, extends from about 1.266to 1.283 μm, and hence overlaps with the O2 airglow emission.While it is possible to model the spectrum of the Venus thermalwindows using radiative transfer models (e.g. Pollack et al., 1993;Meadows and Crisp, 1996), there are still deficiencies in the avail-able lists of hot CO2 and H2O lines, as well as uncertainties in thefar-wing line shapes, and continuum absorption, that mean thatthe models are not, as yet, perfect fits to the observed spectra.A particular issue with the 1.28 μm window is that the modelsshow significant emission shortward of the main window at about1.262 μm that does not appear to be present in our observed spec-tra (Meadows and Crisp, 1996).

For these reasons we prefer to use an empirical model of thethermal window spectral shape. This was derived from our obser-vations by using an observed spectrum from a region of the diskwhere the O2 airglow emission is very weak, and subtracting an

252 J. Bailey et al. / Icarus 197 (2008) 247–259

appropriately scaled spectrum from a region with strong airglowemission to remove the weak airglow component.

A consequence of this approach is that the spectral shape ofthe thermal window was assumed to be the same for all posi-tions on the disk. In reality, small changes might be expected asa function of viewing angle and possibly other parameters. Empir-ically we find that the fixed shape fits reasonably well to spectraacross the disk. The fit is not perfect as shown by larger residualsin the wavelength region where the thermal emission is strongest.However, as described below, our temperature fits were taken fromspectral regions where there is little thermal emission and so arerelatively insensitive to any imperfections in the thermal spectrum.

Because the thermal spectrum model is at the observed IRIS-2resolution, it was added into the emission model after the latterhas been Fourier convolved with the instrument’s spectral pointspread function (PSF). A Gaussian PSF was used and was found toprovide good fits to observed spectra.

3.5. Temperature fitting

The forward model can be adjusted to fit the observed spectraat any point on the disk by varying four parameters. These are theO2 rotational temperature, the intensity scaling factors for the O2emission, and for the thermal emission, and a small wavelengthshift. The wavelength shift is needed to allow for small errors inthe initial wavelength calibration due to flexure in the instrument,as well as shifts arising from uneven illumination of the spectro-graph slit. Three additional parameters are needed that are fixedfor any one observation. These are the radial velocity of Venus, thespectral resolving power of the instrument, and the mean zenithangle of observation.

The model was fitted to the observed spectra using theLevenberg–Marquardt non-linear least squares algorithm (Presset al., 1989). The fit was carried out in two stages. The mainpurpose of the first fit was to determine the level of the ther-mal emission in the 1.28 μm window. In this stage the fit makesuse of data over the wavelength ranges 1.2480–1.2672 and 1.270–1.2880 μm, and all four parameters are adjusted.

The final fit used to determine the rotational temperature usedonly the wavelength range from 1.2480 to 1.2672 μm, and onlythree parameters were fitted, with the thermal component scal-ing factor fixed at its value from the initial fit. The temperaturefit includes the SR, RR and RQ branches of the band. This spectralregion provides the most reliable temperature data as the contri-bution from the thermal emission is small at these wavelengths.The lines of the QQ, QP and QR branches were not used for tem-perature fitting as they are unresolved at IRIS2 resolution and thusprovide no useful rotational temperature data. The PP, PQ and OPwere also not used as this wavelength region is strongly affectedby the underlying thermal emission from Venus.

An example of a fit to an observed IRIS2 spectrum, and thesensitivity of the spectra to temperature change is shown in Fig. 5.

3.6. Temperature maps

Temperature maps were constructed from each observed datacube by carrying out a fit as described above at each spatial po-sition. A mask is constructed from an intensity image of the O2emission to exclude from the fit any points where the airglowemission is too weak for reliable fitting. The original spatial pix-els of size 0.45 arcsec were binned 2 × 2 for fitting, giving pixels0.9 arcsec square in the temperature maps. The temperature mapsand raw intensity data are shown in Figs. 6 and 7.

To estimate the uncertainties in the temperature measure-ments, separate fits were made to individual scans on the samenight, and the results compared. These generally agree well and

Fig. 5. Residuals of model fits to an observed spectrum on 2004 July 9. The effectsof rotational temperature changes on the spectral fit are illustrated. The best fittemperature in this case is 187 K.

show the same temperature structure. Fig. 8 shows the tempera-ture differences from corresponding pixels in such a comparison,and the mean and standard deviation of these values for variousintensity bins. From these results it can be seen that the one sigmaerror on a single pixel temperature measurement is about ±3 K forbright airglow regions (2 mW m−2 sr−1), increasing to ±10 K at in-tensities of ∼0.5 mW m−2 sr−1. Intensities significantly below thisvalue are excluded from the temperature fits by the mask.

3.7. Intensities and emission rates

From the O2 spectral fits, it is also possible to derive integratedintensity measurements over all branches of the band by summingthe high-resolution model spectra, appropriately scaled to matchthe observed data. To obtain intensities, a slightly modified ver-sion of the fitting procedure was used. The full wavelength rangeincluding all branches of the transition was used, and the tem-perature was fixed at a value of 195 K rather than included as aparameter in the fit. Tests have shown that there is no significantdifference in the intensities in the bright regions between fits withthe temperature as a free parameter and with the temperaturefixed. A fixed temperature allowed the intensity to be determined

Temperature of the Venus mesosphere 253

Fig. 6. O2 intensity and rotational temperature maps for July 2004 observations. The cross marks the antisolar point, and meridians are drawn at two hour intervals of localtime. The intensity plotted here is the image extracted from the cube in the O2 Q-branch. For fitted, calibrated intensities see Fig. 9.

in regions where the band was too weak for reliable temperaturemeasurement. An absolute calibration for this measurement wasobtained by making use of the G-type standard star observations,and assuming the star has a solar spectral energy distribution, andusing the J -band magnitude of the star, and J -band photometriczero point from Allen and Cragg (1983). The standard stars usedwere BS 996 ( J = 3.65) for the July 2004 observations and BS 8477( J = 4.28) for the December 2005 observations.

Since the standard star observations were made through a nar-row slit, and typically in seeing from 1 to 2.5 arcsec, there aresignificant slit losses when observing the standard star. A slit lossfactor was calculated on the assumption of a Gaussian point spreadfunction for the seeing, with a full-width at half maximum de-termined from the seeing profile along the slit. Comparison ofdifferent observations of the standard star, indicates that the un-certainty in the absolute calibrations is about 30%.

The calibrations result in an image giving the observed spec-trally integrated airglow intensity (Iobs) in W m−2 sr−1. The ob-served intensity is affected by air mass enhancements, which aredependent on the viewing angle, as well as by reflection from theunderlying clouds. These effects are considered in detail by Crispet al. (1996) who derive the expression (combining their equations14 and 16):

F R = 2.556 × 109π Iobs/(sec θ + 2α).

Here, F R is the emission rate in Rayleighs (where 1 Rayleigh =106 photons cm−2 s−1 into 4π steradians), θ is the local zenith an-gle, and α is the Lambert albedo of the clouds, which Crisp et al.(1996) show can be satisfactorily approximated with a constantvalue of 0.875.

Fig. 9 shows the emission rate after correction for viewingangle, plotted on a cylindrical projection centered on the longi-tude of the antisolar point (i.e. midnight local time). The emissionrate distributions for the July 2004 observations show semicircu-lar structures roughly centered on the antisolar point and mostlyon its morning side. These structures somewhat resemble thosepresent in simulations by Bougher and Borucki (1994, Fig. 12) for acase where zonal winds are dominant. The displacement of theemission to the morning side is however, much greater (∼5 h)in the simulation than in our observations (∼2 h). These obser-vations may therefore represent a case in which zonal winds aresignificant at the airglow altitude, whereas the December 2005 ob-servations, where peak emission is close to the antisolar point, aremore consistent with a dominant subsolar-to-antisolar circulation.

4. Discussion

4.1. Temperature results

Temperature data on the six nights are summarized in Table 2.The intensity weighted mean temperatures (equivalent to what

254 J. Bailey et al. / Icarus 197 (2008) 247–259

Fig. 7. O2 intensity and rotational temperature maps for December 2005 observations. The cross marks the antisolar point, and meridians are drawn at two hour intervals oflocal time. The intensity plotted here is the image extracted from the cube in the O2 Q-branch. For fitted, calibrated intensities see Fig. 9.

Table 2Summary of O2 temperatures, intensities and emission rates

Date Temperatureat intensitypeak (K)

Intensityweighted meantemperature (K)

Peak intensity(mW m−2 sr−1)

Peakemissionrate (MR)

2004 July 9 183 196 1.28 2.32004 July 12 198 196 2.31 4.62004 July 14 192 195 2.16 4.82005 December 9 190 186 2.25 4.02005 December 10 182 181 1.09 1.52005 December 11 190 190 2.31 3.7

would be measured with a large aperture instrument) range from181 to 196 K. If 2005 December 10 is excluded (as the airglowwas weak on this night and there are few temperature points) therange is 186 to 196 K. The temperatures appear to be systemati-cally slightly lower during 2005 December than for 2004 July. Thetemperatures at the intensity peak range from 182 to 198 K. Asnoted in Section 3.6 the typical errors on temperature measure-ments range from ±3 K for bright airglow regions to ±10 K forthe fainter parts.

The mean temperatures are in good agreement with previousO2 rotational temperature measurements from wide aperture FTSdata (185 ±15 K, Connes et al., 1979; 186 ± 6 K, Crisp et al., 1996)

as well as with the value of 193 ± 9 K at the emission peak mea-sured by Ohtsuki et al. (2005).

Fig. 10 shows the variation of temperature with O2 emissionrate (corrected for viewing angle). Crisp et al. (1996) suggestedthat higher temperatures might be expected in bright airglow re-gions as a result of compressional heating of the downwellinggas. Ohtsuki et al. (2005) reported a higher temperature at theO2 emission peak than in surrounding regions, though the mea-surement uncertainties were large. Fig. 10 shows that temperaturedoes indeed appear to vary with emission rate, with the tempera-ture being at a roughly constant high value for emission rates fromabout 1.5 to 4.5 MR, but falling to lower values at the lowest emis-sion rates (0.5 to 1 MR). This behavior is seen consistently on mostnights of observation.

The variation of temperature with position was also investi-gated. Fig. 11 shows the latitude variation of temperature. Thereis a tendency for higher temperatures to be seen at equatorial lat-itudes but this effect is only clearly seen on two nights (July 9and December 11) with other nights showing flatter curves. Simi-lar effects are seen in the local time dependence with trends beingseen on some nights but with opposite slopes on different nights.In general it appears that there is temperature structure that variesrandomly from night to night with no strong overall trends. The O2temperature structure varies from night to night in much the sameway as the O2 intensity distribution.

Temperature of the Venus mesosphere 255

Fig. 8. Differences in fitted O2 rotational temperature for corresponding pixels when the July 14th and December 9th data are split into two sets of scans, plotted against O2

integrated intensity. The mean and standard deviation of the points combined in five intensity bins are also shown and provide a measure of the consistency of the temper-ature fits. The one sigma error in temperature is thus shown to be about ±3 K for bright regions (2 mW m−2 sr−1), increasing to ±10 K at intensities of ∼0.5 mW m−2 sr−1,the lowest included in temperature maps.

4.2. Comparison with VIRA and other past temperature data

A standard model for the atmospheric temperature profile isthe Venus International Reference Atmosphere (VIRA; Seiff et al.,1985; Keating et al., 1985) based on in situ and orbiting spacecraftdata. The VIRA model consists of a single profile for the deep at-mosphere (0–32 km) and five models for different latitude zonesfor the range from 33 to 100 km (Seiff et al., 1985). The modelsat these altitudes include no local time dependence. Only above100 km does a strong diurnal variation become apparent and sep-arate dayside and nightside profiles are used at these altitudes(Keating et al., 1985).

A comparison of our data with the VIRA profile is shown inFig. 12. It can be seen that if our temperature measurements applyto altitudes of around 95 km, as indicated by VIRTIS limb obser-vations of the airglow (Drossart et al., 2007), our temperatures aresignificantly higher than the VIRA values by typically 15–30 K.

At lower altitudes, the VIRA profile is based on data from di-rect in situ probe measurements, and radio occultation data wasused for altitudes up to 80 km. However, in the 80–100 km rangeonly two sources of temperature data were available; radiometermeasurements from the Pioneer Venus Orbiter Infrared Radiometer(OIR) instrument, and measurements of atmospheric decelerationof Pioneer Venus and Venera probes.

The Pioneer Venus OIR (Taylor et al., 1980) was a radiometeroperating in the 15 μm CO2 band. Temperatures at 80–100 km

altitudes come from OIR channels 1 and 2, which have broadweighting functions peaking at about 88 and 102 km (Schofieldand Taylor, 1983), with the 95 km altitude being at the crossoverpoint between these channels. The OIR measurements used forVIRA were also averaged over all local times and over the full72 day duration of the experiment. The probe deceleration datasamples mostly the dayside of Venus. Thus while VIRA is probablya reasonable representation of the globally averaged temperatureprofile at these altitudes, these data sources do not exclude localtemperature enhancements in the vicinity of the antisolar point assuggested by our results. In fact OIR does show a small (∼2 K)temperature enhancement above the mean temperature field at95 km altitude at the antisolar point (Schofield and Taylor, 1983,Fig. 10b). Given the substantial vertical, horizontal and temporalaveraging involved, this might be indicative of substantially largerlocal effects. Also the Pioneer Venus night probe (midnight, lati-tude 28.7 ◦S) recorded a higher temperature at 95 km than theother probes (Seiff and Kirk, 1982).

Subsequent to VIRA, other sources of temperature data at thesealtitudes have become available. The Venera 15 Fourier Transformspectrometer has been used to derive temperature profiles in sev-eral local time zones (Zasova et al., 2006). However, local timesaround midnight are not covered. The 95 km temperature for thezone nearest to midnight is 166.4 K, close to the VIRA value.

Ground-based CO millimeter-wave observations have also beenused to derive temperature profiles at these altitudes. Clancy and

256 J. Bailey et al. / Icarus 197 (2008) 247–259

Fig. 9. O2 emission rate, corrected for viewing angle effects and projected into a cylindrical coordinate system centered on the antisolar longitude.

Table 3Summary of measurements of the Venus nightside temperature near 95 km altitude

Method Temperature (K) Reference

1.27 μm O2 airglow 185 ± 15 Connes et al. (1979)Pioneer Venus night probedeceleration

167.2 Seiff and Kirk (1982)

Pioneer Venus OIR 170–175 Schofield and Taylor (1983)VIRA (based on OIR andprobe deceleration)

168 Seiff et al. (1985)

CO mm lines 165–210 Clancy and Muhleman (1991)1.27 μm O2 airglow 186 ± 6 Crisp et al. (1996)CO mm lines 165–178 Clancy et al. (2003)1.27 μm O2 airglow 193 ± 9 Ohtsuki et al. (2005)Venera 15 IR Fourierspectrometer

166.4 Zasova et al. (2006)

SPICAV Stellar occultation 194–240 Bertaux et al. (2007)1.27 μm O2 airglow(intensity weighted mean)

181–196 This work

Muhleman (1991) reported substantial year-to-year variations innightside temperature, with high temperatures (up to 210 K at95 km) recorded in 1985 and 1986, and values closer to the VIRAlevels being seen in other years. More recent data (Clancy et al.,2003) also shows some variability with 95 km temperatures rang-ing from about 165–178 K. The temperature data from all sourcesis summarized in Table 3.

4.3. Comparison with Venus express results

New temperature profiles for the mesosphere and upper atmo-sphere have been obtained from measurements of UV CO2 absorp-tion during stellar occultations observed with SPICAV on Venus

Express (Bertaux et al., 2007). These show a very strong temper-ature peak at 95–100 km altitudes on the Venus nightside, withpeak temperatures ranging from ∼195 K up to ∼240 K. The aver-age SPICAV profile is shown in Fig. 12.

VIRTIS limb observations have also been used to determine thevertical distribution of the O2 emission intensity (Drossart et al.,2007, Fig. 2c). This vertical profile can be combined with the meantemperature profile from SPICAV to give a weighted mean temper-ature that would be expected over the O2 emission at all altitudes.The result is 198 K. This is in excellent agreement with the inten-sity weighted mean O2 temperatures of 195–196 K we obtained forour July 2004 observations, and is a little higher than the temper-atures of 181–190 K obtained for the December 2005 observations.Given the fact that the observations are not simultaneous and spa-tial and temporal variability is apparent in both datasets, it wouldseem that the O2 and SPICAV results are reasonably consistent andagree in showing temperatures higher than the VIRA values.

Combining the VIRA temperature profile with the VIRTIS inten-sity profile in the same way gives a weighted mean temperatureof 174 K, which is significantly lower than all our results.

4.4. Interpretation of observed temperatures

Our O2 temperature measurements are consistently higher thanthose expected from the VIRA temperature profile at an altitude of∼95 km. The temperatures we see could only be consistent withthe VIRA profile if the bulk of the emission was arising from al-titudes as low as 80 km. The VIRTIS limb observations (Drossartet al., 2007) show that this is unlikely to be the case showing thepeak O2 emission is typically at 96 ± 1 km.

Temperature of the Venus mesosphere 257

Fig. 10. Variation of temperature with emission rate (corrected for viewing angle). Points are individual image pixels. Error bars and lines are mean values for data combinedin emission rate bins. The cross at zero emission rate is the Venus International Reference Atmosphere (VIRA) temperature for 95 km altitude.

We therefore suggest that the VIRA temperature is probablya reasonable representation of the globally averaged temperature,and in particular the dayside, but a local temperature enhance-ment occurs around the antisolar point as a result of compres-sional heating of downwelling gas. Because O2 airglow is onlyexpected to be seen in regions of downwelling gas, the O2 tem-peratures will be consistently higher than the VIRA values as isindeed the case in both the results presented here and past O2

temperature measurements (Connes et al., 1979; Crisp et al., 1996;Ohtsuki et al., 2005). This local enhancement due to compres-sional heating also accounts for the warm temperatures detectedin SPICAV occultation measurements (Bertaux et al., 2007), and thevariability that SPICAV sees between different occultation profiles(since only some of the occultations will sample strongly down-welling parts of the atmosphere). SPICAV also samples smallerspatial scales than we can observe with IRIS2, and therefore thehighest SPICAV temperatures may correspond to small regions ofintense downwelling that are too small to resolve with IRIS2. Thedata shown in Fig. 10 indicates that the O2 temperatures dropin regions of low O2 emission rate (and hence presumably lessstrongly downwelling gas) to levels close to the VIRA value.

This model is also consistent with the CO temperature mea-surements. These are also often higher than the VIRA values, butgenerally not as high as the O2 or some of the SPICAV measure-ments (Clancy and Muhleman, 1991; Clancy et al., 2003). Since theCO measurements will cover the whole of the nightside and notjust the strongly downwelling region where the airglow occurs, theaverage CO temperatures are expected to be lower.

The downflow velocity needed to account for the observed tem-perature enhancements can be estimated as follows. The downflowvelocity (w∗) and net heating rate (Q ) are related by the thermo-dynamic energy equation (e.g. Santee and Crisp, 1995).

w∗ ≈ Q

(dT /dz − g/cp),

where dT /dz is the local temperature lapse rate, and g/cp is theadiabatic lapse rate. In this case dT /dz ∼ 1 K km−1 and g/cp ∼12 K km−1. The hot spots associated with the O2 emission are con-sidered to be local temperature perturbations above a steady-stateVIRA-like temperature structure. The night-to-night variations (e.g.over the period December 9–11, 2005) show that ∼20 K tempera-ture enhancements can appear over ∼ 1 day, requiring a heating

258 J. Bailey et al. / Icarus 197 (2008) 247–259

Fig. 11. Variation of temperature with latitude. Points are individual image pixels. Error bars and lines are mean values for data combined in latitude bins.

rate Q ∼ 20 K day−1 or 2.3 × 10−4 K s−1. This corresponds tow∗ = −21 cm s−1.

Our data and previous O2 emission maps (e.g. Crisp et al., 1996)also show that the spatial structure of the airglow emission, al-though typically showing peaks near the antisolar point, variesdramatically from one night to the next. This indicates that thegas flow at these altitudes is complex and chaotically varying. Thisprobably reflects the fact that we are seeing the transition regionbetween the subsolar-to-antisolar flow that dominates at higher al-titudes, and the zonal retrograde superrotation flow pattern seen atthe cloud tops. Doppler wind measurements using sub-millimeterobservations of CO lines (Clancy et al., 2007) show circulation pat-terns changing from zonal retrograde flow to subsolar-to-antisolarflow on timescales of several days.

5. Conclusions

We have developed a technique for mapping the two-dimen-sional temperature distribution in the Venus mesosphere by for-ward modeling of moderate resolution ground-based spectra ofthe 1.27 μm O2 airglow emission. This airglow emission has beenshown by limb observations with VIRTIS on Venus Express to peak

at an altitude of 96 km. The O2 temperature maps show signif-icant spatial temperature structure and intensity weighted meantemperatures of 195–196 K in July 2004 and 181–190 K in De-cember 2005. The temperatures are in reasonable agreement withearlier wide aperture O2 temperature measurements. The temper-atures are higher than the temperature at this altitude expectedfrom the Venus International Reference Atmosphere (VIRA) profile(168 K at 95 km; 174 K weighted average over observed VIRTIS O2vertical distribution). Our temperature measurements are similarto but slightly lower than those obtained from stellar occultationprofiles observed with SPICAV on Venus Express (198 K weightedaverage over VIRTIS O2 distribution).

The O2 temperatures drop to values nearer the VIRA value atlow O2 emission rates. We suggest that our results are consis-tent with a model in which compressional heating of downwellinggas produces a local temperature enhancement on the nightsideregions at which airglow emission is observed. The temperatureand intensity distribution of the O2 emission is highly variablefrom night to night. This is consistent with a chaotic circulationpattern occurring in the transition region between the high alti-tude subsolar-to-antisolar flow, and the zonal superrotation flowat lower altitudes.

Temperature of the Venus mesosphere 259

Fig. 12. Comparison of O2 temperatures with the Venus International Reference At-mosphere (VIRA) and Venus Express SPICAV occultation profiles. The VIRA profileup to 100 km is the low latitude profile from Seiff et al. (1985). Above 100 km thenightside profile from Keating et al. (1985) is used. The SPICAV profile is the aver-age of the six profiles in Fig. 1 of Bertaux et al. (2007) The histogram shows thedistribution of individual pixel O2 temperature values over all six nights. The his-togram is drawn at 96 km, the approximate altitude at which the airglow emissionis thought to arise. The measured temperatures are typically 15–30 K higher thanthe VIRA temperature at this altitude.

Acknowledgments

This work was supported by the NASA Astrobiology Institute’sVirtual Planetary Laboratory Lead Team, funded by the NationalAeronautics and Space Administration through the NASA Astrobi-ology Institute under Cooperative Agreement No. CAN-00-OSS-01.Part of this work was performed at the Jet Propulsion Laboratory,California Institute of Technology, under contract to the NationalAeronautics and Space Administration. We thank the AAO staff forsupport of the observations at the Anglo–Australian Telescope. Wethank Andrew Orr-Ewing and Colin Western (University of Bristol)for the PGOPHER software and O2 transition data.

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