Mao-Chang Liang1,2, Claire Newman3, Yuk L. Yung3

1 Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan2 Graduate Institute of Astronomy, National Central University, Jhongli, Taiwan

3 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, USA

Modeling the Distribution of Hydrocarbons in the Atmosphere of TitanModeling the Distribution of Hydrocarbons in the Atmosphere of Titan

ReferencesReferencesFlasar et al. Science, 2005; Hong and Pan Monthly Weather Review, 1996; Lebonnois et al. Icarus, 2001; Liang et al. ApJL, 2007; McKay et al. Icarus, 1989; Moses et al. Icarus, 2000; Moses et al. JGR, 2005; Richardson et al. JGR, 2007; Vinatier et al. Icarus, 2007; Yung et al. ApJS, 1984

AbstractAbstractThe chemical and dynamical processes in the atmosphere of Titan are poorly known. Here, we constrain the transport using the data obtained by the Cassini and Voyager spacecrafts. A two-dimensional (2-D) photochemical model is used to model the distribution of hydrocarbons at latitudes from pole to pole and altitudes from the tropopause (~50 km) to ~1500 km. The circulation used in 2-D model is the streamfunction derived from the three-dimensional general circulation model (3-D GCM), TitanWRF. This 3-D GCM model provides transport from the surface to ~400 km. Above that, the vertical eddy mixing coefficients derived previously are the primary transport in this region. Preliminary results and implications of the model are discussed. .

IntroductionIntroductionTitan is Nature's laboratory for organic synthesis. The coupled chemistry between nitrogen and carbon results in a rich suite of nitrogen/carbon compounds, such as hydrocarbons and hydrogen cyanide. The low gravity of the moon allows hydrogen to escape readily and the low temperature in the atmosphere causes hydrocarbons/nitrogen compounds to condense and store in/on the surface. The connection between such rich chemistry and biological evolution has been seriously raised, because of the uniqueness of synthesizing organic matters and possible liquid hydrocarbon oceans underneath the surface. The recent high quality data acquired by the Cassini spacecraft and Cassini-Huygens probe has brought deeper insight into the study of chemical transports in the atmosphere of Titan than that derived previously based on the Voyager I data. A coupled chemistry-transport model has been used by Lebonnois et al. (2001) to reproduce the latitudinal profiles of hydrocarbons and nitriles obtained by Voyager I flybys at spring equinox; their study demonstrates the importance of dynamical transports in the redistribution of photochemical products in the atmosphere of Titan. The adopted circulation fields can also explain the north-south symmetry in albedo (haze distribution), super-rotation at ~1 mbar, and a large equator-pole temperature contrast between 1 and 10 mbar.

ResultsResults1. C2H4

Standard models (black curves of Figure 3; Figure 4) underestimate the abundance by a factor of 10

New kinetics is proposed to explain the observation (blue and red curves of Figure 3)

The newly proposed schemes will be incorporated into 2-D models2. Voyager (Spring equinox)3. Cassini (Winter solstice)

Three-dimensional Global Circulation SimulationThree-dimensional Global Circulation Simulation TitanWRF is a three-dimensional model of Titan's atmosphere from the surface to ~400km. It

was developed from the Earth-based, limited area WRF (Weather Research and Forecasting) model, adapted for global and planetary use (Richardson et al.  2007). TitanWRF includes representations of radiative transfer through a hazy nitrogen-methane atmosphere (using an updated version of the scheme described in McKay et al. 1989, provided by Chris McKay), parameterized surface fluxes of heat and momentum (which depend on local stability via the Richardson number) and boundary layer mixing (including non-local diffusion within the PBL, Hong and Pan 1996). TitanWRF also includes several horizontal diffusion options, though the simulation used in this work was run with zero horizontal diffusion, which produced stronger and far more realistic superrotation and winter hemisphere temperature gradients than in previous simulations published in Richardson et al. 2007. The simulation was run in hydrostatic mode, included the full seasonal and diurnal cycle of solar heating, and was started with zero winds and then allowed to 'spin up' (gain angular momentum from the surface) until it reached a stable state (i.e., one with no net gain or loss of angular momentum when averaged over a Titan year). The derived streamfunction at the spring equinox and the age of air in four seasons are shown in Figures 1 and 2, respectively.

Photochemical ModelinPhotochemical Modelingg

1. Caltech/JPL photochemical model is used for the study2. Standard hydrocarbon kinetics from Moses et al. (2000, 2005) are evaluated; new kinetics that

better match the existing observations are obtained (red curve of Figure 3)3. Caltech/JPL two-dimensional kinetics model is used; only consider C1 and C2 species4. Advection from streamfunction derived from TitanWRF 3-D model5. Vertical eddy mixing from previous work (e.g., Liang et al. 2007)6. Meridional eddy mixing coefficients of 107 cm2 s-1, ~200 years

Contact: see http://www.rcec.sinica.edu.tw/~mcl

Spring equinoxSpring equinox

Figure 1: Streamfunction at Spring equinox Figure 2: Age of air derived from the circulation

Figure 2: Derived 2-D distributions of C2H2, C2H4, and C2H6 in four seasons

CC22HH66CC22HH66

CC22HH22

CC22HH22

CC22HH44

CC22HH44

Spring equinoxSpring equinox Winter solsticeWinter solstice

Figure 3: Vertical profiles of hydrocarbons Figure 4: Meridional profiles corresponding to the epochs of Voyager and Cassini missions

Standard model (Moses et al. 2000)

Proposal #1

Proposal #2