1. X. Zhang 1, R. Shia 1, M. Liang 2, C. Newman 1, D. Shemansky 3, Y. Yung 1, 1 Division of Geological and Planetary Sciences, California Institute of Technology, USA, Pasadena, CA 91125 2 Research Center for Environmental Changes, Academia Sinica, Taiwan 3 Planetary and Space Science Div., Space Environment Technologies, Pasadena, CA 91107 Modeling the Distribution of Hydrocarbons in the Atmosphere of Titan References Flasar 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 Introduction Titan 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. Results 1D eddy diffusion test Better fit by Moses (2000) chemistry scheme CH 4 LOS abundances not sensitive to the eddy diffusion at the low atmosphere 2D modeling Simple test. The trend of the latitudinal profiles from cassini observations (Winter solstice) can be reproduced Mixing ratios are order of magnitude different from the cassini value Streamfunction effect can be represented by comparing fig.2 (no streamfuntion) and fig.5 (modified streamfunction) Three-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 super-rotation 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 streamfunction above 200km is based on analogy with the winter mesospheric circulation in the Earth’s atmosphere. The streamfunction at the winter solsitice is shown in Figures 1. Photochemical Modeling 1. Caltech/JPL photochemical model is used for the study 2. Idealized chemistry and idealized single cell circulation used above 200 km 3. Advection modified based on the streamfunction derived from TitanWRF 3-D model 4. Vertical eddy mixing from previous work (e.g., Liang et al., 2007) 5. Meridional eddy mixing coefficients of 10 7 cm 2 s -1, ~200 years Figure 1: Streamfunction at Winter Solstice (above 1mbar, ~200km) Figure 2: “C 2 H 6 ” mixing ratio computed with Kzz only (no streamfunction) to illustrate the effect of the sun. Figure 4: Model meridional profiles of idealized hydrocarbons at ~200 km (left), compared with Cassini observations (right) at winter solstice Abstract The chemical and dynamical processes in the atmosphere of Titan are poorly quantified. In this presentation, we constrain the transport using the data obtained by the Cassini and Voyager spacecrafts, with emphasis on Cassini measurements. A two-dimensional 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. Above ~500 km, Cassini UVIS data are used; Cassini CIRS measurements are preferred for lower altitudes. No GCM is available to provide transport of species over such wide a range of altitudes. The transport is obtained from TitanWRF below 400 km and by analogy with the Earth’s mesosphere above 400 km. An idealized chemistry is used to illustrate the causes of latitudinal distribution of hydrocarbons. Figure 3: Eddy sensitivity tests for Line-of-sight (LOS) abundances of the main hydrocarbons based on different chemistry schemes (Left: Moses et al., 2000; right: Moses et al., 2005). All the squares are taken from cassini αVir (open) andλSco (filled) occultations (Shemansky et al., 2008) Figure 5: Derived 2-D distributions of idealized hydrocarbons CH 4, C 2 H 2, C 2 H 4, and C 2 H 6 in four seasons Contact: xiz@gps.caltech.edu C2H4C2H4 C2H4C2H4 C2H2C2H2 C2H6C2H6 C2H6C2H6 C2H2C2H2