A 3–D model of the atmospheric chemistry of Saturn's moon Titan has been constructed which incorporates some of the latest advances of 1–D Titan chemistry models as well as 3–D GCMs for Titan in simplified-enough way that integrations covering many Earth years are numerically feasible. The thermosphere, with its shorter transport and chemistry time scales, reasonably simple circulation, and basic 3–D coverage of the moon with Cassini INMS observations, is the primary focus of this study.
The model is first validated as far as possible against the observations, showing that it performs quite well for most species, given the general margins of error and uncertainties inherent in Titan modeling. Winds are not seen to have the same overriding influence on concentrations as an earlier study using a pre-Cassini GCM wind field suggested (Doege et al., 2008), but nonetheless many species such as ethane exhibit sensitivity to advection and a chemistry transport model with advection does improve the fit to observations over a model with only diffusive transport. Overall it is found that state-of-the-art GCMs for Titan deliver winds realistic enough for the chemistry model to correctly reproduce the basic shapes of chemical species distributions with a variety of chemical lifetimes. In some instances, the limitations of the Cassini measurements become apparent and model results point to difficulties and uncertainties with the INMS data retrieval process (e.g. for C4H6) that would be less conspicuous by analyzing only Cassini observations without a model.
Then some specific scientific topics are explored, namely the influence of the Solar cycle and of vertically-propagating tidal waves. As the Cassini measurements so far cover a period of decreasing and generally below-average Solar activity, Solar minimum conditions are particularly of interest, because chemistry models for Titan have so far normally been run for Solar average UV output. It is found that some species such as propane react strongly to changes in Solar irradiance and the resulting concentrations of primary photolysis products. However, chemical distributions remain recognizable from the Control experiment and accounting for the Solar cycle does not resolve remaining issues with chemistry schemes for Titan's atmosphere.
Atmospheric tidal waves which are forced mainly in the lower atmosphere also influence the thermosphere significantly, therefore the possbility that chemical concentrations might be measurably affected by these waves is explored with the model. The conclusion is drawn that the effects of such waves through their temperature perturbations alone, while they are easily detectable in the model, might be too weak to identify them in Cassini measurements
