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E.Y. Adams, S.K. Atreya, W.R. Kuhn (Univ. Michigan)
In light of Huygens measurements, we present our improved model of thermal and photochemical evolution of Titan�s atmosphere. Atreya et al. (1) demonstrated that photolysis of ammonia on primordial Titan is capable of producing a nitrogen atmosphere substantially thicker than that measured by Voyager. Wilson (2) carried this calculation one step further by including methane and water vapor explicitly in the ammonia photochemistry model, and arrived at a preliminary estimate required to accumulate different amounts of nitrogen. However, both models assumed an isothermal atmosphere. Since chemistry leading up to nitrogen occurs in the stratosphere, both the thermal structure and saturation effects are important for determining time constants and amounts of nitrogen production. In this presentation, we discuss preliminary results of a radiative- convective equilibrium model for the primordial middle and lower atmosphere of Titan. For its initial composition, the model includes NH3 and H2O limited by saturation, and CH4 as a free parameter due to unknown outgassing rates. The temperature structure in the stratosphere is controlled by haze, and we explore effects of several haze layers at various altitudes for accelerating conversion of ammonia to nitrogen. The possible composition of the primordial haze is also studied by the photochemical model of Wilson and Atreya (3). Furthermore, we include the effects of enhanced solar flux during the T Tauri phase, which could speed up both the loss of nitrogen and conversion of ammonia to nitrogen. The Cassini-Huygens results on the isotopes of nitrogen and argon are used to constrain the nitrogen production and accumulation rates (4). We are in the process of coupling the radiative transfer model to a comprehensive photochemical model (3) to access the roles of trace species other than those included in this calculation. (1) Atreya et al., Science, 201, 611, 1978. (2) Wilson, thesis U Michigan, 2001. (3) Wilson, Atreya, JGR 109, E06002, doi:10.1029/2003JE002181, 2004. (4) Niemann et al., submitted to Nature, 2005.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.