Thursday, February 25, 2016

How Titan's Early Atmosphere Evolved

Evolution of an early Titan atmosphere


Johnson et al


Rapid escape from a proposed early CH4/NH3 atmosphere on Titan could, in principle, limit the amount of NH3 that is converted by photolysis into the present N2 atmosphere. Assuming that this conversion occurred, a recent estimate of escape driven by the surface temperature and pressure was used to constrain Titan's accretion temperature. Here we show that for the range of temperatures of interest, heating of the surface is not the primary driver for escape. Atmospheric loss from a thick Titan atmosphere is predominantly driven by heating of the upper atmosphere; therefore, the loss rate cannot be used to easily constrain the accretion temperature. We give an estimate of the solar driven escape rate from an early atmosphere on Titan, and then briefly discuss its relevance to the cooling rate, isotope ratios, and the time period suggested to convert NH3 to the present N2 atmosphere.

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