The Methane Cycle on Titan: Surface-Atmosphere Thermochemical Coupling (Invited)

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[5210] Planetary Sciences: Astrobiology / Planetary Atmospheres, Clouds, And Hazes, [5405] Planetary Sciences: Solid Surface Planets / Atmospheres, [6207] Planetary Sciences: Solar System Objects / Comparative Planetology, [6281] Planetary Sciences: Solar System Objects / Titan

Scientific paper

The cycle of methane on Titan is often considered to be similar to Earth’s hydrological cycle, complete with surface evaporation, cloud formation, and precipitation, although on quite different seasonal and dynamical time scales. However, unlike water on Earth, methane plays a critical role in maintaining Titan’s very atmosphere (of nitrogen). The fate of Titan’s methane is controlled by atmospheric photochemistry - neutral chemistry in the upper stratosphere below 900 km, and ion chemistry above. As a consequence, methane is irreversibly destroyed in tens to hundred Myr, a relatively short time on geologic time scale. Thus, it is essential that methane be replenished from time to time for continued presence of the nitrogen atmosphere. There is evidence from Cassini-Huygens measurements that the atmospheric photochemical products of methane - hydrocarbons, nitriles and aerosols - eventually end up at Titan’s surface, together with rained-out components, methane and ethane. The photochemical species have seasonal dependence, but their concentrations in the surface would build up over time. The surface material is expected to undergo gradual change, largely by radiolysis. However, this is of little consequence to the recycling of methane destroyed in the stratosphere (85%) and the ionosphere (15%) of Titan, unlike the H2-rich atmosphere of Jupiter in whose hot interior similar (hydrocarbon) products of methane can be converted back into methane. Thus, methane on Titan must be replenished from the vast methane-clathrate reservoir that is presumably present in the moon’s interior. The destabilization of clathrates can occur by one of several processes including cryovolcanism and impacts. [www.umich.edu/~atreya]

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