Palaeoclimate on Saturn moon Titan under methane-enriched conditions
Astrophysics and Astronomy
Final Report Abstract
This project investigated the characteristics of the possible palaeoclimate of Saturn’s moon Titan in the far past with a global climate model under the premise that there was a substantially larger methane reservoir than today. Two fundamentally different scenarios concerning the temporal variation in methane reservoir on Titan were assumed for the numerical simulations of the palaeoclimate. The first part of the project assumed that Titan’s atmosphere was depleted in methane over extended periods, but thereafter became methane-enriched by volcanic outgassing of a few hundred million years duration. Under this condition the climate model predicted dramatic climate changes, where the global-mean surface temperature oscillated by up to 15 K mainly due to variable intensity of the greenhouse effect and ice-albedo feedback. A large portion of the volcanically emitted methane was deposited as surface frost, which caused global cooling instead of a runaway greenhouse effect. Eventually, the entire globe was covered by methane frost given the huge amount of emitted methane. After methane outgassing ceased, the global methane frost cover gradually shrank due to sublimation and subsequent photolysis. Merely the methane polar cap survived until geologically recent past and then melted, which may explain the present methane-rich polar seas. The second part of the project assumed that in the far past Titan possessed a huge methane reservoir, which was lost by photolysis without being substantially resupplied in between. Under this condition Titan never became so cold in such a way as to enable surface deposition of methane frost. Instead, the majority of the methane reservoir existed as liquid oceans, which gradually shrank due to methane destruction. Depending on the past topography of Titan, the palaeoocean may have been global or partial. In the project the palaeoclimate was simulated considering global or partial oceans and two representative ocean compositions. For this purpose, the climate model was coupled to a simplified thermodynamic ocean model. If the methane content in the ocean was small, the atmosphere was relatively dry and calm climate conditions with weak winds over both oceans and continents prevailed. If the methane content in the ocean was high, large differences emerged between maritime and continental climate. The climate over methane-rich oceans was very moist, although the precipitation was weak. If large continents and methane-rich oceans coexisted, sea breeze caused strong precipitation on the wind-ward slope of continents, while there were rain shadows in the middle of oceans.
Publications
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Paleoclimate Evolution on Titan After Episodic Massive Methane Outgassing Simulated by a Global Climate Model. Journal of Geophysical Research: Planets, 126(12).
Tokano, Tetsuya & Lorenz, Ralph D.
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Titan Stratospheric Haze Bands Observed in Cassini VIMS as Tracers of Meridional Circulation. The Planetary Science Journal, 3(5), 114.
Kutsop, N. W.; Hayes, A. G.; Corlies, P. M.; Le Mouélic, S.; Lunine, J. I.; Nixon, C. A.; Rannou, P.; Rodriguez, S.; Roman, M. T.; Sotin, C. & Tokano, T.
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Paleoclimate of Titan with hydrocarbon oceans and continents simulated by a global climate model. Icarus, 389, 115253.
Tokano, Tetsuya
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Taking Titan’s Boreal Pole Temperature: Evidence for Evaporative Cooling in Ligeia Mare. The Astrophysical Journal, 961(2), 191.
Sultana, R. E.; Le Gall, A.; Tokano, T.; Bonnefoy, L. E.; Coutelier, M. & Lorenz, R. D.
