Palaeoclimate on Saturn moon Titan under methane-enriched conditions

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Titan, the largest moon of Saturn, possesses a cold dense nitrogen atmosphere with a few percent methane and numerous minor species. The atmospheric methane content is likely to have evolved significantly in the past because of irreversible photochemical destruction in the upper atmosphere and episodic outgassing from subsurface reservoirs such as methane clathrates. The atmospheric methane content controls the strength of the greenhouse effect, which in turn affects the phase change of the two major atmospheric constituents, nitrogen and methane. A higher methane inventory in the past may have resulted in wide-spread methane oceans and thereby affected the climate by ocean-atmosphere feedback. This project aims at investigating the past climate of Titan under methane-enriched conditions during the past 1 Gyr to fill the gap in our knowledge of the evolution of the atmosphere and its impact on the surface. An investigation of the palaeoclimate under methane-enriched conditions in the last 1 Gyr is important since there are geomorphologic features from this epoch, which are hardly consistent with the climate under the present atmospheric composition. The project shall address the possible range of long-term fluctuations of atmospheric pressure and temperature as well as methane ocean distribution. The climate feedback between ocean and atmosphere and any climate features related to ocean-continent distributions such as monsoon circulation shall be scrutinized. Another particular focus is the variable nitrogen dissolution in methane oceans and its impact on meteorology, e.g. winds induced by changing partial pressure of nitrogen in the atmosphere. The project shall also help understand the history of possible standing bodies of liquids in the study area of the upcoming Dragonfly mission on Titan. The palaeoclimate of Titan under methane-enriched condition shall be simulated first by a radiative-convective model and subsequently by a general circulation model with a built-in methane hydrology scheme coupled to a slab ocean model. The greenhouse effect due to collision-induced absorption by nitrogen, methane and hydrogen is calculated by a spectrally resolved radiative transfer model. The slab ocean model takes into account the strongly temperature-dependent solubility of nitrogen in liquid methane and the observed global topography. Major observational constraints for the simulations are observed geomorphologic features of past climates such as palaeo-seas or palaeo-rivers whose age is estimated to be at most 1 Gyr.