Palaeoclimate of Titan with methane oceans and continents simulated by a global climate model

The limited photochemical lifetime of methane in Titan’s atmosphere and the lack of obvious strong methane sources at present allow the idea that Titan experienced past epochs in which the total inventory of outgassed methane was much larger than today. Much of the methane inventory would have existed as surface oceans because the atmospheric methane content is limited by condensation. The observed topography and estimated evolution of the methane inventory imply that a combination of oceans and continents as on Earth was more likely than a global ocean within the past 1 Gyr. Titan’s palaeoclimate in the presence of hypothetical methane-rich oceans and continents is simulated in an exemplary way by a global climate model coupled to a slab ocean model. The model also takes into account the exchange of N₂ between the ocean and atmosphere as the N₂ solubility in the ocean changes with the ocean temperature. If the ocean is global and there are no continents, the tropospheric climate is globally very moist but calm. N₂ is released from the ocean in spring and is absorbed by the ocean in autumn. The global imbalance between the N₂ release and absorption causes a semi-annual oscillation of global-mean surface pressure by 0.1 hPa, by analogy with the semi-annual oscillation of surface pressure on Mars caused by the polar CO₂ condensation and sublimation. In warm seasons, oceans are colder than continents because of evaporative cooling of the ocean. The ocean-land temperature contrast in warm seasons induces a sea breeze circulation, which carries moist air onshore and thereby causes substantial orographic rainfall in coastal areas and elevated terrains of continents. Therefore, the total precipitation is larger on continents than over oceans regardless of the geographic location of continents. This may have implications for the geomorphology. For instance, the possible presence of circum-Xanadu ridges surrounded by methane-rich oceans may have been conducive to substantial seasonal rainfall around Xanadu, which partly drained to the desiccated Xanadu basin and caused the formation of the observed numerous fluvial networks.