Palaeoclimate on Saturn moon Titan under methane-enriched conditions

土卫六是土星最大的卫星，拥有寒冷而稠密的氮气大气，含有少量甲烷和许多次要物种。由于高层大气不可逆转的光化学破坏，以及甲烷笼状化合物等地下储气库时不时地放出气体，大气中的甲烷含量可能在过去发生了重大变化。大气中的甲烷含量控制着温室效应的强度，而温室效应反过来又会影响两种主要大气成分--氮气和甲烷的相变。过去较高的甲烷库存可能导致了甲烷海洋的广泛分布，从而通过海洋-大气反馈影响了气候。该项目旨在调查过去1年来在甲烷丰富条件下土卫六的过去气候，以填补我们对大气演变及其对地表影响的知识空白。研究近1Gyr富甲烷条件下的古气候具有重要意义，因为该时期的地貌特征与目前大气成分下的气候很难吻合。该项目应处理大气压力和温度长期波动的可能范围以及甲烷在海洋中的分布。应仔细审查海洋和大气之间的气候反馈以及与海洋-大陆分布有关的任何气候特征，如季风环流。另一个特别关注的问题是甲烷海洋中氮的可变溶解及其对气象学的影响，例如，改变大气中氮的分压所引起的风。该项目还将有助于了解即将在土卫六上执行的蜻蜓任务研究区域内可能存在的液体的历史。首先用辐射对流模式模拟富甲烷条件下土卫六的古气候，然后用大气环流模式和板状海洋模式耦合的甲烷水文方案来模拟。用光谱分辨辐射传输模型计算了氮气、甲烷和氢气碰撞吸收引起的温室效应。平板海洋模式考虑了氮在液态甲烷中的高度依赖温度的溶解度和观测到的全球地形。模拟的主要观测约束是观察到的过去气候的地貌特征，如古海洋或古河流，其年龄估计至多为1Gyr。

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.