Palaeoclimate on Saturn moon Titan under methane-enriched conditions

土卫六是土星最大的卫星，拥有寒冷的稠密氮气大气层，含有百分之几的甲烷和许多次要物种。由于高层大气中不可逆转的光化学破坏以及甲烷包合物等地下储层的间歇性释气，大气中的甲烷含量在过去很可能发生了显著变化。大气中的甲烷含量控制着温室效应的强度，而温室效应又影响着两种主要大气成分氮和甲烷的相变。过去较高的甲烷存量可能导致广泛的甲烷海洋，从而通过海洋-大气反馈影响气候。该项目的目的是调查过去1 Gyr期间土卫六在甲烷富集条件下的过去气候，以填补我们对大气演变及其对地表影响的知识方面的差距。在过去1 Gyr的甲烷富集条件下的古气候的调查是重要的，因为有地貌特征，从这个时代，这是很难在目前的大气成分下的气候一致。该项目应研究大气压力和温度长期波动的可能范围以及甲烷海洋分布情况。应仔细研究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溶解的变化及其对气象学的影响，例如大气中氮分压变化引起的风。该项目还将有助于了解即将在土卫六上进行的蜻蜓使命的研究区域中可能存在的液体的历史。在富甲烷条件下的土卫六古气候应首先用辐射-对流模型模拟，然后用大气环流模型模拟，该模型内置与平板海洋模型耦合的甲烷水文方案。利用光谱分辨辐射传输模型计算了氮、甲烷和氢碰撞诱导吸收的温室效应。板状海洋模式考虑到氮在液态甲烷中的溶解度与温度的强烈相关性和观测到的全球地形。模拟的主要观测约束是观测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1 Gyr。

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.