土卫六链烷中液态烃的垂直组成变化
Vertical compositional variations of liquid hydrocarbons in Titan's alkanofers
摘要：
根据卡西尼号任务留下的线索，土卫六，两个具有水文循环的太阳系天体之一，可能含有我们陆地含水层的液态烃类类似物，称为“烷醇”。在地球上，石油和天然气储层在化学成分上显示出在地质时间尺度上建立的垂直梯度。在这项工作中，我们的目标是调查土卫六的过程可能导致类似情况的条件。本文建立了N_2+CH_4+C_2H_6液体混合物中的压力扩散和热扩散（Soret效应）的数值模型，这些模型与Titan上可能存在的烷氧化物有关。我们的主要假设是存在的储层中的液体被困在一个多孔基质与低渗透率。由于分子的小尺寸，氮似乎比乙烷对重力更敏感，即使后者的质量稍大。这种行为，注意到一个等温地壳，是加强了地热梯度的存在。垂直成分梯度，形成的时间尺度之间的一个百万年的分数到几十个百万年，不受分子扩散系数。我们发现，乙烷不积累在底部的烷醇的扩散下，留下的问题，为什么没有观察到泰坦的表面上未解决。当烷醇液体与水-冰接触时，N_2一般不妨碍C_2H_6的包合，但在某些层中除外。有趣的是，我们发现稀有气体可以很容易地聚集在链烷醇的底部。
英文摘要：
According to clues left by the Cassini mission, Titan, one of the two Solar System bodies with a hydrologic cycle, may harbor liquid hydrocarbon-based analogs of our terrestrial aquifers, referred to as "alkanofers". On the Earth, petroleum and natural gas reservoirs show a vertical gradient in chemical composition, established over geological timescales. In this work, we aim to investigate the conditions under which Titan's processes could lead to similar situations. We built numerical models including barodiffusion and thermodiffusion (Soret's effect) in N_2+CH_4+C_2H_6 liquid mixtures, which are relevant for Titan's possible alkanofers. Our main assumption is the existence of reservoirs of liquids trapped in a porous matrix with low permeability. Due to the small size of the molecule, nitrogen seems to be more sensitive to gravity than ethane, even if the latter has a slightly larger mass. This behavior, noticed for an isothermal crust, is reinforced by the presence of a geothermal gradient. Vertical composition gradients, formed over timescales of between a fraction of a mega-year to several tens of mega-years, are not influenced by molecular diffusion coefficients. We find that ethane does not accumulate at the bottom of the alkanofers under diffusion, leaving the question of why ethane is not observed on Titan's surface unresolved. If the alkanofer liquid was in contact with water-ice, we checked that N_2 did not, in general, impede the clathration of C_2H_6, except in some layers. Interestingly, we found that noble gases could easily accumulate at the bottom of an alkanofer.
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