Vertical compositional variations of liquid hydrocarbons in Titan’s alkanofers

Context. According to clues left by the Cassini mission, Titan, one of the two solar system bodies having an hydrologic cycle, may harbor liquid hydrocarbon-based analogs of our terrestrial aquifers, referred to as “alkanofers”. Aims. On the Earth, petroleum and natural gas reservoirs show vertical gradient in chemical composition, established over geological timescales. In this work, we investigate the conditions under which Titan’s processes could lead to similar situations. Methods. We built numerical models including barodiusion and thermodiusion (Soret’s eect) in N2+CH4+C2H6 liquid mixtures, relevant for Titan’s possible alkanofers. Our main assumption is the existence of reservoirs of liquids trapped in a porous matrix with low permeability. Results. Due to the small size of the molecule, nitrogen seems to be more sensitive to gravity than ethane, even if the latter has a mass slightly larger. This behavior noticed for an isothermal crust, is reinforced by the presence of a geothermal gradient. Vertical composition gradients, formed over timescales between a fraction of mega-year to several tens of mega-years, are not influenced by molecular diusion coecients. If the alkanofer liquid is in contact with water-ice, we have checked that N2 does not, in general, impede the clathration of C2H6, except in some layers. Interestingly, we found that noble gases could easily accumulate at the bottom of an alkanofer.