Abstract
Context.According to clues left by theCassinimission, 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”.Aims.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.Methods.We built numerical models including barodiffusion and thermodiffusion (Soret’s effect) in N2+CH4+C2H6liquid 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.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 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 N2did 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.
Highlights
Since the detection of its thick atmosphere by G
We subsequently include the effect of the geothermal gradient, and we describe our in-depth study of the role of molecular diffusion coefficients
We addressed the possibility of an ethane storage in the deepest part of an alkanofer, due to the effect of molecular diffusion over geological timescales
Summary
Since the detection of its thick atmosphere by G. In addition to seas and lakes, dry fluvial channels have been detected (Le Gall et al 2010; Coutelier et al 2021) and evidence of the presence of evaporite deposits (Barnes et al 2011; Cordier et al 2013, 2016; MacKenzie et al 2014) and underground alkanofers has been found (Corlies et al 2017; Hayes et al 2017). These latter geological structures were proposed to be the Titanian analogs for terrestrial aquifers. While a porous water-ice matrix plays the role of Earth’s porous rocks, the liquid contained in the pores is a mixture of methane and ethane, complemented by some amount of dissolved nitrogen (Cordier et al 2017)
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