Sequestration of noble gases in clathrate during the open ocean phase of Titan

  

Titan, visited by the Huygens probe in 2005, possess a nitrogen-rich atmosphere, surprisingly depleted in primordial noble gases such as ³⁸Ar, Kr, and Xe. Since these gases would be expected to be present in Titan’s primordial composition if its ice content was delivered by volatile-rich planetesimals and solids, a mechanism should have occurred to explain such depletion. One plausible _scenario is their sequestration in clathrate hydrates. This process could have occurred either after the formation of Titan's ice crust or shortly after the moon's accretion, during the “open-ocean” phase, when Titan’s surface was initially liquid.

Our work focuses on the impact of clathrate formation on the composition of the primordial atmosphere during Titan's early history. To do this, we first compute the composition of Titan's primordial hydrosphere, assuming a cometary-like volatile bulk composition. We take into account the vapor-liquid equilibrium between water and various volatiles, as well as the CO₂-NH₃ chemical equilibrium that occurs in the ocean at shallow depths. We then use a statistical thermodynamic model to study the effect of surface clathrate formation on the volatile distribution in the primordial atmosphere. If the stability conditions for clathrates are met, we calculate their composition and whether or not they could have sufficiently depleted the atmosphere of noble gases. In particular, we estimate the thickness of the clathrate crust required to explain the absence of noble gases in the primordial atmosphere.

Our calculations suggest that Titan should have possessed a thick CO₂- and CH₄-rich primordial atmosphere if Titan's water budget was supplied by icy planetesimals with a comet-like composition. Despite being delivered in a significant fraction, NH₃ should remain mostly dissolved as ammonium ions in the water ocean due to the chemical equilibrium with CO₂ in the ocean. Furthermore, we calculated that clathrates could begin to form when the surface temperature drops below 280 K. Specifically, at 273.15 K we calculate that both krypton and xenon can be sufficiently trapped to be undetectable by Huygens' GCMS for a clathrate crust of tens of kilometers. However, since ³⁸Ar is not trapped as efficiently as other noble gases, our calculations emphasize that such a sequestration process could not justify its absence from the atmosphere.