Clathrate hydrates as a sink of noble gases in Titan's atmosphere

TL;DR

This study uses thermodynamic modeling to show that clathrate hydrates could explain the observed depletion of xenon and krypton in Titan's atmosphere, but not argon, suggesting additional processes are involved.

Contribution

It provides a detailed thermodynamic analysis of noble gas trapping in Titan-like conditions, highlighting the selective trapping efficiency of clathrate hydrates.

Findings

Xenon and krypton are highly trapped in clathrates, reducing their atmospheric abundance.

Argon is poorly trapped, remaining abundant in the atmosphere.

The trapping mechanism explains the observed noble gas deficiencies in Titan's atmosphere.

Abstract

We use a statistical thermodynamic approach to determine the composition of clathrate hydrates which may form from a multiple compound gas whose composition is similar to that of Titan's atmosphere. Assuming that noble gases are initially present in this gas phase, we calculate the ratios of xenon, krypton and argon to species trapped in clathrate hydrates. We find that these ratios calculated for xenon and krypton are several orders of magnitude higher than in the coexisting gas at temperature and pressure conditions close to those of Titan's present atmosphere at ground level. Furthermore we show that, by contrast, argon is poorly trapped in these ices. This trapping mechanism implies that the gas-phase is progressively depleted in xenon and krypton when the coexisting clathrate hydrates form whereas the initial abundance of argon remains almost constant. Our results are thus compatible with the deficiency of Titan's atmosphere in xenon and krypton measured by the {\it Huygens} probe during its descent on January 14, 2005. However, in order to interpret the subsolar abundance of primordial Ar also revealed by {\it Huygens}, other processes that occurred either during the formation of Titan or during its evolution must be also invoked.