Noble gas depletion on Titan: Clathrate sequestration during the open ocean phase

TL;DR

This study models how clathrate hydrate formation during Titan's early ocean phase could have sequestered noble gases, explaining their absence in Titan's current atmosphere.

Contribution

It introduces a thermodynamic model to estimate clathrate crust thickness needed for noble gas depletion on Titan.

Findings

A thick clathrate crust can deplete xenon and krypton from Titan's atmosphere.

Primordial argon may be absent due to partial de-volatilization of accreted materials.

Titan's initial atmosphere was likely rich in CO2 and CH4, with NH3 retained in water.

Abstract

A plausible explanation for the absence of primordial argon, krypton, and xenon in Titan's current atmosphere is that these gases were sequestered in clathrate hydrates during Titan's "open-ocean" phase. We examine how clathrate hydrate formation at Titan's ocean surface in its early history may have contributed to noble gas depletion in the primordial atmosphere. Starting with vapor-liquid equilibrium modeling between water and volatiles, we used a statistical thermodynamic model to determine the clathrate hydrate crust thickness needed to deplete the primordial atmosphere of noble gases. Our computations suggest that if Titan's volatile budget was delivered by icy planetesimals with a comet-like composition, its primordial atmosphere should be rich in CO$_2$ and CH$_4$, with NH$_3$ largely retained in water as ions. We show that at 273.15 K, a clathrate crust tens of kilometers thick would deplete the primordial atmosphere of xenon and krypton. The lack of primordial argon in Titan's atmosphere may result from the partial de-volatilization of its accreted materials.