Clathrate as a noble gas reservoir from the primordial hydrosphere of Titan

Titan, visited by the Huygens probe in 2005, is the only moon in the solar system known to have a dense, nitrogen-rich atmosphere. It is also suspected to possess a subsurface global ocean beneath an ice crust. A striking characteristic of Titan’s atmosphere is the absence of primordial noble gases such as argon, krypton, and xenon. If Titan’s ice content—estimated to be between 30% and 50%—was delivered by volatile-rich planetesimals and solids, it would be expected that these noble gases would have been incorporated into the moon’s hydrosphere during its formation. A plausible explanation for the depletion of these noble gases in Titan’s current atmosphere 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 modeling the ocean-atmosphere equilibrium during Titan’s early history. To achieve this, we begin with a bulk composition and calculate how volatiles are distributed between the vapor and liquid phases. We take into account the vapor-liquid equilibrium between water and various volatiles, as well as the CO₂-NH₃ chemical equilibrium occurring within the ocean at shallow depths. Additionally, using a statistical thermodynamic model, we explore the potential impact of clathrate formation at the ocean's surface. If the stability conditions for clathrates are met, we investigate how their formation could influence the composition of Titan’s primordial atmosphere. Specifically, we assess the required thickness of the clathrate crust necessary to deplete the primordial atmosphere of noble gases.

Our computations suggest that if Titan's water budget was delivered by icy planetesimals with a comet-like composition, a thick, CO₂- and CH₄-rich primordial atmosphere would form above the ocean. We also highlight that the equilibrium of the primordial hydrosphere leads to a significant depletion of NH₃ in both the atmosphere and the ocean, as it is converted into ions due to the chemical equilibrium with CO₂. Furthermore, we show that a clathrate crust just a few kilometers thick would be sufficient to completely deplete the primordial atmosphere of xenon at 273.15 K. In contrast, to retain most of the krypton in the atmosphere, a much thicker clathrate crust—on the order of tens of kilometers—would be required. Argon, however, is not trapped as efficiently as other noble gases. Our calculations show that argon can only be captured in significant amounts at much lower temperatures, after Titan's surface has cooled.