On the possible noble gas deficiency of Pluto's atmosphere

TL;DR

This study models how noble gases like Ar, Kr, and Xe could be trapped in Pluto's surface clathrates, potentially explaining their atmospheric deficiency and suggesting observational tests for future space missions.

Contribution

It introduces a thermodynamic model showing noble gas trapping in Pluto's surface clathrates as a mechanism for atmospheric depletion, a novel explanation for observed compositions.

Findings

Noble gases can be efficiently trapped in Pluto's surface clathrates.

A few centimeters of clathrates could sequester all noble gases if initially abundant.

Trapping explains the potential depletion of noble gases over geological timescales.

Abstract

We use a statistical-thermodynamic model to investigate the formation and composition of noble-gas-rich clathrates on Pluto's surface. By considering an atmospheric composition close to that of today's Pluto and a broad range of surface pressures, we find that Ar, Kr and Xe can be efficiently trapped in clathrates if they formed at the surface, in a way similar to what has been proposed for Titan. The formation on Pluto of clathrates rich in noble gases could then induce a strong decrease in their atmospheric abundances relative to their initial values. A clathrate thickness of order of a few centimeters globally averaged on the planet is enough to trap all Ar, Kr and Xe if these noble gases were in protosolar proportions in Pluto's early atmosphere. Because atmospheric escape over an extended period of time (millions of years) should lead to a noble gas abundance that either remains constant or increases with time, we find that a potential depletion of Ar, Kr and Xe in the atmosphere would best be explained by their trapping in clathrates. A key observational test is the measurement of Ar since the Alice UV spectrometer aboard the New Horizons spacecraft will be sensitive enough to detect its abundance $\sim$10 times smaller than in the case considered here.