Atmospheric escape explains diverse surface compositions of Pluto vs Sedna

Abstract

Kuiper Belt Objects are thought to be remnants of early solar system materials, which have retained volatiles such as CH₄ and C₂H₆. Spectroscopic data from various sources reveals that while Pluto retains CH₄ and C₂H₆, Sedna shows a lack of CH₄ but an abundance of C₂H₆ (Emery et al. 2024). We hypothesized that CH₄, while stable on Pluto, is able to escape from less massive Sedna sufficiently rapidly to be depleted over the age of the solar system, while C₂H₆ has been retained due to its larger molecular mass.

Utilizing models for Jeans escape and hydrodynamic escape, as appropriate for a given mass of object and of escaping species, we analyzed the stability and escape rates of CH₄ and C₂H₆ on both Pluto and Sedna. We explored a wide range of masses of Sedna, ranging from ½ to ⅒ the mass of Pluto, and three different abundances of CH₄ and C₂H₆ using measured values from Comet 67-P, and Enceladus, and an upper limit assuming clathrate hydrate.

We find that CH₄ remains stable on Pluto, but escapes from Sedna due to its lower mass, whereas C₂H₆ remains stable when using both 100 % and 10 % outgassing rates. This result is in agreement with the observed spectra and leads to a tighter mass constraint for Sedna. Our model also explains the observed absence of methane on another KBO, Gonggong.