Lifetime of a transient atmosphere produced by Lunar Volcanism

TL;DR

This study models the escape processes of a transient lunar atmosphere produced by volcanic outgassing, estimating its lifetime to be between hundreds of years and about a million years depending on temperature and solar conditions.

Contribution

It provides a detailed analysis of atmospheric escape mechanisms and lifetime estimates for a volcanic lunar atmosphere, incorporating non-thermal and thermal escape processes.

Findings

Thermal escape dominates if exobase temperature exceeds 400 K.

Atmospheric lifetime can be as short as a few hundred years at high temperatures.

Non-thermal escape can remove a millibar-level atmosphere in about 1 million years.

Abstract

Early in the Moon's history volcanic outgassing may have produced a periodic millibar level atmosphere (Needham and Kring, 2017). We examined the relevant atmospheric escape processes and lifetime of such an atmosphere. Thermal escape rates were calculated as a function of atmospheric mass for a range of temperatures including the effect of the presence of a light constituent such as H2. Photochemical escape and atmospheric sputtering were calculated using estimates of the higher EUV and plasma fluxes consistent with the early Sun. The often used surface Jeans calculation carried out in Vondrak (1974) is not applicable for the scale and composition of the atmosphere considered. We show that solar driven non-thermal escape can remove an early CO millibar level atmosphere on the order of 1 Myr if the average exobase temperature is below 350 - 400 K. However, if solar UV/EUV absorption heats the upper atmosphere to temperatures > 400 K thermal escape increasingly dominates the loss rate, and we estimated a minimum lifetime of 100's of years considering energy limited escape.