Lunar Outgassing, Transient Phenomena & the Return to the Moon, II: Predictions and Tests for Outgassing/Regolith Interactions

TL;DR

This paper models lunar outgassing effects on regolith and transient phenomena, proposing observational tests and exploring implications for water ice accumulation and detection methods.

Contribution

It provides detailed models of outgassing impacts on lunar regolith and suggests new observational strategies for detecting outgassing and water ice.

Findings

Explosive outgassing can explain observed TLPs.

Water can freeze in the regolith, especially near poles.

Outgassing detection can be enhanced with remote sensing techniques.

Abstract

We follow Paper I with predictions of how gas leaking through the lunar surface could influence the regolith, as might be observed via optical Transient Lunar Phenomena (TLPs) and related effects. We touch on several processes, but concentrate on low and high flow rate extremes, perhaps the most likely. We model explosive outgassing for the smallest gas overpressure at the regolith base that releases the regolith plug above it. This disturbance's timescale and affected area are consistent with observed TLPs; we also discuss other effects. For slow flow, escape through the regolith is prolonged by low diffusivity. Water, found recently in deep magma samples, is unique among candidate volatiles, capable of freezing between the regolith base and surface, especially near the lunar poles. For major outgassing sites, we consider the possible accumulation of water ice. Over geological time ice accumulation can evolve downward through the regolith. Depending on gases additional to water, regolith diffusivity might be suppressed chemically, blocking seepage and forcing the ice zone to expand to larger areas, up to square km scales, again, particularly at high latitudes. We propose an empirical path forward, wherein current and forthcoming technologies provide controlled, sensitive probes of outgassing. The optical transient/outgassing connection, addressed via Earth-based remote sensing, suggests imaging and/or spectroscopy, but aspects of lunar outgassing might be more covert, as indicated above. TLPs betray some outgassing, but does outgassing necessarily produces TLPs? We also suggest more intrusive techniques from radar to in-situ probes. Many of these approaches should be practiced in a pristine lunar atmosphere, before significant confusing signals likely to be produced upon humans returning to the Moon.