Volcanically Induced Transient Atmospheres on the Moon: Assessment of Duration, Significance, and Contributions to Polar Volatile Traps

Abstract

A transient lunar atmosphere formed during a peak period of volcanic outgassing and lasting up to about~70 Ma was recently proposed. We utilize forward-modeling of individual lunar basaltic eruptions and the observed geologic record to predict eruption frequency, magma volumes, and rates of volcanic volatile release. Typical lunar mare basalt eruptions have volumes of~10 2-10 3 km 3 , last less than a year, and have a rapidly decreasing volatile release rate. The total volume of lunar mare basalts erupted is small, and the repose period between individual eruptions is predicted to range from 20,000 to 60,000 years. Only under very exceptional circumstances could sufficient volatiles be released in a single eruption to create a transient atmosphere with a pressure as large as~0.5 Pa. The frequency of eruptions was likely too low to sustain any such atmosphere for more than a few thousand years. Transient, volcanically induced atmospheres were probably inefficient sources for volatile delivery to permanently shadowed lunar polar regions. Plain Language Summary Could gas emitted from volcanic eruptions during the most intense and voluminous period of lunar mare volcanism produce a temporary lunar atmosphere? Could the presence of such an atmosphere enable volatiles to reach the cold traps in the permanently shadowed regions at the lunar poles? We use information from lunar geology and sample analyses to predict the number of eruptions with time, the volume of individual eruptions, the rates of volcanic gas release during each eruption, and the time between eruptions. We find that only under rare circumstances could a single eruption or two eruptions closely spaced in time release enough gas to create a transient atmosphere with a pressure as large as~0.5 Pa. Furthermore, it is difficult to sustain such an atmosphere for more than a few thousand years. These results suggest that volcanically produced atmospheres are inefficient source mechanisms for delivery of volatiles to form deposits in permanently shadowed polar regions of the Moon; this favors volatile-rich impactors as the major source of polar ice.

Stratigraphy of Ice and Ejecta Deposits at the Lunar Poles

“…In addition to impacts, volcanic activity is suspected to be an important pathway for the transport of volatiles to the lunar surface (Milliken & Li, 2017; Needham & Kring, 2017; Saal et al, 2008; Wilson & Head, 1981). We applied new eruption frequency estimates from Head et al (2020) to calculate the delivery of volcanically outgassed water to the poles (the supporting information). Water formed from interactions between the regolith and solar wind was ignored in this work because it likely contributes many orders of magnitude less water than asteroids and volcanism (Hurley et al, 2017; Lucey et al, 2020) and may be outweighed by loss processes including UV photolysis and solar wind sputtering at the very surface (Farrell et al, 2019).…”

Water Group Exospheres and Surface Interactions on the Moon, Mercury, and Ceres

“…Endogenic outgassing of primordial water is more likely in the distant past than in recent history (Needham and Kring 2017;Deutsch et al 2019;Head et al 2020). At this point, the major source of molecular water has not been determined on either Mercury or the Moon.…”

Temperatures Near the Lunar Poles and Their Correlation With Hydrogen Predicted by LEND

“…Needham and Kring (2017) estimated that volcanically derived volatiles alone, degassed during mare basalt forming eruptions, could account for all currently observed hydrogen deposits in lunar PSRs. However a more recent study by Head et al (2020) suggests volatile-rich impactors to be the main source of polar volatiles rather than volcanically derived volatiles. Crider and Vondrak (2002) showed that water deposits implanted by the solar wind proton flux within 100 Ma could also account for all hydrogen detected by the Lunar Prospector Neutron Spectrometer (LPNS; Feldman et al, 1998).…”

“…However a more recent study by Head et al. (2020) suggests volatile‐rich impactors to be the main source of polar volatiles rather than volcanically derived volatiles. Crider and Vondrak (2002) showed that water deposits implanted by the solar wind proton flux within 100 Ma could also account for all hydrogen detected by the Lunar Prospector Neutron Spectrometer (LPNS; Feldman et al., 1998).…”