Exogenic origin for the volatiles sampled by the Lunar CRater Observation and Sensing Satellite impact

Abstract

Returning humans to the Moon presents an unprecedented opportunity to determine the origin of volatiles stored in the permanently shaded regions (PSRs), which trace the history of lunar volcanic activity, solar wind surface chemistry, and volatile delivery to the Earth and Moon through impacts of comets, asteroids, and micrometeoroids. So far, the source of the volatiles sampled by the Lunar Crater Observation and Sensing Satellite (LCROSS) plume has remained undetermined. We show here that the source could not be volcanic outgassing and the composition is best explained by cometary impacts. Ruling out a volcanic source means that volatiles in the top 1-3 meters of the Cabeus PSR regolith may be younger than the latest volcanic outgassing event (~1 billion years ago; Gya).

Fig. 1. Elemental composition of the Lunar regolith in the top 1–3 m of the Cabeus Crater Permanently Shaded Region sampled by the Lunar CRater Observation and Sensing Satellite compared to the elemental composition of possible sources.

The regolith elemental composition (black squares) of (a) N/C, (b) O/C, and (c) C/H compared to C/S is determined based on the assumption that the volatiles are either stored as Clathrates or are condensed onto the regolith as Condensates. All sources are identified by name in the figures next to their symbol. Uncertainties are extrapolated from reported measurements according to standard methods. Note that no single source exactly matches all of the elemental ratios. Data are provided in Supplementary Table S2.

Fig. 2. Stability curves for clathrates stored in the permanently shaded regions.

Clathrates are stable above and to the left of the curve. Comparison of (top blue line) the pressure-temperature profile, or P = f(T), in the upper lunar regolith (0.2–5 m) to stability curves for clathrates with SO₂, H₂S, CO, and mixtures of these species as noted in the legend^,,. The P = f(T) profile is calculated based on a temperature profile extrapolated from ref. and pressure based on a 1.66 g/cm³ lunar regolith. Mixed clathrate stability curves are based on clathrates formed from gas mixture of CO + SO₂ or H₂S (with a cometary C/S from ref. ); such clathrate is dominated by SO₂ or H₂S. Because the P = f(T) for the lunar regolith falls in the area above and to the left of all stability curves, the regolith is within the clathrate stability domain.