Consequences of a volatile-rich bulk silicate Moon for its core and transient atmosphere

It is widely accepted that the Moon lost most of its volatiles during formation by a catastrophic impact and subsequent accretion from a hot debris disk.[1] However, analyses of primitive lunar samples (e.g., olivine-hosted melt inclusions) indicate that portions of the lunar silicate mantle (bulk silicate Moon; BSM) may retain significant amounts of volatiles.[2] A recent compilation [3] provides best estimates for BSM volatile abundances, including S, H, O, and C, with hydrogen showing the greatest variability. In parallel, remote sensing data reveal water ice deposits in permanently shadowed polar regions of the Moon, implying the presence of water reservoirs today.[4]

Despite these observations, the implications of a volatile-rich BSM for the Moon’s differentiation and resulting reservoirs (core-mantle-atmosphere) remain poorly explored. Here, we apply a state-of-the-art differentiation model developed in our lab [5] inspired by recent work [6, 7] that tracks volatile partitioning using experimental volatile solubility laws for silicate melt, metal, and gas. The model is benchmarked against proposed BSM volatile inventories.[3] We assess the impact of a range of mantle volatile contents on the composition of the Fe-dominated lunar core. We deduce plausible volatile abundances (in wt% of the core) of S = 0.4–1.1, H < 10^-4; O ≈ 0.1, and C = 0.05–0.16. We further evaluate composition and mass of an atmosphere generated during lunar magma ocean degassing. Such an atmosphere is CO and H₂-dominated, with total pressures of 0.5–6 bar, PH₂O/PH₂ ≈ 0.05 and PCO/PCO₂ = 63.7–64.6. Our results provide new constraints on volatile redistribution during lunar differentiation and support a magmatic contribution to the formation of lunar polar ice.

 

1 Kato et al. 2015 Nature Communications (6) 7617, 2 Saal et al. 2008 Nature (454) 192-195, 3 McCubbin et al. 2023 Reviews in Mineralogy and Petrology (89) 729-786, 4 Li et al. 2018 PNAS (115) 8907-8912, 5 https://calcul-isto.cnrs-orleans.fr/apps/magworld_III/, 6 Gaillard et al., 2021 Space Science Reviews (217), 7 Gaillard et al., 2022 Earth and Planetary Science Letters (577) 117255