Composition and speciation of volcanic volatiles on the Moon

Lunar volcanic volatiles are crucial for understanding eruption dynamics on the Moon as well as the potential formation, life span, and dissipation of a lunar secondary atmosphere. We review literature concerning volatile content, degassing extent, and speciation during the mare eruption period on the Moon from 4.0 to 1.2 Ga, providing a realistic summary of degassed compositions for the traditional volcanic elements C-O-H-S-F-Cl. The most reliable estimates of lunar volcanic volatiles come from high‑titanium (high-Ti) glass beads sampled during the Apollo 17 mission. Analysis of these samples demonstrates that hydrogen is the most abundant element by mole in erupted volcanic gases, so a hydrogen species should be the most abundant molecule in the lunar gas, rather than carbon monoxide. This hydrogen is expected to speciate mostly as H₂, rather than H₂O, at the predicted oxygen fugacity for lunar magma. This difference is important because H₂ more easily escapes from the Moon, whereas H₂O could freeze out on the lunar surface, and potentially persist within permanently shadowed regions near the poles. We also find that sulfur, rather than carbon, is the third most abundant element in lunar volcanic gas, after hydrogen and oxygen.