VOLATILES RELEASED DURING EMPLACEMENT OF MARE BASALTS: IMPLICATIONS FOR A LUNAR ATMOSPHERE

The lunar atmosphere currently has an extremely low density and is designated a surface boundary exosphere (SBE). Although the lunar environment has most likely been a stable SBE for 3 Gyr, enhanced impact and volcanic activity early in lunar history may have contributed to an ancient, thicker atmosphere around the Moon. To determine how the ancient lunar environment may have been affected by more intense volcanic activity, this study investigates the volume of mare eruptions and the associated mass of volatiles released on the Moon as a function of time.

We compiled estimates of mare volumes in 25 lunar basins and noted ages of individual mare units as interpreted by Hiesinger et al. (2011). These observations indicate mare was emplaced from ~3.9 Ga to ~1.1 Ga, with the largest volumes emitted ~3.5 Gyrs ago. We then used recently detected concentrations of mare volatile abundances to determine the mass of volatiles released over lunar history. The most prevalent volatile species released were CO (0.2 – 2.0 × 10¹⁹ g), S (0.5 – 1.4 × 10¹⁹ g), and H₂O (0.5 – 2.6 × 10¹⁷ g). Contents of F and Cl in mare basalts have not yet been explicitly reported and are, therefore, assumed to have been released in amounts smaller than anticipated for pyroclastic deposits (e.g., less than 2 – 9 × 10¹⁴ g of F and 0 – 4 × 10¹³ g of Cl).

The mass of volatiles released during peak mare emplacement can be used to estimate the surface pressure of an atmosphere that might have developed as a result of these eruptions. The maximum surface pressure during peak eruption activity 3.5 Ga would have been ~1 kPa, or 0.01 atm. This pressure is ~1% of Earth’s current surface pressure and ~1.5 times greater than Mars’ current surface pressure. This thicker lunar atmosphere would have dissipated in ~70 Myrs. After erupting, some volatiles may have migrated towards the poles, getting trapped in permanently shadowed regions (PSRs). If 0.1% of the vented water (~10¹⁷ g) is trapped in PSRs, volcanically-derived volatiles could account for all water currently observed in PSRs. These results suggest transport models need to account for periods with higher fluences of indigenous volatiles to properly evaluate their contribution to PSR volatile deposits. As indigenous volatiles will have a distinct isotopic signature, those model calculations can be tested with future lunar surface missions.