Controls on determining the bulk water content of the Moon

The Moon is much wetter than previously thought. The estimated bulk H₂O concentrations based on the analyses of H₂O in lunar materials show a wide range from 5 to 1650 ppm. To better constrain bulk H₂O in the lunar magma ocean (LMO), we model LMO crystallization and vary D_H (concentration of H₂O in LMO mineral/concentration of H₂O in melt), interstitial melt fraction, and initial LMO depth. We take the highest and lowest values of D_H reported in the literature for the LMO minerals. We assess the bulk H₂O content required in the initial magma ocean to satisfy two observational constraints: (1) H₂O measured in plagioclase grains from ferroan anorthosites and (2) crustal mass from GRAIL. We find that the initial bulk LMO H₂O that best explains the H₂O content in crustal plagioclase is strongly dependent on D_H rather than interstitial melt fractions or initial LMO depths, with a drier magma ocean (10 ppm H₂O) being favored with higher D_H and a wetter magma ocean (100–1000 ppm H₂O) with lower D_H. This underscores the importance of constraining D_H specific to lunar conditions in future studies. We also demonstrate that crustal mass is not an effective hygrometer.