Water Release during Lunar Magma Ocean Crystallization and the Potential Endogeneous Origin of Water-Ice in the Permanently Shadowed Regions of the Moon

Abstract

Introduction:The detection of trapped water ice in permanently shadowed regions of the Moon [1] is of interest for water harvesting to support life and generate fuel [2, 3]. The origins of the cold trapped water ice are debated and affect the distribution and abundance of ice, the knowledge of which is required for water harvesting [4]. Here we investigate a previously unexplored possibility that some fraction of the cold trapped water ice may have originated from lunar magma ocean (LMO) outgassing. Using H2-H2O solubility laws and thermodynamic modeling of coupled degassing and crystallization, we provide estimates on the range of water masses that might have been outgassed during LMO crystallization.Methods: We model LMO crystallization using the softwares SPICEs [5] and alphaMELTS [6] which have been successfully employed in recent studies [7, 8, 9]. The estimated mass of water outgassed during LMO crystallization depends on the initial bulk H2O content of the LMO, the partition coefficients of H2O between minerals crystallized from the LMO and LMO melt, the initial LMO depth, and the fraction of interstitial liquid trapped during LMO crystallization [8, 9]. The bulk H2O content of the LMO and the partition coefficients of H2O (for relevant LMO minerals and conditions) are currently poorly constrained [e.g. 8, 9, 10]. Using the measured H2O content in plagioclase from ferroan anorthosites (FAN) [11] as the observational constraint to validate their models, [8, 9] demonstrated that the model outputs are not sensitive to either the fractions of trapped interstitial liquid or the initial LMO depth. Accordingly, an initial LMO depth of 600 km and 0% interstitial liquid are considered in this study. We vary the initial bulk LMO H2O from 1-5000 ppm and the partition coefficients between the maximum and minimum values reported in the literature. We consider two species of hydrogen dissolved and eventually outgassed from the LMO: H2O and H2. Their proportions depend on the fO2 of the system, which we varied from IW to IW-2 [12]. We use the solubility laws of [13] and [14] to model water outgassing during LMO crystallization. By integrating volatile exsolution over depth, the total amount of degassed volatiles from the LMO at a given temperature is calculated. We consider that the vigor of convection in the LMO affects the outgassing efficiency by varying the number of degassing cycles (1-50) per cooling step during crystallization and assess the effect on our model results. We bracket the range of realistic LMO crystallization scenarios based on the conditions required to explain the H2O in FAN plagioclase, calculate the total H2O mass released under such conditions, and compare it with the polar ice inventory.Results and Discussion: We find that when the mineral-melt partition coefficient of H2O approaches the minimum (Dmin), the number of degassing cycles (i.e. the contribution of LMO convection to outgassing efficiency) has no effect when bulk H2O ≤ 100 ppm, but is important at higher bulk H2O contents. The H2O contents in crustal plagioclase are best explained by bulk H2O contents ≥ 100 ppm. For Dmax the amount of H2O degassed in each cycle is small, hence, the crustal H2O is not very sensitive to degassing cycles. However, only drier LMO (≤ 10 ppm bulk H2O) can explain the crustal H2O contents. Accordingly, we provide estimates of the total amount of H2O released during LMO crystallization for ≥ 100 ppm bulk H2O, 1-50 degassing cycles/K for Dmin, and ≤ 10 ppm and only 50 degassing cycles/K for Dmax. For Dmin, the outgassed H2O ranges from 1016-1021 kg (up to 7 orders of magnitude higher than mare volcanic H2O outgassing estimates of ~1014 kg [15] and 1016 kg [16, 17]), and the outgassed H2 ranges from 1015-1020 kg. For Dmax, the outgassed H2O ranges from 103-104 kg, and the outgassed H2 ranges from 1012-1013 kg. We find that the species and mass of outgassed volatiles are very sensitive to the mineral-melt partition coefficient of H2O, which emphasizes the need to determine these partition coefficients specifically for lunar conditions in future studies. For Dmin, if the outgassed H2 does not oxidize to H2O and only the outgassed H2O contributes to water-ice,