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

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 (D_min), 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 D_max 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 D_min, and ≤ 10 ppm and only 50 degassing cycles/K for D_max. For D_min, 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 D_max, 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 D_min, if the outgassed H₂ does not oxidize to H₂O and only the outgassed H₂O contributes to water-ice, <0.001 % of the total H₂O released due to LMO outgassing, if condensed, can explain the entire estimated polar ice inventory of ~10¹¹ kg [18]. For D_max, if all the outgassed H₂O condenses to water-ice, it may explain an insignificant amount of the estimated ice inventory.

Future studies need to evaluate the lifetime of the transient atmosphere that would be formed by outgassing during LMO crystallization and the effectiveness of the atmospheric water to be cold trapped in shadowed craters. However, this study raises an intriguing possibility that primordial outgassed endogenic volatiles during the very early stages of lunar chemical differentiation (~4370 Ma [19]) contributed to the polar ice inventory and thus may be partially preserved to some extent on the Moon today. Cryogenic sample return (as proposed by the Artemis III Science Definition Team Report) from permanently shadowed regions and their geochemical analyses could validate the origin and delivery of volatiles by magma ocean crystallization.

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