O-H kinetic isotope fractionation model to assess lunar polar water ice reserves & origin

Abstract O-H isotope signatures are essential for unravelling the lunar water cycle, pivotal for a sustainable water exploitation and management (SDG6) on the Moon, Earth and beyond, and for better understanding the Earth-Moon system’s history and the Earth’s cryosphere. Sublimation alters the initial isotope signature on the lunar and Earth surface/subsurface and during robotic missions’ sampling chain, hindering the assessment of lunar water resources for ISRU and the interpretation of scientific planetary data. The scarce existing studies conclude on different trends: (i) Rayleigh-like HDO isotope fractionation; (ii) no relevant HDO isotope fractionation for water ice mass loss below 40%. Here we present a kinetic isotope fractionation model for pure hexagonal ice and planar specific surface area to correct lunar missions’ raw data and to help tracking back lunar and terrestrial secondary processes at geological timescales. Our model follows a Rayleigh-like trend, more pronounced at lower temperatures and higher abundances of heavy water isotopologues. At 120 K, HDO isotope fractionation becomes relevant for water ice mass loss greater than 10% (δD enrichment: 9.5 to 85.7‰ for δD 0 : -800 to + 800‰), while H 2 18 O and H 2 17 O isotope fractionations become relevant for water ice mass loss greater than 0.5% (δ 18 O and δ 17 O enrichments below 0.6 and 0.3‰, respectively, for δ 18 O 0 : +5.64 and δ 17 O 0 : +2.94‰).