Abstract
Projectiles striking the Moon have modified its crust and delivered volatile elements to its interior and surface. Direct evidence of impactor origins is recorded by the rare occurrence of sub-cm sized meteorite fragments identified in Apollo samples and lunar meteorites. The Bench Crater meteorite is a millimetre-sized carbonaceous chondrite collected in regolith on the rim of Bench impact crater at the Apollo 12 landing site. Transmission electron microscopy has previously shown that Bench Crater contains abundant hydrated silicates, establishing the survivability of hydrated material impacting the lunar surface. To provide further information on the volatile inventory of the Bench Crater meteorite, we report here the isotope compositions of hydrogen, nitrogen, carbon and oxygen. This is the first direct isotopic analysis of meteoritic material delivered to the lunar surface and provides context for volatile and organic element signatures in lunar regolith samples, and the survivability of volatile material delivered to planetary surfaces during impact bombardment. The Bench Crater meteorite is characterised by δD values ranging between −36 ± 40 and 200 ± 40‰, and bulk average δ13C of −13 ± 30‰, and δ15N of −40 ± 36‰ (all uncertainties at the 2σ confidence level). The oxygen isotope compositions measured in situ in matrix silicates and magnetite in Bench Crater are consistent with those measured in matrix and magnetite in CI and CM chondrite falls. Altogether, these new H, C, N and O isotope data, coupled to mineralogical and geochemical observations, suggest that Bench Crater may have been derived from an asteroidal parent body not represented in the terrestrial meteorite collection. This is a crucial outcome in the current context of sample-return missions to carbonaceous asteroids, and more broadly for investigating the flux of material delivered to the Earth-Moon system through time.
Highlights
A key driver of future lunar scientific exploration is to determine the source(s) of water ice and volatiles found on the Moon’s surface at the present day (Li et al, 2018), and to understand past origins of water and volatiles in the Earth-Moon system (NRC, School of Natural Sciences, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK.2007)
Microtomed sections were prepared and analysed using the NASA Johnson Space Center (JSC) JEOL 2200FS field emission gun (FEG) transmission electron microscope (TEM) coupled to an Oxford energy-dispersive X-ray (EDX) spectroscopy system to check the presence of hydrated phases (Supplementary note S-2.2) and determine the composition of matrix materials (Table S-1)
Bench Crater provides critical constraints for these exploration activities that aim to identify the sources of different volatile species with depth and time
Summary
A key driver of future lunar scientific exploration is to determine the source(s) of water ice and volatiles found on the Moon’s surface at the present day (Li et al, 2018), and to understand past origins of water and volatiles in the Earth-Moon system (NRC, School of Natural Sciences, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK.2007). Potential caches of water and other volatiles at the lunar surface are being targeted by spacecraft missions for both scientific and exploration objectives (e.g., Carpenter et al, 2014; Colaprete et al, 2016; Barber et al, 2017). Such volatile reservoirs at the lunar surface are of key astrobiological interest (Cockell, 2010; Martins et al, 2013) and may prove to be useful for in situ resource utilisation (Anand et al, 2012) for the generation of lunar surface exploration. The origin of these different reservoirs is poorly constrained: they are likely a mixture of both indigenous and exogenous sources (e.g., McCubbin et al, 2015 and references therein; Barnes et al, 2016)
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