Jessica J. Barnes

An asteroidal origin for water in the Moon

The Apollo-derived tenet of an anhydrous Moon has been contested following measurement of water in several lunar samples that require water to be present in the lunar interior. However, significant uncertainties exist regarding the flux, sources and timing of water delivery to the Moon. Here we address those fundamental issues by constraining the mass of water accreted to the Moon and modelling the relative proportions of asteroidal and cometary sources for water that are consistent with measured isotopic compositions of lunar samples. We determine that a combination of carbonaceous chondrite-type materials were responsible for the majority of water (and nitrogen) delivered to the Earth–Moon system. Crucially, we conclude that comets containing water enriched in deuterium contributed significantly <20% of the water in the Moon. Therefore, our work places important constraints on the types of objects impacting the Moon ∼4.5–4.3 billion years ago and on the origin of water in the inner Solar System.

Understanding the origin and evolution of water in the Moon through lunar sample studies

A paradigm shift has recently occurred in our knowledge and understanding of water in the lunar interior. This has transpired principally through continued analysis of returned lunar samples using modern analytical instrumentation. While these recent studies have undoubtedly measured indigenous water in lunar samples they have also highlighted our current limitations and some future challenges that need to be overcome in order to fully understand the origin, distribution and evolution of water in the lunar interior. Another exciting recent development in the field of lunar science has been the unambiguous detection of water or water ice on the surface of the Moon through instruments flown on a number of orbiting spacecraft missions. Considered together, sample-based studies and those from orbit strongly suggest that the Moon is not an anhydrous planetary body, as previously believed. New observations and measurements support the possibility of a wet lunar interior and the presence of distinct reservoirs of water on the lunar surface. Furthermore, an approach combining measurements of water abundance in lunar samples and its hydrogen isotopic composition has proved to be of vital importance to fingerprint and elucidate processes and source(s) involved in giving rise to the lunar water inventory. A number of sources are likely to have contributed to the water inventory of the Moon ranging from primordial water to meteorite-derived water ice through to the water formed during the reaction of solar wind hydrogen with the lunar soil. Perhaps two of the most striking findings from these recent studies are the revelation that at least some portions of the lunar interior are as water-rich as some Mid-Ocean Ridge Basalt source regions on Earth and that the water in the Earth and the Moon probably share a common origin.

Investigating the History of Magmatic Volatiles in the Moon Using NanoSIMS

For decades the Moon was considered an anhydrous planetary body, significantly depleted in volatiles, including water, compared to the Earth [1]. It wasn’t until recently that water (H2O equivalent) and other volatiles were confidently measured in a variety of lunar samples (e.g., [2-5]), particularly in lunar volcanic glass beads, melt inclusions, and apatite [Ca5(PO4)3(F,Cl,OH)] from samples returned by the Apollo missions. The Earth and Moon are isotopic twins in many respects (e.g., [6-7]), however, they have very different chlorine isotopic compositions [8-9]. [The chlorine isotopic composition is reported in permil using a delta notation where: δCl (‰) = (Cl/Cl sample/ Cl/Cl standard)-1 × 1000, where the standard ratio is relative to standard mean ocean chloride (SMOC, Cl/Cl ratio = 0.31977])]. When compared to terrestrial rocks and chondritic (carbonaceous and ordinary) meteorites (δCl ~0 ‰, e.g., [10]), lunar samples show highly variable chlorine isotopic compositions ranging from ~ -4 to +24 ‰ [8-9]. The cause(s) for this heavy chlorine isotopic signature is most likely related to magmatic degassing (loss) of light Cl in metal chlorides, a process that would enrich the remaining melt in Cl [8]. However it remains unclear at what stages(s) in the Moon’s history such fractionation(s) may have occurred.