Abstract
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.
Highlights
The origin of the Moon remains a subject of considerable debate, as evidenced by emergence of several new ideas [1,2] as well as refinements of previously proposed models [3,4]
Lunar science is presently going through a renaissance period as evidenced by a surge in lunar exploration by several space agencies as well as renewed interest in laboratory-based analysis of lunar samples for their volatile inventories using the latest advancements in analytical instrumentation and techniques. These efforts have already resulted in several new discoveries about the Moon, one of which has been an unambiguous detection and quantification of water in lunar samples
Our understanding of complex magmatic processes influencing the volatile inventory of lunar samples is in its infancy
Summary
The origin of the Moon remains a subject of considerable debate, as evidenced by emergence of several new ideas [1,2] as well as refinements of previously proposed models [3,4]. It is a reasonable assumption that any inherited primordial water and other volatiles present originally in the Moon-forming material would have experienced significant loss, exchange and/or processing before being sequestered in the lunar interior [5] This primordial volatile inventory of the Moon has almost certainly been modified through planetary impacts during the geological history of the Moon spanning at least a period of more than 4 billion years. Most lunar samples in our collections have been available for over four decades, but it is only recently that technological advancements in modern analytical instrumentation have permitted unambiguous detection of water and other volatiles, and investigation of their isotopic characteristics, in lunar samples In this contribution, we provide a comprehensive review and current status of our knowledge and understanding of water in the Moon as a result of laboratory-based investigations of lunar samples. Current limitations and challenges in this area of research and discuss future prospects for improving our understanding of the history of water in the Earth–Moon system
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