Remotely distinguishing and mapping endogenic water on the Moon
Dr Rachel Klima, Johns Hopkins University Applied Physics Laboratory, USA
Water and/or hydroxyl detected remotely on the lunar surface may originate from several sources: 1) comets and other exogenous debris; 2) solar-wind implantation; 3) the lunar interior. While each of these sources are interesting in their own right, distinguishing among them is critical for testing hypotheses for the origin and evolution of the Moon and our solar system. Existing spacecraft observations are not of high enough spatial and spectral resolution to uniquely characterise the bonding energies of the hydroxyl molecules that have been detected. Nevertheless, the spatial distribution and associations of H, OH-, or H2O with specific lunar lithologies provide some insight into the origin of hydrous materials. Further inferences can be made on the basis of laboratory measurements and computational models that have been developed to constrain the production of solar wind hydroxyl and the migration and stability of any OH- or H2O molecules on the surface. The current understanding, limitations, and future outlook for mapping and characterising water and hydroxyl on the lunar surface will be discussed.
History of water and other volatiles in the Moon and implications for the evolution of the inner Solar System
Dr Mahesh Anand, Open University, UK
Recent sample studies have demonstrated that the lunar interior likely contains appreciable quantities of water, although estimates for the bulk-water content of the Moon vary considerably. The origin and evolution of this water remain unresolved with a range of possibilities, including retention of primordial water to a later addition of water following the crystallisation of the putative Lunar Magma Ocean.
In addition to water, recent investigations focussing on other volatiles (e.g., C, N, Cl), have brought about a paradigm shift in our understanding of the history of lunar volatiles and their wider implications for volatiles in the inner Solar System.
We have been investigating the volatile inventory of the Moon through laboratory analyses of lunar samples. Our main target has been measuring volatiles, such as OH, F and Cl, in apatite using NanoSIMS. These studies are complemented by bulk analyses of lunar samples for their C, N and noble gas signatures using stepped-combustion.
Some highlights from our work carried out on lunar apatite include the revelation of significant effects of degassing on the initial magmatic H- and Cl-isotopic compositions, and the possibility of a common source of water in the Earth-Moon system. Whilst the dominant source for lunar water appears to be carbonaceous chondrite-type material, it is becoming apparent that the Moon may have received volatiles from multiple sources at different stages of its geological history. Unravelling this history will require further study of existing lunar samples, but also ‘new’ samples from regions of the Moon not sampled previously.
Hydrogen isotope ratios in the inner Solar System
Dr Lydia Hallis, University of Glasgow, UK
Based on meteorite measurements, the ratio of hydrogen (1H) to deuterium (2H or D) differs for each type of rocky body in the inner Solar System. In astrophysical environments water is D-rich, but exchange with D-poor H2 gas can decrease the D/H ratio. Thus, water close to the Sun, where higher temperatures drive ion exchange, has low D/H ratios relative to water further away from the Sun, where colder temperatures mean ion exchange is sluggish. Therefore, a meteorites D/H ratio can indicate how and where its parent body formed in the Solar System. For example, could Earth have formed from carbonaceous chondrites? However, hydrogens volatility and incompatibility in silicate melts means that separate water and hydrogen reservoirs can form on the same planetary bodies (e.g., surface vs. interior), with processes such as evaporation, sublimation, atmospheric stripping, impact shock events, aqueous alteration and plate tectonic recycling (on Earth) all affecting the D/H ratio of different reservoirs and/or individual meteorites over geological time. For example, water on the surface of Mars and Earth appears to be more D-rich than water from the interior of these planets. Therefore, if D/H ratios are to tell us anything about the Solar System’s formation it is important to unravel the processes each meteorite has experienced, and how these processes are likely to have affected the D/H ratio.
Global water cycle and the coevolution of Earth’s interior and surface environment
Professor Jun Korenga, Yale University, USA
The bulk Earth composition contains only less than 1% of water, but this trace amount of water can affect the long-term evolution of the Earth in a number of different ways. The foremost issue is the occurrence of plate tectonics, which governs almost all aspects of the Earth system, and the presence of water could both promote and prevent the operation of plate tectonics, depending on where water resides. Global water cycle, which circulates surface water into the deep mantle and back to the surface again, is thus suggested to have played a critical role in the Earth history. In this contribution, we first review the present-day water cycle and discuss its uncertainty as well as its secular variation. The evolution of mantle dynamics and surface environment are linked through global water cycle, and we explore a range of co-evolutionary scenarios to take into account various uncertainties. We survey relevant geological and geochemical observations to distinguish between different possibilities. The initiation of plate tectonics in the early Earth is suggested to be possible with abundant surface water and dry mantle, and the gradual introduction of water into the mantle by subduction could potentially explain a major geological transition from the Archean to the Proterozoic and even the uprise of oxygen in the early Proterozoic.