Abstract
Water ice, abundant in the outer solar system, is volatile in the inner solar system. On the largest airless bodies of the inner solar system (Mercury, the Moon, Ceres), water can be an exospheric species but also occurs in its condensed form. Mercury hosts water ice deposits in permanently shadowed regions near its poles that act as cold traps. Water ice is also present on the Moon, where these polar deposits are of great interest in the context of future lunar exploration. The lunar surface releases either OH or H2O during meteoroid showers, and both of these species are generated by reaction of implanted solar wind protons with metal oxides in the regolith. A consequence of the ongoing interaction between the solar wind and the surface is a surficial hydroxyl population that has been observed on the Moon. Dwarf planet Ceres has enough gravity to have a gravitationally-bound water exosphere, and also has permanently shadowed regions near its poles, with bright ice deposits found in the most long-lived of its cold traps. Tantalizing evidence for cold trapped water ice and exospheres of molecular water has emerged, but even basic questions remain open. The relative and absolute magnitudes of sources of water on Mercury and the Moon remain largely unknown. Exospheres can transport water to cold traps, but the efficiency of this process remains uncertain. Here, the status of observations, theory, and laboratory measurements is reviewed.
Highlights
The abundance of water ice in permanently shadowed craters on the Moon is key to the future exploration of the lunar surface
Molecular hydrogen was first detected in the lunar exosphere by the Lyman Alpha Mapping Project (LAMP) UV spectrograph on Lunar Reconnaissance Orbiter (LRO) (Stern et al 2013), and later confirmed by the Chandrayaan-1 Altitudinal Composition Explorer (CHACE) mass spectrometer (Thampi et al 2015)
Lunar surface hydroxyl or water was assessed globally through analyzing the absorption features of OH and H2O near 3 μm seen by three different missions/instruments, namely Chandrayaan-1 Moon Mineralogy Mapper (M3) (Pieters et al 2009), EPOXI near infrared spectrometer (Sunshine et al 2009), and Cassini Visual and Infrared Mapping Spectrometer (VIMS) (Clark 2009)
Summary
The abundance of water ice in permanently shadowed craters on the Moon is key to the future exploration of the lunar surface. The underlying idea was formulated a long time ago (Watson et al 1961a,b): water from exogenic or endogenic sources is transported laterally through the exosphere and trapped in permanently shadowed regions (PSRs) near the lunar poles. No part of this process has yet been confirmed. A large asteroid that has PSRs, was visited by the Dawn spacecraft from 2015 to 2018 These three airless bodies of the inner solar system (Mercury, the Moon, and Ceres) each have (nearly) ice-free surfaces, PSRs, and (at least potentially) gravitationally-bound water exospheres. We review recent theoretical studies aimed at examining how exogenic hydrogen sources are partitioned within surficial and exospheric inventories of the lunar environment
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