Water Group Exospheres and Surface Interactions on the Moon, Mercury, and Ceres

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.

Variability of Hydration Across the Southern Hemisphere of the Moon as Observed by Deep Impact

“…The exact source and transport processes are unknown with the current understandings and hypotheses presented in Schörghofer et al. (2021). Spectrally, the hydration feature has been suggested to result from OH/H 2 O molecules that could range from ∼70 (Lin et al., 2022) to 1,000 ppm (Pieters et al., 2009), potentially consisting of a fraction of a monolayer of molecules on the lunar regolith (Hendrix et al., 2019).…”

Molecular Dynamics Simulation of Solar Wind Implantation in the Permanently Shadowed Regions on the Lunar Surface

“…This could potentially change the original view of volatile transport. Current volatile transport models mostly focused on thermal transport (see (Schörghofer et al., 2021) and reference therein). However, the dynamics of charged particles H 3 O + is controlled not only by the temperature but also by the electric and magnetic fields.…”

Particles and Photons as Drivers for Particle Release from the Surfaces of the Moon and Mercury

“…These photo-desorption, photo-destruction and photo-formation cross sections are relatively high and indicate that surficial water will not persist in Sun-lit regions of Mercury and the Moon unless a persistent source term exists. More discussion on the formation of H 2 O, its distribution and migration over the planet, and its contribution to the exosphere are given in (Schörghofer et al 2021).…”