A tale of two poles: Toward understanding the presence, distribution, and origin of volatiles at the polar regions of the Moon and Mercury

Abstract

The Moon and Mercury both have permanently shaded regions (PSRs) at their poles, which are locations that do not see the Sun for geologically long periods of time. The PSRs of the Moon and Mercury have very cold temperatures (<120 K) and as a consequence act as traps for volatile materials. Volatile enhancements have been detected and characterized at both planetary bodies, but the volatile concentrations at Mercury's poles are significantly larger than at the Moon's poles. This paper documents the study of PSR volatiles at the Moon and Mercury that has taken place over the past 60 years. Starting with speculative ideas in the 1950s and 1960s, the field of PSR volatiles has emerged into a thriving subfield of planetary science that has significant implications for scientific studies of the solar system, as well as future human exploration of the solar system. While much has been learned about PSRs and PSR volatiles, many foundational aspects of PSRs are still not understood. One of the most important unanswered questions is why the PSR volatile concentrations at the Moon and Mercury are so different. After describing the initial predictions and measurements of PSR volatiles, this paper documents a variety of PSR measurements, summarizes the current understanding of PSR volatiles, and then suggests what new measurements and studies are needed to answer many of the remaining open questions about PSR volatiles.

The Temporal and Geographic Extent of Seasonal Cold Trapping on the Moon

“…After decades of exploration, the preponderance of data from a host of remote sensing and in situ investigations point toward an enrichment of volatiles at polar latitudes of Earth's moon (Lawrence, , and references therein). Attempts to map the abundance and geographic distribution of volatiles, in particular H 2 O water ice and its derivatives, have been made using orbital neutron spectroscopy as well as ultraviolet and infrared (IR) reflectance measurements.…”

Unravelling the Mystery of Lunar Anomalous Craters Using Radar and Infrared Observations

“…Impact craters over the lunar polar regions, if permanently shaded from direct sunlight, can act as cold traps where the physical temperature remains extremely low (for instance, mean temperature 50 K in; Paige et al, ), allowing volatiles such as water ice to remain stable over geological timescales (e.g., Arnold, ; Lawrence, ; Lucey, ). Recent remote‐sensing observations have provided multiple lines of evidence for the presence of lunar surface water (H 2 O) and hydroxyl (OH − ) that could originate from solar wind implantation, impact delivery, and the interior of the Moon (e.g., Li & Milliken, ; Lucey et al, ; Pieters et al, ).…”

“…However, these surficial deposits are not volumetrically significant from the viewpoint of resource usage. Therefore, the detection of a large quantity of water ice delivered by water‐bearing debris or meteorites within the permanently shadowed regions (PSRs) near the lunar poles has been, and remains, a high‐priority challenge in lunar science (for a review, the reader is referred to; Lawrence, ). Recently, two orbital miniature synthetic aperture radars (India's Chandrayaan‐1 Mini‐SAR and NASA's Lunar Reconnaissance Orbiter (LRO) Miniature Radio Frequency [Mini‐RF]) found a class of anomalous craters with high circular polarization ratio (CPR) only in their interior regions, but not exterior to their rims (Spudis et al, ; Spudis et al, ).…”

Examining the Potential Contribution of the Hokusai Impact to Water Ice on Mercury

“…The relative purity of Mercury's water‐ice deposits, with less than about 5% silicates by volume (Butler et al, ), supports deposition over a short time interval (such as potentially by a single large impact) rather than a steady‐state process. Although evidence of water‐ice and other volatile enhancements has been found at the lunar poles (e.g., Colaprete et al, ; Fisher et al, ; Hayne et al, ; Schultz et al, ), the inferred concentrations of water ice are significantly lower than, and distribution of water‐ice patchier than, those at Mercury (e.g., Lawrence, ). The substantial differences between the water‐ice inventories observed in the polar regions of Mercury and the Moon could be explained by the lack of a recent, large, water‐delivering impact at the Moon.…”

“…Recent studies have suggested that additional water-ice reserves may exist in small-scale cold traps distributed across rough, intercrater terrain in the polar regions Neumann et al, 2017;Rubanenko et al, 2018). enhancements has been found at the lunar poles (e.g., Colaprete et al, 2010;Fisher et al, 2017;Hayne et al, 2015;Schultz et al, 2010), the inferred concentrations of water ice are significantly lower than, and distribution of water-ice patchier than, those at Mercury (e.g., Lawrence, 2017). The substantial differences between the water-ice inventories observed in the polar regions of Mercury and the Moon could be explained by the lack of a recent, large, water-delivering impact at the Moon.…”