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

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.