Current Theories of Lunar Ice
Norbert Schorghofer
TL;DR
The article surveys current theories of lunar ice, focusing on how polar cold traps, especially Permanently Shadowed Regions (PSRs), can host long-lived H$_2$O ice and how various storage, transport, and source mechanisms interact with observational constraints. It contrasts the classical (Watson/Murray/Brown) and modern (Feldman et al. and successors) views, highlighting key ideas such as surface cold traps, relic buried ice, subsurface cold-trapping via vapor pumping, exospheric transport, and multiple water sources, while evaluating measurements from LPNS/LEND, LCROSS, ShadowCam, and Diviner. The findings suggest the standard model remains broadly consistent with many constraints, predicting ice concentration trends that align with some measurements but not all, and it acknowledges challenges from hydrogen outside PSRs and regional heterogeneity that require definitive in-situ tests. The work emphasizes that future missions should target old PSRs and subsurface depths around the thermal skin depth to resolve unresolved questions about ice abundance, distribution, and the viability of subsurface cold-trapping and exospheric transport for broader lunar regions.
Abstract
The classical theory of cold-trapped ice on the Moon and some modern theories are reviewed and compared with observational constraints. The "standard model" for lunar ice posits that ice has accumulated in polar cold traps after the spin axis orientation became small enough for polar craters to be permanently shadowed. Its predictions are consistent with major observational constraints. Only a few less established observational claims are unaccounted for. The text focuses on fundamental theoretical concepts and assumes some pre-existing familiarity with the topic of lunar polar volatiles.