The History of Lunar Polar Ice Accumulation

The lunar polar regions harbor water ice detected directly and indirectly by remotely sensed data. The deposits are concentrated within perennially shadowed regions where water molecules are thermally stable. Owing to the gradual decrease in the lunar spin axis obliquity with time, these regions have grown in extent monotonically since the passage of the Moon through the Cassini State Transition ~4 Ga ago. Combining spectral observations of exposed ice with the theoretical predictions of regions of ice stability constrains the history of accumulation. Using reflected ultraviolet starlight, observations from LAMP exhibit a strong correlation between exposed ice fraction and the age of permanent shadow within which it resides.  This is the first such age relationship that has been identified for the lunar ice, and it indicates ice has accumulated quasi-continuously at least over the last ~1.5 Ga. The exposed ice area ratio of ~5% in the youngest PSRs that have been in shadow over the last ~100 Ma suggests that regolith gardening effectively balances the source and loss rates such that a 1 m layer at least partly equilibrates with the surface over that relatively short timescale.

Finally, we developed a simple model that simulates the sources, sinks, and mixing within the lunar regolith, and show that this model can successfully account for the observations. The model thus provides constraints on the physical parameters and characteristic timescales of the relevant processes.

Figure 1: The LAMP identifications of exposed ice (red, Hayne et al., 2015) superimposed on the PSR ages (blue-white, Schörghofer & Rufu, 2023) with shaded relief in the background.

Figure 2: Cartoon illustrating the simplified regolith gardening and ice accumulation processes our model assumes. At the surface, there is a time-dependent source and a proportional loss term. The domain is assumed to be mixed by a diffusive process.