Impact Gardening as a Constraint on the Age, Source, and Evolution of Ice on Mercury and the Moon

Abstract

We update an analytic impact gardening model (Costello et al., 2018, https://doi.org/10.1016/j.icarus.2018.05.023) to calculate the depth gardened by impactors on the Moon and Mercury and assess the implications of our results for the age, extent, and source of water ice deposits on both planetary bodies. We show that if the water presently on the Moon has a primordial origin, it may have been 4-15 m thick. If ice deposits are buried, they may be as shallow as 3 cm or as deep as 10 m and provide a gradient of probability for ice gardened into a column. Our calculations for gardening on Mercury show that thermal lag deposits will be reworked into the background over 200 Myr, and, thus, the most recent large-scale deposition of ice on Mercury must have occurred no more than 200 Myr ago. We also find that gardening mixes incremental layers of ice with underlying regolith and prevents the growth of pure ice deposits by continuous supply. We conclude that ice deposits on the Moon and Mercury are likely the result of sudden and voluminous deposition. Plain Language Summary The Moon and Mercury have water ice in permanently shadowed regions at their poles; however, while Mercury's poles are rich in water ice, the Moon's are relatively sparse. Impact cratering mixes surface material, bringing buried ice upward to a relatively hostile surface in a process called impact gardening. We apply a revised impact gardening model to investigate the depth from which ice is mixed upward by impacts as a function of time and discover the following: (1) Ice on the Moon may have been mixed with underlying regolith or buried under many meters of dry regolith; (2) the most recent large-scale delivery of water to Mercury happened no more than 200 Myr ago; (3) what ice exists at the poles of the Moon and Mercury was probably delivered suddenly and voluminously; and (4) the differences between the abundance of water ice on the Moon and Mercury will only become more pronounced with time as impact gardening destroys surface ice on the Moon and only scratches the surface of Mercury's extensive deposits.