Impact Gardening as a Constraint on the Age, Source, and Evolution of Ice on Mercury and the Moon
E. S. Costello, R. R. Ghent, Masatoshi Hirabayashi, P. G. Lucey
Abstract
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.