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.

Stratigraphy of Ice and Ejecta Deposits at the Lunar Poles

“…The model captures four major features: (1) ice delivered by hydrated asteroids and volcanic outgassing; (2) ejecta emplaced by the large craters that we age‐dated; (3) a simplified estimate of ice loss; and (4) a proxy for smaller‐scale gardening. Our model is dissimilar to those explored by Cannon et al (2020), Costello et al (2020), or Hurley et al (2012), which operated at much smaller scales.…”

“…Impact cratering has been the dominant process affecting the lunar poles, but the effects of large polar craters on nearby ice deposits have not been previously addressed. Impact effects have been considered for micrometeoroids (e.g., Farrell et al, 2019) and small impactors (Cannon & Britt, 2020; Costello et al, 2020; Crider & Vondrak, 2003; Hurley et al, 2012). The large polar craters (>20 km diameter) each formed at a distinct point in lunar history (Deutsch et al, 2020; Tye et al, 2015), and during crater formation, these impacts would have emplaced ejecta out to significant distances (e.g., McGetchin et al, 1973), affecting existing ice deposits in the surrounding terrains.…”

Impact Gardening on Ceres

“…Gardening by primary impacts falls short of the apparent mixing of space‐weathering products by orders of magnitude (Costello et al., 2018; Gault et al., 1974). In contrast, gardening by lunar secondary impacts represented by the Moon saturation contour has been shown to reproduce the depth of impact gardening on the Moon as evidenced by the distribution of surface‐correlated space weathering produces in the Apollo cores (Costello et al., 2020). The saturation contour shows that secondary impacts on the Moon thoroughly mix the top 1 m of lunar regolith over 1 Ga. On Ceres, the high secondary production model predicts that only the top 1 mm has been thoroughly reworked in 1 Ga.…”

“…Equilibrium gardening describes gardening where 2% of the points in a plane at the gardening depth have been inside the ejected volume of an impact crater at least once and can be used to understand less complete mixing or capture the mixing effects of impacts beyond the excavation zone. Modeled saturation gardening on the Moon has reproduced the homogenous depth‐distribution of cosmogenic radionuclides and space weathering products observed in the Apollo cores (Morris, 1978), and equilibrium gardening has reproduced the rate at which density anomalous cold spots and albedo anomalous rays fade into background regolith (Costello et al., 2020; Hawke et al., 2004; Williams et al., 2018). On Ceres, saturation gardening represents thorough mixing of surface material, while equilibrium gardening represents degradation by impact effects, but not necessarily homogenous vertical mixing.…”

“…Gardening data plotted in figures can be reproduced using the equations provided in the text and values which can be found in Costello et al. (2020).…”