Secondary Impact Burial and Excavation Gardening on the Moon and the Depth to Ice in Permanent Shadow

Abstract

AbstractImpact gardening is a mixture of excavation by impacts and burial under continuous proximal ejecta. An existing analytical model describes the rate at which impacts excavate material on the Moon (Gault et al., 1974; Costello et al., 2018, https://doi.org/10.1016/j.icarus.2018.05.023; Costello et al., 2020, https://doi.org/10.1029/2019je006172). We expand the model to include a treatment of burial under proximal ejecta. Using the models for excavation and burial, we explore the effects of impacts in the evolution of the lunar surface over the last few billion years. We find that excavation of material by gardening outpaces burial in all reasonable ejecta coverage test scenarios. Thus, gardening does not act as a shield for ice in permanent shadow. However, gardening fails to eradicate the surface expression of compositional contrasts, such as those associated with pyroclastic deposits and compositional rays, which are not vulnerable to removal by thermal or ionization processes. Explorers seeking ice at the lunar poles should not expect regions of permanent shadow to have pure ice within the top 1–10 m because that ice will have been disrupted by gardening.