Buried Ice Deposits in Lunar Polar Cold Traps Were Disrupted by Ballistic Sedimentation

The NASA Artemis program will send humans to the lunar south polar region, in part to investigate the availability of water ice and other in situ resources. While trace amounts of ice have been detected at the surface of polar permanently shadowed regions (PSRs), recent studies suggest that large ice deposits could be stable below cold traps in the PSRs over geologic time. A recent study modeling the rate of ice delivery, ejecta deposition and ice loss from cold traps predicted that gigatons of ice could be buried below 100s of meters of crater ejecta and regolith. However, crater ejecta vigorously mix the target on impact through ballistic sedimentation, which may disrupt buried ice deposits. Here, we developed a thermal model to predict ice stability during ballistic sedimentation events. We then modeled cold trap ice and ejecta stratigraphy over geologic time using Monte Carlo methods. We found that ballistic sedimentation disrupted large ice deposits in most cases, dispersing them into smaller layers. Ice retention decreased in most cases, but varied significantly with the sequence of ejecta delivery, particularly from basin-forming events. Over many model runs, we found that south polar craters Amundsen, Cabeus, and Cabeus B were most likely to retain large deposits of ice at depths up to 100 m, shallow enough to be detectable with ground-penetrating radar. We discuss these findings in the context of the imminent human exploration activities at the lunar south pole.