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.Plain Language Summary Some craters near the south pole of the Moon contain permanently shadowed regions (PSRs) which stay cold enough to trap water vapor as ice. Recent studies have predicted that large amounts of ice could be buried under thick protective layers of lunar soil in the PSRs. Lunar soil is mainly transported by large impacts which launch soil and boulders to distances up to hundreds of kilometers. However, when these projectiles land they have destructive effects and may melt or redistribute buried ice. We simulated this process, called ballistic sedimentation, and predicted the amount of ice it removes. We also simulated ice and soil deposition over billions of years to test how much ice is lost to ballistic sedimentation over time. We predicted which PSRs are most likely to have ice near enough to the surface to detect in future missions. The upcoming Artemis program will send crewed and robotic missions to the lunar south pole region, and our work will help with planning where to land, what instruments to bring, and how much ice we might find.