Lobate Forms Around Craters on the Moon and Mercury: Origin From Landslides, Ejecta Flows and Modification Stage Collapse

Lobate forms emanate from impact craters and have morphologies indicating ground-hugging emplacement via flow. We present global surveys of lobate forms on the Moon and Mercury. Very few examples have previously been reported on either body, with debate over their interpretation as either crater rim landslides or ejecta flows. We identify 92 lobate forms on the Moon, and 98 on Mercury, a significant increase on previously reported examples. 98% extend downslope into an adjacent or overlapped antecedent crater. Lobate forms are directly linked to impact cratering processes: evidence for syn-impact formation, including impact melt on top of lobate forms, suggests ∼71% formed due to exterior collapse of source crater material into antecedent craters during source crater modification. 24% of examples instead appear to be landslides on antecedent crater rims, triggered by the newer impact. Understanding the formation of these features therefore provides new insight into impact cratering and mass movements processes. Equivalent lobe mobilities and occurrence rates on the volatile-depleted Moon and the somewhat volatile-enriched Mercury suggest volatiles are not necessary for formation. However, lobate form mobilities on the Moon and Mercury are lower than for comparable martian features, thought to require ground-ice to form, potentially supporting the idea that very high volatile content enables more mobile mass movements. Two lobate forms on flat ground (Hokusai and Piazzolla craters, Mercury) are morphologically similar to martian layered ejecta, possibly indicating a more significant influence of volatiles for these examples or a more enigmatic formation process.