Occurrence and mechanisms of impact melt emplacement at small lunar craters

•Impact melt at small lunar craters can experience gravity-driven flow and ponding.•Melt occurrence correlates with crater diameter, target porosity and impact velocity.•Melt deposits occur at twice as many non-mare craters as at comparable mare craters.•Pre-existing topography and degradation affect impact melt distribution.•More small craters produce and retain impact melt than were previously recognized. Using observations from the Lunar Reconnaissance Orbiter Camera (LROC), we assess the frequency and occurrence of impact melt at simple craters less than 5km in diameter. Nine-hundred-and-fifty fresh, randomly distributed impact craters were identified for study based on their maturity, albedo, and preservation state. The occurrence, frequency, and distribution of impact melt deposits associated with these craters, particularly ponded melt and lobate flows, are diagnostic of melt emplacement mechanisms. Like larger craters, those smaller than a few kilometers in diameter often exhibit ponded melt on the crater floor as well as lobate flows near the crater rim crest. The morphologies of these deposits suggest gravity-driven flow while the melt was molten. Impact melt deposits emplaced as veneers and “sprays”, thin layers of ejecta that drape other crater materials, indicate deposition late in the cratering process; the deposits of fine sprays are particularly sensitive to degradation. Exterior melt deposits found near the rims of a few dozen craters are distributed asymmetrically around the crater and are rare at craters less than 2km in diameter. Pre-existing topography plays a role in the occurrence and distribution of these melt deposits, particularly for craters smaller than 1km in diameter, but does not account for all observed asymmetries in impact melt distribution. The observed relative abundance and frequency of ponded melt and flows in and around simple lunar craters increases with crater diameter, as was previously predicted from models. However, impact melt deposits are found more commonly at simple lunar craters (i.e., those less than a few kilometers in diameter) than previously expected. Ponded melt deposits are observed in roughly 15% of fresh craters smaller than 300m in diameter and 80% of fresh craters between 600m and 5km in diameter. Furthermore, melt deposits are observed at roughly twice as many non-mare craters than at mare craters. We infer that the distributions and occurrences of impact melt are strongly influenced by impact ve