Impact Melt Emplacement on the Moon: New Results from Mini-RF on LRO

Flow-like deposits of impact melt are commonly observed on the Moon, typically around young fresh craters. These flows are thought to be mixtures of clasts and melted material that are emplaced during the late stages of impact crater formation. Lunar impact melts have been primarily studied at optical wavelengths, but complementary information can be obtained by observing impact melts at radar wavelengths. Radar data is sensitive to surface and subsurface roughness, and thus can highlight these features even when they are not easily seen in optical data due to burial or imperfect lighting conditions. Impact melts have been identified in radar data on the lunar near side, but they have yet to be studied in depth on the lunar far side, given the lack of global radar data prior to the launch of NASA's Mini-RF instrument on the Lunar Reconnaissance Orbiter (LRO). We have therefore undertaken a global mapping project to identify impact melt flows in the Mini-RF data set. We seek to use this data to determine how the emplacement of melt deposits is influenced by pre-existing topography and impact direction. We have thus far identified fourteen new impact melt flow candidates, ranging in diameter from 2.8 to 37 km, and confirmed these as impact melt in corresponding LRO NAC imagery. Of the known lunar impact melt flows (those reported here combined with those reported in the literature), over 90% of complex crater flows and 55% of simple crater flows have directions that are within 45° of rim crest low. In comparison, only 50% of complex crater flows and 25% of simple crater flows have directions that are within 45° of the downrange direction of impact. This suggests that for many of these melt flows, pre-existing topography plays an important role in final melt flow morphology. This is likely a result of movement during crater modification (complex craters) or a breached crater rim causing melt to escape and flow outwards (simple craters).