IMPACT CRATER MELT RHEOLOGY

Impact melt is observed over a range of impact crater sizes on the Moon and other silicate bodies. It occurs as the floor filling of basins and large craters, as ponds and flows along terrace blocks, as isolated ponds, and as flows. Quantification of the volume and rheologic properties of the melt res is critical to understanding the details of impact cratering energy partition, excavation, and ejecta emplacement As impact-melt is a molten silicate systems, its rheology can be understood in a context similar to that of lava flows and modeling of lava flow rheology can be used as a basis to study impact melt flows. Impact melt is initially deposited on the rim and begins to flow downslope. It can continue downslope as a broad sheet flow or coalesce into a well-defined flow. Multiple pulses are indicated by numerous overlapping lobes. Individual lobes have sufficient momentum to ride up and over topographic obstacles and to bulldoze boulders. The presence of melt flows around simple craters demonstrates that its ejection from the interior does not require a mechanism associated with a central uplift. Cooling of the melt occurs by radiation; ingestion debris can significantly influence cooling. Impact melt flows exhibit a range of morphologies and a range of rheologic properties. Yield strengths vary over three orders of magnitude from 10 to 10000 Pa. Modeled yield strengths overlap calculated and measured yield strengths of basaltic lavas at Kilauea and Mauna Loa. The lunar values are also consistent with previous estimates of impact melt rheology. Modeled yield strengths do not correlate with crater diameter or with target type. Yield strengths do correlate with flow morphology. Low yield-strength values are associated with the thin coatings. The high values are associated with large, bulbous morphology of some flows (e.g., the north flank of Tycho). Lunar impact melt flows have rheologies similar to that of terrestrial basalts, however yield strengths range over three orders of magnitude. The variations likely reflect small-scale heterogeneities the amount of melting of target rocks, the initial temperature of the melt and the impact conditions (e.g., impact angle).