IMPACT MELT VOLUMES IN SIMPLE LUNAR CRATERS: CONSTRAINTS ON THEORETICAL MODELING

Impact melt is a typical characteristic of impact crater formation and is observed in both simple and complex craters on the Earth and other planets. Melt forms pools on the crater floor, flows beyond the crater rim, and coats the inner crater walls. Significant melt deposits have been assumed to only be associated with larger (multi-kilometer) simple and complex craters. However, we have observed pools of impact melt on the floors of craters as small as 200 m in the lunar highlands. Theoretical models of impact melt generation suggest that only small volumes would be produced in small craters and most of melt would be ejected from the crater interior; yet such melt is observed. This raised the question of whether the theoretical models provide valid order of magnitude estimates of impact melt. It is recognized that such models have, to date, been only weakly constrained by estimates of the volume of impact melt from terrestrial craters (which are themselves significantly eroded). To test these models, we have used Lunar Reconnaissance Orbiter images and laser altimeter data, to estimate the volume of impact melt in simple craters. Simple craters are used as their shape can easily and accurately modeled as a polynomial surface. A variable order polynomial function is fit to the exposed crater wall and that function represents the original crater shape and volume. The present volume is subtracted from the modeled original volume to indicate the amount of melt. An axi-symmetric geometry is assumed. These estimates, in turn, can be used to validate the theoretical models and also to explore possible variations in the amount of melt produced as a function of target characteristics (mare vs. highlands) and impact energy (crater size). The lunar data show that the calculated volumes are typically less than the theoretical volumes, although the data have significant scatter. There is some suggestion that craters formed on the mare have greater melt volume than those formed in the highlands. The estimates represent minimum values as any melt that is ejected and melt mixed with the subfloor breccia are not included.