Impact Melt Deposit Morphology and Distribution on the Moon and Mars

Melt-bearing deposits associated with lunar craters were first identified as an impact product from Apollo era photography (e.g Dence, 1971) and occur as ponds, flows, and veneers on the crater floor, crater walls and on the near-rim ejecta (Howard and Wilshire, 1975). Lunar Reconnaissance Orbiter Camera (LROC) images are providing new details regarding small-scale impact melt related features. Similar scale images from the High Resolution Imaging Science Experiment (HiRISE) and the Context Imager (CTX) have revealed similar features such as ponds and viscous flows in and around martian impact craters (McEwen et al., 2007; Tornabene et al., 2007). The comparable scales between these spacecraft instruments allow a valuable comparison of impact-melt features within fresh and well-preserved craters on these two bodies. This study will help us to understand the formation, emplacement and modification of impact melts, and investigate the influence of target properties on the impact process. Melt pools on the Moon and Mars exhibit both fractured and smooth surfaces. In addition to this, martian impact melt-bearing bodies are often pitted, which has been suggested to be a consequence of water/ice in the target (Tornabene et al., 2007; Bray et al., 2009); a comparison with lunar analogs may help confirm or refute this theory. Thus far, pitting has not been observed in lunar melt-bearing bodies, suggesting that the pits in martian craters may be a consequence of the difference in target properties (volatile rich vs. volatile poor). Melt ponds on the floor of lunar craters floor also differ from martian ponds in that they have a more blocky surface of partially submerged blocks. This may be the result of crater rim collapse debris falling onto a still molten melt sheet during the modification stage of crater formation. Recognizing such characteristics within martian craters may be key to understanding erosion, deposition and other geologic processes on Mars that obscure such primary crater deposits over time. References: Dence, M. R. (1971). J. Geophys. Res. 76:5552-5565. Howard and Wilshire (1975). J. Geophys. Res. U. S. Geol. Survey, 3, no. 2:237-251. McEwen et al. 2007, Science 317. Tornabene et al., 2007 Mars 7 abs. 3288. Bray et al. (2009). 40th LPSC abs. 1389.