IMPACT CRATER: NEW INSIGHTS FROM THE MOON

Impact cratering is a primary geologic process on any solid planetary surface. Study of the cratering process is hampered by the lack of well-preserved terrestrial field examples. While images of other planetary surfaces provide numerous examples, they are viewed only from above and under variable lighting and resolution. The Lunar Reconnaissance Orbiter provides a variety of data that allow greater insight into the details of the impact cratering process. High-resolution imaging (0.5 m/px), thermal data, and altimetry allow details of the morphology and morphometry to be examined.

Images of impact craters formed during the last few years show that ejecta and secondary craters extend over much larger areas and extend for greater distances than previously assumed. Study of ejecta and impact melt at craters such as Giordano Bruno provides a clear temporal sequence of emplacement of clastic ejecta and impact melt. Relatively fine-grained (< 1 m) ejecta is deposited in a proximal deposit beyond the crater rim with variable surface morphology, then bouldery debris is deposited immediately beyond the rim, finally impact melt is deposited embaying and burying the earlier deposits. Some ejecta deposits display distinct flow-like morphology with overlapping lobes. The presence of self-secondary craters on the proximal ejecta is indicative of debris launched into near-vertical trajectories at relatively high speed (but less than the escape velocity). This debris remains aloft sufficiently long that ejecta and most of the melt are emplaced before that debris impacts the surface. The presence of melt sheets around the rim of GB (a simple crater lacking a central peak) demonstrates that significant melt can be emplaced beyond the rim without requiring a central peak phenomena to eject the melt. Impact melt not only forms coatings and ponds, but distinct lava-like flows with lobate margins and channels. The morphology and morphometry of the flows provide insight into the rheologic properties of melt. Altimetric data allow detailed morphometric data for crater depth, rim height, and ejecta thickness to better understand excavation and deposition changes as a function of size and target material.