|This fresh crater in the north-central Oceanus Procellarum basin has a "bench" along its crater wall, and boulders are strewn among its ejecta blanket. What does this crater tell us about the local geology? LROC Narrow Anle Camera (NAC) observation M160363812RE, orbit 8767, May 18, 2011, incidence angle after local sunrisei was 57.87° from the southeast; image field of view is 500 meters with an original resolution of 47.8 centimeters per pixel from 40 kilometers. View the original 1000 x 1000 pixel LROC Featured Image HERE [NASA/GSFC/Arizona State University].|
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Bench craters form in terrains where two layers exist with substantially different strengths. On the Moon this is normally interpreted as a loose regolith covering a more cohesive bedrock. Because less energy is needed to penetrate the regolith than the bedrock, the crater develops a bench at the boundary between regolith and bedrock. Using this interpretation, we can estimate the depth of regolith. In the case of today's Featured Image we can interpret that a thin layer of regolith is covering the layered mare deposits within Oceanus Procellarum.
|LROC QuickMap mosaic view at 2 meters per pixel resolution shows the granularity of ejecta making up the relatively fresh bench crater's ray system, 30.24°N, 62.35°W [NASA/GSFC/Arizona State University].|
But how meaningful is a regolith thickness estimate from one crater? Regolith is created as small impacts churn up the top layer of a surface. As more and larger impacts occur, the regolith grows in thickness. However, impact events are not evenly distributed, and regolith thicknesses can vary in a small area. One way to more accurately determine the regolith thickness is to then document all the bench craters in a given area. From this data an isopach map can be made, showing the thickness of the regolith for that area!
What other types of simple craters can you find in the full NAC frame?
Bench Crater in Plato
Bouldery crater near Mare Australe