|High-reflectance boulders interspersed among rubble eroding out from impact melt deposited on Green M crater floor. Illumination from right incidence angle 38.4° on a field of view 570 meters wide; LROC Narrow Angle Camera (NAC) observation M159080552L, orbit 8577, May 3, 2011 - resolution 0.62 meters per pixel from 60.09 kilometers. View the full size LROC Featured Image HERE [NASA/GSFC/Arizona State University].|
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Impact craters come in all sizes and morphologies due to different impacting conditions (e.g., impactor velocity, impactor material, angle of impact) and target materials (e.g., rock type, coherent or fragmented material). Green M (0.336°N, 133.117°E, southeast of Mendeleev crater) is a 35 km diameter crater with beautiful wall slumps, several terraces, and ponded impact melt deposits on the crater floor. In the opening image, both impact melt and fractured target material are visible. The two high-reflectance, fractured blocks are approximately 25 m wide and about 40 m in length; about the width of two school buses and the length of three school buses. These blocks are partially covered by a thin layer of impact melt or regolith, so it has been some time since these multiple-schoolbus-sized rocks last moved substantially (probably seconds after impact), unlike the likely more-recent rolling boulders observed elsewhere.
|Full 5.8 km width of both left and right frames of LROC NAC M159080552L, reduced one-third from 4 meters per pixel resolution as viewed using LROC QuickMap, HERE. Square shows approximate field of view seen in Featured Image further above[NASA/GSFC/Arizona State University].|
|Forty kilometer wide view of Green M, with the location of the Featured Image (0.426°N, 133.236°E) indicated by arrow. Range of elevations above global mean derived from LROC QuickMap. Image LROC Wide Angle Camera (WAC) M149648971CE (643 nm); orbit 7187, January 14, 2011. Illumination is from the west at an incidence angle of 72.33° Resolution 79.22 meters per pixel from 58.26 km [NASA/GSFC/Arizona State University].|
A thin layer of regolith does not completely obscure the irregular shallow depressions, or negative relief features, surrounding the high-reflectance boulders or the much smaller (<10 m) rocks eroding nearby. These depressions probably represent collapsed pockets in the impact melt sheet, similar to those observed in the King crater impact melt. However, enough time has passed to accumulate a layer of regolith that obscures what was originally a distinct rim, smoothing the surroundings while also enhancing small rock erosion - due to micrometeorite impacts - on the melt sheet over time.
What other interactions between target material and impact melt can you find in the full LROC NAC image? Can you find evidence for additional negative relief features or pits in the impact melt deposit?
Impact melt in Anaxagoras crater
Ejecta in Tycho crater
The Chicken or the Egg?