On the floor of Green M

High-reflectance boulders are interspersed among rubble eroding out of impact melt deposited on the Green M crater floor. Illumination from right, image is 570 m across, LROC NAC M159080552L [NASA/GSFC/Arizona State University].

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 schoolbuses and the length of three schoolbuses. 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.

LROC WAC monochrome mosaic of Green M. Illumination is from the left and the asterisk (0.426°N, 133.236°E) notes the location of the opening image [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?

Related Posts: Impact melt in Anaxagoras crater
Ejecta in Tycho crater
The Chicken or the Egg?

Published by Lillian Ostrach on 7 December 2011