Slumping rim of Darwin C

Illumination from a high angle of incidence (83°) accentuates the slumping rim of Darwin C. The parallel fractures along the crater rim are slump blocks pulling away from the rim toward the interior of the crater, in shadow at lower right. LROC Narrow Angle Camera (NAC) observation M148624404R; LRO orbit 7036, January 2, 2011. Frame field of view is 720 meters across. See the full-size LROC Featured Image HERE [NASA/GSFC/Arizona State University].

Lillian Ostrach
LROC News System

Darwin C (20.5°S, 288.9°E) is one of several satellite craters associated with the crater Darwin. Compared to its sister satellite craters, this one is less degraded. However, the rim of Darwin C provides an excellent example of post-impact modification of a crater rim. Previously, a Featured Image highlighting Klute W explored crater degradation, specifically focusing on the fracturing and slumping of the crater rim. Similar to Klute W, the rim of Darwin C has experienced slope retreat and slumping as observed in the opening image. However, based on the LROC WAC context image below, Darwin C has not experienced as much slumping as Klute W.

The Lunar Reconnaissance Orbiter has orbited the Moon since June 2009, long enough to catalog NAC views of many areas more than once, including overlapping observations of the area highlighted in the LROC Featured Image above. Six thousand orbits ago, on December 2, 2009, LRO captured the slumped rim of Darwin C under very different lighting conditions. While the earlier image does show us many of the details shadowed in the image above, the area in evening shadows provides topographic detail [NASA/GSFC/Arizona State University].

LROC Wide Angle Camera (WAC) monochrome mosaic of Darwin and associated satellite craters. The mosaic is centered on Darwin C and an asterisk marks location of opening image. See the full-size LROC WAC mosaic context image HERE [NASA/GSFC/Arizona State University].

The slumping and downslope movement of material from a crater rim increases the size of the crater. This type of post-impact modification is important to note because scientists use crater counting to determine the relative ages of different units, since older surfaces will have more craters than younger surfaces, in addition to figuring out where the unit belongs in the lunar timescale. In crater counting, the specific diameter of the craters is very important because complex mathematical equations rely on the crater size and density (how many craters of a certain size there are in a given area) to derive a model absolute age. In addition to having more craters, older surfaces have more large craters than younger surfaces. So, if crater diameters substantially increase due to post-impact modification, the model age derived from crater counts may be anomalously old.

Explore the slumping rim of Darwin C in the full LROC NAC image!

For more information on crater counting methods and the mathematical equations used to derive model absolute ages, please download the Neukum, et al. (2001) paper.

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Post-impact modification of Klute W
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Copernicus Crater and the Lunar Timescale