Boulder on the Edge

A boulder on the edge of a crater rim, partially covered with a veneer of impact melt. LROC NAC M169738975L, image width is 600 m [NASA/GSFC/Arizona State University].

A boulder rests on the rim of an unnamed fresh crater, whose center is located at 66.615°S, 128.611°E. The boulder is partially covered in a layer of impact melt (dark smooth material). Small pieces of bare rock are visible in the impact melt as high reflectance dots in the relatively lower reflectance melt deposit. The boulder lies between fractured impact melt (outside the crater rim - left side of the image) and the interior rim of the crater (right side of the image). It appears that pieces of the main boulder broke off and tumbled down the interior crater wall leaving a trail of debris. Some of the impact melt deposit has also crumbled away and fallen towards the interior crater wall.

The sequence of events leading to this geologic scene probably progressed very quickly. First, the impact which formed the crater occurred, generating both the impact melt and the boulders. The boulders and the impact melt were emplaced almost simultaneously. Usually geologists use stratigraphic relationships to determine the order in which events took place, but in this case it is impossible to tell whether the boulder was emplaced first and then the impact melt, or vice versa. In the case where the boulder was emplaced second, it is easy to imagine the impact melt splashing or flowing to partially cover the boulder with a thin veneer of impact melt. 

LROC WAC 100 m/px context image of Grotrian crater. The white asterisk marks the location of the Featured Image [NASA/GSFC/Arizona State University].

Explore the entire NAC frame!

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Published by Sarah Braden on 10 May 2012