Lunar Reconnaissance Orbiter Camera

The Chicken or the Egg?

Building-sized boulders are partially coated in impact melt. LROC NAC frame M140840176R, incidence angle is 39.6°, sun is from the south, image width is 540 meters [NASA/GSFC/Arizona State University].

Which arrived first, the impact melt or the boulders in this image? In other words, did these boulders arrive after melt had pooled, displacing the melt while still molten; or were they there in advance of the melt, which flowed around and partially over their southwest flanks? While there are no obvious signs of a 'splash' in this melt that might indicate the late arrival of the blocks, this question still seems like a bit of a puzzle at this image scale. Zooming out a little provides the information required to answer the timing question ...

This wider context image shows the melt as part of a shallow flow, not a pool; image width is 2.7 km, white square shows Featured Image area [NASA/GSFC/Arizona State University].

In the above image, we can see how the melt flowed down this slope and around the boulders, which just happened to be in its path. The flow occurs on the portion of the Klute W crater wall adjacent to a neighboring crater, and along a portion of the rim where extensive mass wasting and other complex adjustments have occurred following the Klute W impact event.

LROC WAC monochrome mosaic of Klute W and its neighboring environment. Red square indicates Featured Image location. Image is 100 km wide. [NASA/GSFC/Arizona State University].

The full NAC image shows abundant additional features of high geologic interest. Other examples of lunar impact melt occurrence and behavior can be found here, here, and here.

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