LROC: "The Chicken or the Egg"

Building-sized boulders partially coated in impact melt on the southeast rim of Klute W (38.0°N, 216.7°E), from LROC Narrow Angle Camera (NAC) frame M140840176R, LRO Orbit 5889, October 4, 2010, incidence angle is 39.6° and a field of view width of 540 meters from 63.03 km. View the full size LROC Featured Image HERE [NASA/GSFC/Arizona State University].

James Ashley
LROC News System

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, and not a pool; image field of view is 2.7 km, white square shows area within the Featured Image. See the full size context image HERE [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.

A high degree of granularity is compromised when viewing the contact zone between Klute W and wider, much older Klute (lower right) using the 250 meter resolution LROC WAC Digital Terrain Model (DTM) (available through the LROC QuickMap) at 64 meter per pixel close-up. The smaller area in the Featured Image is just northeast of the 4855 meter elevation mark. Still, the range of color contrast hints at Klute W superimposition on the northwest rim of Klute [NASA/GSFC/Arizona State University].

LROC Wide Angle Camera (WAC) mosaic situated on the USGS/JAXA digital elevation model (DEM) installed in the Google Earth application. The blur rectangle shows the general area within LROC NAC frame M140840176R. Mosaic of five sequential observation opportunities August 10-11, 2010, LRO orbits 5192-5196; average incidence angle 61.5° from 62 km [NASA/GSFC/Arizona State University].

LROC WAC monochrome mosaic of Klute W and its neighboring 100 kilometer-wide environment. Red square indicates Featured Image location . View the full size LROC context image HERE [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 and here.

Related posts:

Impact melt on Klute W wall
Post-impact modification of Klute W