Off-center impact on the wall of Guthnick

A small 600 meter crater inside the rim of Guthnick, a Copernican impact integral to the Mendlel-Rydberg basin immediately south of Mare Orientalis. This small impact crater exhibits boulders clustered off center, along with a poorly defined rim. Drew Enns asks, "what could be the cause of these distinctive features?" - Crop from LROC Narrow Angle Camera (NAC) M1117124706L, spacecraft orbit 16850, March 5, 2013; 0.60 meters per pixel resolution, above field of view 3 km [NASA/GSFC/Arizona State University].

Drew Enns
LROC News System

Small impact craters are normally bowl-shaped depressions in a planetary surface. Because of this, boulders and impact melt will also fill in the center of the crater. Yet this is not what we observe in today's Featured Image. Why does this small crater have boulders that are off center? Why is the northern portion of the rim undefined? Is it some sort of dynamical fluke? Probably not. It is more likely that there is some uneven terrain influencing the crater. We can zoom out for a larger view.

Asymmetric craters tend to form when the impact angle is greater than 15° The LROC WAC context mosaic helps a lot! We now see that our small crater formed on the wall of the much larger Guthnick crater.

LROC Wide Angle Camera (WAC) context for the small crater (arrow) on the wall of Guthnick crater at 48.27°S, 266.157°E. Though Guthnick is not the subject of this post, the 36 km crater has been identified as one of two that satisfy requirements for sampling intact basin melt sheets. (Science Concept 2: "The structure and composition of the lunar interior provide fundamental information on the evolution of a differentiated planetary body;" CLSE, 2012, pg 115) - LROC WAC observation M112231731CE (604nm), spacecraft orbit 1673, November 7, 2009; resolution 74.25 meters per pixel from 52.51 km [NASA/GSFC/Arizona State University].

The slope of the Guthnick crater's wall had a big effect on the morphology of this simple crater. During the impact event the steep slope resulted in collapse of the downhill portion of the crater, thus the asymmetric shape and collection of boulders on the downhill side.

Still image taken from HDTV feed from Japan's SELENE-1 (Kaguya) orbiting north over the Moon's west limb. The edge of Mare Orientalis has just appeared on the horizon and long chains of impact craters radiate from is central basin. Guthnick, on the right of the two largest craters at the center probably impacted upon one of the long chains, as much as two billion years after the Orientalis event [JAXA/NHK/SELENE]..

The wide-spread and lasting influence of the Orientale basin-forming-impact event can more easily be seen in this LROC WAC digital elevation model. Perhaps at one time the Mendel-Ryder basin, home of Guthnick (white arrow, lower right) though smaller, had an influence nearly as wide spread, wiped away - on the surface at least - 3.1 billion years ago [NASA/GSFC/Arizona State University/DLR].

Explore more of the Guthnick crater interior in a full resolution reproduction of the original LROC NAC, HERE.

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Figure 2.43 (A Global Lunar Landing Site Study to Provide the Scientific Context for Exploration of the Moon, 2012) Topographic profile of Guthnick. Black arrows indicate the transition from upper crater wall to slumped material, as shown by an inflection in the slope. The map uses a polar projection centered on 48°N, 266°E, and the vertical projection of the elevation profile is about 2:1 [CLSE/NLSI/LPI].