Friday, October 7, 2011

LROC: At the top of an avalanche in Langrenus

Bedrock benches near the top of an avalanche on the central peak of Langrenus (8.79°S, 61.22°E). LROC Narrow Angle Camera (NAC) observation M139504224L, LRO orbit 5692, September 19, 2010; downslope is to lower left, image field of view is ~302 meter. From the full size LROC Featured Image HERE [NASA/GSFC/Arizona State University].

James Ashley
LROC News System

Do these "stair steps" represent true rock layers that somehow survived the compression and uplift which accompanied the formation of this central peak, or are they expressions of jointing in the fractured bedrock? Many examples of lava flow layering have been identified within mare deposits across the Moon (see related Featured Image post links below), but these have been as exposures in crater walls or the sides of vertical pits within the mare. Rock layering in a central peak may be more difficult to see.

The physics of impact cratering describes the way a rock may respond to high-energy shockwave formation and decay. The central peak of a complex crater represents material that had once been deep beneath the lunar surface, but was later exposed by the rebound that followed compression during the impact event. The phenomenon of central peak formation in rock targets is often likened to the way a droplet of liquid creates a splash in a saucer. It represents a kind of snapshot of the splash phenomenon that results from impact into a target with an appreciable gravity field.

While considerable deformation of target layers is expected, vestiges of that layering can survive impact and become preserved in the central peak. The features in today's image are exposed by erosion caused by landslide activity, and reveal the bedrock that once lay underneath (see context image below).

The wider view of the NAC frame shows granular material which flowed down (toward the lower left) the steep central peak wall and away from the featured bedrock in Langrenus. From the full width of LROC NAC M139504224L, image field of view is ~2.0 kilometers wide. View the full size LROC context image HERE [NASA/GSFC/Arizona State University].
The central peak of Langrenus and surrounding crater floor shows the location of the ridge and slope highlighted in the LROC Featured Image. From LROC Wide Angle Camera (WAC) observation M131249971CE (566 nm Band), LRO orbit 4475, June 15, 2010; from an altitude of 38 km, resolution 55 meters per pixel, illumination from the east with an incidence angle of 76.63° [NASA/GSFC/Arizona State University].
From an early local morning illumination to late afternoon brings the highlighted slope out of shadows though unfortunately from an orbital pass at higher altitude. LROC WAC observation M134788704CE (566 nm Band), LRO orbit 4996, July 26, 2010; from an altitude of 50 kms with a resolution of 70 meters per pixel, illumination from the west with an incidence angle of 64.48° [NASA/GSFC/Arizona State University].
On the Moon, rock jointing can be caused by tidal stresses working over time to weaken the rocks. Once jointed, the fragments are more easily eroded, and tend to break away in blocks - which may or may not give the appearance of true stratigraphic layering. Because the energy of impact is expected to at least partially destroy whatever fine structure might have been in pristine layers, it may be difficult to differentiate the two types of stair step benches in an outcrop. The WAC mosaic shows the central peak and Langrenus crater, with the Featured Image location indicated as the red square.

From the LROC Quickmap WAC mosaic shows Langrenus crater with its greater anatomy, field of view is roughly 180 km [NASA/GSFC/Arizona State University].
Langrenus at its best when seen through an earth-bound telescope, in this case a Celestron C14 XLT under the steady guidance of Mario Wegand (, who swept up this view January 12, 2009 (Central European Time) from Offenbach am Main, Germany.
Take a look at the full NAC image.

There are many details visible in the Langrenus crater floor deposits that warrant further scrutiny. What other clues might aid in determining whether this apparent laying is the result of lava flows or of rock jointing? Related Featured Image posts include Layering in Euler Crater, Layering in Messier A, and Rock Avalanche in Robinson Crater.

Langrenus stands out at the center of the bow, half way between limb and terminator, in this three-and-a-half-day-old Crescent Moon. From a 5000 pixel deep mosaic assembled by Mario Weigand, captured the early evening of January 29, 2009.

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