LROC: Outcrops in Laplace A

Debris flows and outcrops exposed in the walls of Laplace A crater, offset from Sinus Iridum. Illumination from southwest over a field of view (FOV) approximately 525 meters across, down-slope to the right. LROC Narrow Angle Camera (NAC) M137725771R, orbit 5430, August 29, 2010; incidence angle 52.91° with a resolution 0.52 meters per pixel from 49.72 kilometers. View the original LROC Featured Image HERE [NASA/GSFC/Arizona State University].

James Ashley
LROC News System

As with many mare deposit exposures when viewed at the Narrow Angle Camera imaging scale of 0.5 m/pixel, the bedrock outcrops in the walls of Laplace A appear as layered ledges. The layers are most noticeable in the north-northeast to south-southwest trending rocks visible in the northeast quadrant of the frame. Additional indications of layering can be found in the full NAC image. 

The prominent V-shaped outlines to some of the outcrops are suggestive of constructional deposition. Granular debris flows cascading down the crater walls following excavation appear to have encountered the obstructing ledges and "dammed up" behind them. Resistance to flow in such situations takes on a tapered shape in the direction of flow as the material becomes compacted. Minor occurrences of impact melt may be contributing to the smooth appearance of the flow deposits between the outcrops.

Featured Image FOV in context of the full 2.5 kilometers wide LROC NAC frame M137735771R [NASA/GSFC/Arizona State University].

The north wall of Laplace A is similarly characterized by the outcrops visible in very high resolution in the LROC NAC frame. The crater and surrounding Sinus Iridum were closely examined as a likely landing site for the Chang'e 3 mission by 2014 by China's lunar orbiter Chang'e 2. Nine kilometer-wide Laplace A (43.64° N, 333.33°E) is a familiar nearside feature because of its place in the largely "featureless" landscape along the frontier of the northwest Mare Imbrium and Sinus Iridum. The crater excavated Imbrium mare over the inundated "missing" southeastern outer ring of the Iridum impact zone. A rewarding 7000 pixel-wide, very detailed version of the image is available from tantaonews.com [CNSA/CLEP].

Nine-kilometer Laplace A is a familiar nearside feature in the mare where Sinus Iridum meets Mare Imbrium. If you have access to a small telescope, it is a challenging target for the eyepiece best viewed two days after First Quarter or a day after Last Quarter. Context information for the LROC NAC Featured Image and the Chang'e 2 study from LROC Wide Angle Camera (WAC) observations gathered during last August's low periapsis period (from an average 33.06 kilometers, at a resolution of 47.6 meters per pixel), orbits 9946 through 9949, August 19, 2011 [NASA/GSFC/Arizona State University].

A 95 km-wide FOV from the WAC monochrome (566 nm) mosaic collected over 4 orbital passes, August 29, 2011 [NASA/GSFC/Arizona State University].

Additional examples of both layering and debris flow in crater wall exposures can be found in the following Featured Image posts: Dawes, Pytheas, and Layering in Messier A.

The apparent isolation of Laplace A (center right) is misleading in this HDTV still of Sinus Iridum, embayed by the vast northwest floor of Mare Imbrium, though there is an authentic paucity of similarly-sized craters in the vicinity, unlike the lunar Highlands. HD video captured by Japan's lunar orbiter SELENE-1 (Kaguya) in 2009. A larger version of this frame can be viewed HERE [JAXA/NHK/SELENE].