Friday, April 12, 2013

Crater on Crater Debris

Evidence of erosion and mass-wasting abound in lunar craters. Boulders and a variety of aggregate debris  intrude on the melt pool on the floor of a small crater which, in turn, is neatly nested on the rim of a larger and older crater in the Planck crater group on the southern far side (53.821°S, 139.639°E). Detail from LROC Featured Image released April 12, 2013. [NASA/GSFC/Arizona State University].
Mass-wasting abounds in lunar craters. LROC Featured Image, April 12, 2013; rescaled from LROC Narrow Angle Camera (NAC) mosaic  M18731958LR, LRO orbit 12673, March 25, 2012; original resolution 53 cm per pixel, angle of incidence 55.75° over a field of view approximately 810 meters across, from 51.01 kilometers [NASA/GSFC/Arizona State University].
Lillian Ostrach
LROC News System

The high resolution and stunning detail of LROC NAC images reveal evidence of recent erosion on the Moon, particularly within crater interiors. How do we know that the erosion is recent?

Boulder trails are one reason; micrometeorite bombardment of the lunar surface creates and churns up the regolith over millions of years resulting in erasure of surficial features. Absence of many superposed impact craters is another reason; over geologic time, craters accumulate on lunar surfaces and the absence of many superposed craters suggests that mass-wasting landslides in craters are young. Today's Featured Image highlights the southeastern portion of an unnamed 1.5 km diameter crater (53.819°S, 139.652°E) that shows evidence of mass-wasting.

Full 6.1 kilometers field of view covered by the footprint of the LROC NAC mosaic, source of the LROC Featured Image. From the full image browser exploration tool [NASA/GSFC/Arizona State University].
Just as on Earth, gravity promotes downhill movement and thus erosion of landforms. The crater floor (opening image, upper left) is a mix of pooled impact melt, fragmented blocks coated in melt, and boulders (>1 m diameter) that migrated downhill after the melt solidified. Piles of material (often called talus on Earth) are located at the change in slope between crater walls and floor. The piles of material have boulders of various sizes with smoother material in between. One might call this smoother material "fine-grained", but we are unable to quantitatively characterize the size distribution of the debris below the pixel size of the NAC images, here about 50 cm. Indeed, the term "fine-grained" is particular to a grouping of particle sizes when used in Earth-based sedimentary geology (1/16 to 1/256 mm).

LROC WAC monochrome mosaic centered on an unnamed crater on the farside that formed on the outside rim of another crater. Asterisk notes location of the crater in opening image [NASA/GSFC/Arizona State University].
Generally speaking, an object on a planetary surface is "detectable" when considering two or more pixels in a remotely sensed image. In today's NAC, two pixels represent 1.1 m on the lunar surface. However, simply because an object occupies two pixels does not mean that the object is "resolvable" so that scientists can confidentally determine the type of feature (is it a car or a boulder?). Thus, a good rule of thumb is to use at least three to five pixels when determining the true nature of a feature in an image. So, it is possible that the "fine-grained" material is not so "fine-grained" at all, and the material looks smooth because the individual particles are smaller than both a detectable and resolvable "grain".

The deepest material excavated by an impact crater usually ends up on its rim, which is also its highest elevation. Thus, as we see in this LROC Global Wide Angle Camera mosaic, the relatively fresh small crater (yellow arrow) is nested on the rim of a larger unnamed crater that is, itself in proximity to the rim of the ancient Planck impact and the rim of Planck C. It might be a good place to look for samples of the Moon originally excavated by Planck, if such samples are not shocked beyond usefulness. LROC Lunaserv tool [NASA/GSFC, Arizona State University].
Explore the erosive products in this crater for yourself in the full LROC NAC image, HERE.

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