Crater Debris

Evidence of erosion and mass-wasting abound in lunar craters! Crater rim at the bottom of the image, crater floor at upper left, NAC M18731958LR, image width ~810 m [NASA/GSFC/Arizona State University].

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.

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].

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).

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".

Explore the erosive products in this crater for yourself in the full LROC NAC image! 

Related Posts:
Physics is Fun!
Weaving boulder trails on the Moon
Sampling a Central Peak

Published by Lillian Ostrach on 12 April 2013