LROC: The lavish lobes of Necho R

What geologic process created these mounds (5.72°S, 122.09°E)? LROC Narrow Angle Camera (NAC) observation M106074338R, LRO orbit 779, August 28, 2009; Solar illumination incidence angle 35°, from the west (north is up), field of view roughly 600 meters. View the full-sized LROC Featured Image HERE [NASA/GSFC/Arizona State University].

James Ashley
LROC News System

As space enthusiasts and students of the Solar System, we all know that the tremendous energies unleashed by a hypervelocity impact vaporize much of the impactor, fracture the target subsurface, and send cubic kilometers of pulverized and melted ejecta sailing over the horizon. When conditions are right, however, some impacts also produce strange and counter-intuitive phenomena. Dry ejecta can sometimes behave like a fluid and travel as a ground-hugging debris flow great distances from the impact site. The exact physics of this ground-hugging flow behavior is not well understood, but several theories exist. The most popular of these employs the idea of acoustic energy waves keeping particles in motion -- bouncing off each other at just the right frequencies to keep everything suspended much longer than one might anticipate. With friction reduced by this resonant internal energy, the flow can move great distances before finally losing momentum to become part of a distal ejecta deposit.

On Earth, this type of debris flow is known as a sturzstrom, most often associated with avalanches. Evidence for similar behavior is observed on the Moon in the vicinity of relatively recent craters. The odd mounds shown here were caused by the Necho impact (see context image below), and are the result of such flows bunching up and overriding each other as they slowed and came to their final resting place in nearby Necho R crater upon reaching its floor and steep southwestern wall; hence the technical term "deceleration lobes" for these landforms.

A portion of a LROC Wide Angle Camera (WAC) monochrome (643 nm) mosaic swept during three consecutive opportunities (LRO orbits 2676-2678, January 25, 2010) showing the Necho crater region of the farside lunar highlands. See the original LROC WAC context image HERE [NASA/GSFC/Arizona State University].

A virtual view of the same LROC WAC mosaic as an overlay upon the lunar digital elevation model available in later versions of Google Earth. Both Necho and Necho R are situated on the north wall of the much older and larger Necho P, a fact which may somewhat explain the unusual slumping seen in Necho. The scene within the LROC Featured Image released July 21, 2011 is indicated by the small arrow [NASA/GSFC/Arizona State University/JAXA/Google].

Another example of deceleration lobes can be found in the King crater region to the north of Necho crater. Examine the full NAC image for an even more dramatic display of these features.