Symmetry in Asymmetry

A beautiful example of an asymmetric impact feature (27.674°S; 125.465°E). LROC Narrow Angle Camera (NAC) frame M110771566R, LRO orbit 1458, October 21, 2009; illumination angle of incidence 31.51° from the northeast, image field of view roughly 1.2 km across, resolution 63 cm per pixel from 60.63 km [NASA/GSFC/Arizona State University].

James Ashley
LROC News System

While the total energy of an impact depends on the projectile velocity and mass, low-angle (oblique) impacts can distribute this energy in ways that differ from that of a higher angle trajectory.

By definition oblique impacts strike at an angle of 15° or less, producing something more akin to a 'glancing blow' rather than a 'hard smack,' and often result in asymmetrical 'wing-shaped' ejecta patterns.

Based on the ejecta surrounding this small feature in Neujmin crater in the lunar farside highlands, a case can be made for a southwestern approach from the impacting object. Debris was tossed in the down-track direction and splayed at right angles to the flight path on either side. The ejecta is actually quite symmetrical with respect to this flight axis (axial symmetry). The notion of asymmetry really applies to rotational symmetry in the case of many oblique impacts.

One of three dark halo craters (DHC) on the floor of Deutsch crater and recently proposed as one of many newly surveyed potential landing sites picked to fulfill high-priority science goals. A 4-km-wide field of view from LROC NAC observation M185169501R,  spacecraft orbit 12373, February 29, 2012; angle of incidence 29.7° resolution 1.46 meters per pixel from 148.15 km [NASA/GSFC/Arizona State University].

Smaller impacts created markings on the ejecta blanket, and these events excavated through the high-reflectance ejecta bringing up lower reflectance, mature materials -- producing dark-haloed craters. These small dark halo craters likely formed seconds after the high reflectance material was emplaced as slower, larger pieces of ejecta landed.

LROC Wide Angle Camera (WAC) GLD100 mosaic of the ancient, possibly highly "disrupted" terrain in and around Neujmin crater, southeast of the more distinctive farside landmark Tsiolkovskiy crater, presented as context for the 'winged' crater in the LROC Featured Image, released August 6, 2013. It's location is marked by the arrow [NASA/GSFC/Arizona State University].

The full NAC frame can be explored HERE.

Additional examples of oblique impacts are available in the Featured Image browse gallery, including "Not Your Average Crater," "A Tiny, Glancing Blow," and "Crash or Coincidence?"