Lunar Reconnaissance Orbiter Camera

Symmetry in an Asymmetric Pattern?

A beautiful example of an asymmetric impact feature (27.674°S; 125.465°E). NAC frame M110771566R, illumination is from the northeast, north is up, image is ~1.2 km wide [NASA/GSFC/Arizona State University].

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' 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.

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.

The WAC mosaic context image is just over 109 km wide [NASA/GSFC/Arizona State University].

The full NAC frame can be explored below. Additional examples of oblique impacts are available in the Featured Image browse gallery, and include Not Your Average Crater, A Tiny, Glancing Blow, and Crash or Coincidence?.


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