Clam Shell on the wall of Lalande C

An oblique impact crater on the wall of Lalande C crater. LROC Narrow Angle Camera (NAC) M170606751LR, LRO orbit 10276, September 14, 2011; incidence angle 11.82° at 49 cm per pixel resolution revealing a field of view 600 meters across, viewed from 44.83 km [NASA/GSFC/Arizona State University].

Lillian Ostrach
LROC News System

Oblique impact craters can form when the angle of impact is less than 15° from the horizontal or from impact into sloped terrain.

Oblique impact craters typically exhibit specific morphologies: asymmetric ejecta and non-circular (more elliptical) crater shapes. The approximately 220 meter impact crater in today's Featured Image (5.668°S, 353.148°E) formed on the interior crater wall of Lalande C and is a nice example of a small, oblique impact crater that formed due to target slope as opposed to impact angle.

The oblique crater in Lalande C does have an asymmetric ejecta blanket (see the full NAC image) and a poorly defined zone of avoidance. The crater shape is not circular; instead, the irregular crater shape is reminiscent of a clam shell. There is also a blocky jumble of mostly high-reflectance material collected downslope of the crater, where the rim would be. What could explain the presence of this material?

LROC WAC monochrome mosaic centered on Lalande C crater (10.5 km diameter, 5.596°S, 353.041°E), where the asterisk marks the location of the field of view shown at high resolution in the LROC Featured Image released January 29, 2013 [NASA/GSFC/Arizona State University].

Circular, or bowl-shaped, craters form when impact occurs greater than 15° from the horizontal (the most probable angle of impact is 45°), and material from within the crater is ejected ballistically to form an expansive, symmetric ejecta blanket. However, because the bolide impacted into the sloped crater wall, the material ejected from the crater in today's Featured Image did not form a symmetric ejecta blanket. What this means is that the jumble of high-reflectance material is probably excavated material that was ejected at low velocity from the crater during crater formation. LROC NAC images of oblique craters such as this one show similar features and certainly require additional study!

Take a look for yourself in the full LROC NAC image! Can you identify the extent of asymmetric ejecta superposed on the wall of Lalande C?

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