Here we observe a portion of an impact crater that is elliptical rather than circular. How do we know this is a crater and not a volcanic or tectonic feature?

First, a raised rim around the edge of the depression is characteristic of all impact craters. Secondly, in this image from MRO’s Context Camera, there is a distinct ejecta blanket deposited to the northwest and southeast of the cavity, referred to as “butterfly” ejecta.

Why is the crater so oblong and the ejecta distributed thus? Most craters are generally circular. The ejecta distribution and oblong crater shape are due to a lower impact angle. Most impactors hit the surface around 45 degrees, yet they still form circular craters. Models show at the lowest impact angles (less than 15 degrees) that we get an elliptical shape and ejecta that is not equally distributed around the entire cavity.

The impactor likely originated from the southwest. A lack of ejecta, referred to as a “forbidden zone,” tells us which direction the impactor came from. However, in this case ejecta is lacking in both the southwest and northeast. Fortunately, another indicator that tells us about the impact direction is the smaller more circular cavity that comprises the northeast portion of the crater. This cavity likely formed when a piece of the impactor broke off, referred to as a “decapitated impactor,” and struck the surface downrange.

Written by: Will Yingling, Eric Pilles, and Livio L. Tornabene  (15 February 2020)