The Case of the Missing Crater Rim
In this observation from NASA's Mars Reconnaissance Orbiter, these two craters perched at the edge of an outflow channel, appear to have lost a portion of their crater rims during a flood event.
Alternatively, it is also possible that the craters impacted the edge of the outflow channel after the flood occurred and we are seeing the difference in the strength of the material impacted.
The map is projected here at a scale of 50 centimeters (19.7 inches) per pixel. [The original image scale is 53.4 centimeters (21 inches) per pixel (with 2 x 2 binning); objects on the order of 160 centimeters (63 inches) across are resolved.] North is up.
The University of Arizona, Tucson, operates HiRISE, which was built by Ball Aerospace & Technologies Corp., Boulder, Colo. NASA's Jet Propulsion Laboratory, a division of Caltech in Pasadena, California, manages the Mars Reconnaissance Orbiter Project for NASA's Science Mission Directorate, Washington.