Not Quite Round

The two craters in this image are not circular, but instead are more diamond-shaped. Meteor Crater in Arizona is also more square than round. Meteor Crater formed in a limestone rock that had already been faulted in two directions. The preexisting faults controlled some of the stress induced by the impacting event, resulting in the crater not ending up perfectly round. The same thing likely occurred with the formation of the martian craters - a preexisting fault or fracture system in the rocks.