The shape of a crater can depend on factors including the angle of impact and the pre-impact slope and topography.
This image shows a roughly 3-kilometer impact crater, formed on the sloping walls of Tithonium Chasma, part of the large Valles Marineris canyon system. We can see that this crater is non-circular, measuring about 3 by 4 kilometers. The ejecta—the debris that is generated and thrown out by an impact—will typically distribute itself evenly… [More at link]