At some time in the past, an object hit Mars near its south pole. The result was a crater, 45 km in diameter, part of which is evident as the green crescent at the left (west) of this image compiled from Mars Orbiter Laser Altimeter data. But why a crescent? As he describes in the Journal of Geophysical Research (106, 10075–10085; 2001), James W. Head concludes that the crescent shape resulted from considerable movement of the martian polar cap within the past few million years. Head constructed profiles of the crater and neighbouring areas, and took especial account of the pattern of an 'ejecta lobe' and secondary ejecta craters seen on higher-resolution pictures.

The south pole of Mars is thought to be covered by seasonally shifting deposits of water-ice, usually with a layer of CO2 on top. These deposits overlie a much more stable 'polar layered terrain' of ice and dust. Head's thesis is that, in the comparatively recent past, polar layered terrain swept from the south of the crater to occupy a large part of it, leaving only the original crater floor, seen here in green. This conclusion is largely based on the pattern of surviving secondary craters — those on the ground underlying the layered terrain (brown) close to the impact, or in the path of the part of the polar cap that moved, being obliterated. From features known as mantled deposits — residues of polar layered terrain activity — to the north and east of the crater, Head also surmises that the layered terrain later retreated partially.

The other large impact crater, seen on the right of this picture, lies 300 km from that painstakingly investigated by Head. Its western part likewise contains a lobe of polar layered terrain, a further indication of the possible occurrence of dynamic processes at the martian south pole.