The formation is elliptical, check out the map projected image. This usually happens from an oblique impact. It is also oblique because the impact is near the pole and a meteor would normally come from the angle of the ecliptic.

Another interesting thing, the line between the foci is roughly perpendicular to north. A meteor would be somewhat more likely to come from outside Mars orbit so to impact at this angle to North this would translate to more likely early morning or afternoon.

The crater may have filled with water melted from the ice when it hit, and then the layers have been forming over it. Craters like this are really rare, perhaps because the pole was in a different position when meteors were more frequent or they have all been covered over.

This is another crater on the pole. It looks like a hill from the shading but I eventually found out the sun angle on it at MSSS was wrong by someone checking the Viking image.

http://ida.wr.usgs.gov/html/e13019/e1301971.html