Small Crater on Planum Boreum (ctx frame - natural colors; credits: Lunexit)

Impact craters on the surface of Planum Boreum, popularly known as the North Polar Cap, are rare. This dearth of craters has lead scientists to suggest that these deposits may be geologically young (a few million years old), not having had much time to accumulate impact craters throughout their lifetime.

It is also possible that impacts into ice do not retain their shape indefinitely, but instead that the ice relaxes (similar to glass in an old window), and the crater begins to disappear.