This observation shows part of an unnamed crater located in the Northern plains.

The intriguing landforms in the floor of this crater are known as "concentric crater fill." Such landforms are found at high latitudes (approximately above 30 degrees from the equator), where theoretical calculations indicate that ice may exist under the surface, mixed with rocks and soil. Examples of concentric crater fill were first observed in the 1970s, in images acquired by cameras on board the Viking orbiters.

The roughly concentric ridges and throughs in the crater's floor are believed to result from compression caused by viscous flow of a thick mixture of rocks, soils, and ice inward from the crater's walls.

Impact craters with concentric fill are usually shallower than other craters. The crater in this image is approximately 12 kilometers (7.5 miles) in diameter, and 200-400 meters (220-440 yards) deep; other Martian craters of similar diameter but without concentric fill may be as deep as 700 meters (765 yards). Unlike in "regular" craters, the slopes of the walls of craters with concentric fill tend to be convex, and the crater's rim is more rounded.

All these characteristics are consistent with deformation of an ice-rock mixture similar to what's observed in rock glaciers on Earth.

Written by: Sara Martinez-Alonso   (25 August 2010)