The circular feature is 2.5 miles (4 kilometers) across and lies near the south pole of Mars – a mystery when first seen in Mariner 9 and Viking Orbiter images during the 1970s. Now the Mars Reconnaissance Orbiter has taken new high resolution images that may help scientists interpret this Martian feature.

Bright areas are covered by carbon dioxide frost, with a "swiss cheese" pattern common at the south polar residual cap. The circular evenness of the depression suggests an impact crater, but there is no evidence of a crater rim or debris unless it lies beneath the frost. While the depression may have formed by collapse, the image is missing the typical ground fractures that form around a collapse pit. Further analysis of the data may eventually uncover more clues.

--NASA/JPL/University of Arizona and SPACE.com Staff

Credit: NASA/JPL/University of Arizona