NASA's Mars Global Surveyor shows raised-rimmed, circular features sometimes described as boulder rings. These are located on the vast martian northern plains and are, basically, somewhat filled and somewhat buried meteor impact craters.

2 October 2004
This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows raised-rimmed, circular features sometimes described as "boulder rings." These are located on the vast martian northern plains, and they are, basically, somewhat filled and somewhat buried meteor impact craters. The small, dark dots on these rings are boulders derived from the craters' ejecta and perhaps from erosion of the rock in which the craters formed. This image is located near 70.4°N, 310.4°W, and covers an area about 3 km (1.9 mi) across. Sunlight illuminates the scene from the lower left.

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