Medusa Fossae Formation

Credit: NASA/JPL/University of Arizona, Posted on: Friday, 11 May 2007, 05:47 CDT Download full size image

This image shows a pedestal crater located in a geologic unit on Mars called the Medusa Fossae Formation.

Pedestal craters are produced by differential erosion around impact craters. If the ejecta (material thrown out of the crater) is more resistant to erosion, then the crater and surrounding ejecta will be preserved while the surface is eroded nearby. This causes the ejecta blanket surrounding the crater to form a "pedestal," standing out in relief rather than gradually merging into its surroundings.

There appear to be at least two resistant layers in the material around this pedestal crater, as there are two "steps" in the topography of the pedestal. The cutout (2196x1442, 3 MB), from the long ridge near the top center of the image, shows these steps as well as possible smaller-scale layering.

Despite the detail resolved by HiRISE, it is not clear why the step-forming layers are more resistant. Much of the scene is coated with a mantle of dust which obscures details. Dark slope streaks, likely produced by small avalanches in the dust, are common here. Dust deposition and erosion are also likely the reason for the scalloped texture of mantling material in the crater.

Observation Geometry

Image PSP_003253_1880 was taken by the High Resolution Imaging Science Experiment (HiRISE) camera onboard the Mars Reconnaissance Orbiter spacecraft on 06-Apr-2007. The complete image is centered at 7.7 degrees latitude, 196.2 degrees East longitude. The range to the target site was 276.8 km (173.0 miles). At this distance the image scale is 55.4 cm/pixel (with 2 x 2 binning) so objects ~166 cm across are resolved.

The image shown here has been map-projected to 50 cm/pixel and north is up. The image was taken at a local Mars time of 03:35 PM and the scene is illuminated from the west with a solar incidence angle of 58 degrees, thus the sun was about 32 degrees above the horizon. At a solar longitude of 214.3 degrees, the season on Mars is Northern Autumn.

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