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This HiRISE sub-image is located on the southern margin of Elysium Planitia, in the equatorial region of Mars. A scarp (cliff) winds through the scene, dividing the lower knobby terrain to the southwest from the higher terrain to the northeast. The scarp offers a glimpse into the material that underlies the higher terrain. No prominent layers are seen in the vertical face of the scarp, and boulders have not accumulated around its base. This suggests that the elevated northeastern terrain is not made of hard rock; however, it is also possible that rocks are present but buried under sediments. Several of the impact craters in the northern part of the sub-image are "pedestal craters," which have fragmented material that was thrown out of the crater upon impact. But the ejecta was not always raised like this. Being more resistant to erosion, it was left high-standing after the surrounding material was removed, probably by wind. In addition to being raised, the ejecta around these craters is asymmetric - it is skewed towards the southeast. This might be because the craters formed when objects struck the surface of Mars at an angle, or perhaps erosion has preferentially removed the ejecta on the northwest sides of the impact craters. The knobby terrain southwest of the scarp is riddled with wind-blown dunes. The dunes radiate out around the bases of the knobs indicating that they are more strongly influenced by local topography than regional winds. Small boulders on the flanks of a few knobs reveal that they contain rocky material. The image scale is 59 cm/pixel (with 2 x 2 binning) so objects ~178 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 3:28 PM and the scene is illuminated from the west with a solar incidence angle of 54.9 degrees, thus the sun was about 35.1 degrees above the horizon.
01/14/09

 



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