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The 110-kilometer-wide impact crater at center, Rabe, is typical of those in the cratered highlands west of the Hellas impact basin. Most of these craters appear to be very degraded. Raised rims, terraces, central peaks, and other morphologic features and structures that characterize relatively fresh craters (slide #16) are virtually absent, and have been heavily eroded or blanketed. Only one crater, at the bottom of the scene, looks deep and fresh. The degradation of topography on Mars is termed “terrain softening.” Terrain softening may be due to the slow but steady downslope movement of regolith and soil, a process called creep. This process may indicate the presence of ground ice on Mars, which would enhance creep of the soil. Alternatively, the softened appearance could result from a long history of deposition by airfall. In the martian highlands, dark spots are common within the floors of craters. Here, a dark spot is resolved into a field of dunes. The dunes may be the remnants of crater-filling deposits or accumulations of windblown sand that formed in the topographic trap at the bottom of the crater. Viking
1 images 510A29, 094A49. Right click here to download a high-resolution version of the image (650 KB)
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