Small impact craters usually have simple bowl shapes, but in some cases surface properties or processes can alter this shape in unusual ways.
This image shows an approximately 300-meter impact crater that appears to have narrow terraces around the rim. How could these different crater morphologies form? One explanation is that the impact occurred into a surface with layers of differing strengths. However, the clearest example of this type of crater are better-preserved than this one.
Additional clues come from other craters that have a raised mound in the center as opposed to a depression and are sometimes referred to as inverted craters due to their topography. These craters were filled with sediment (or some material stronger than the surrounding material), and subsequent erosion removed the terrain around the filled material, leaving a small mound behind.
Candor Chasma in central Valles Marineris is filled with light-toned layered deposits thought to be sandstones, perhaps formed in an ancient wet and potentially habitable environment.
The CRISM instrument on MRO has acquired thousands high-resolution spectral images across Mars, often with simultaneous coverage by HiRISE, but sometimes, for a variety of reasons, without HiRISE coverage. We are now trying to complete coordinated coverage over such locations, to enable geologic interpretations based on both the compositional information of CRISM and the high-resolution imaging of HiRISE.
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