Impact craters have fairly simple, geometric shapes that easily lend themselves to both qualitative and quantitative analyses. Impact craters have also formed throughout the entire history of Mars. By carefully analyzing the geology and physical structure of fresh and modified impact craters in the highlands, it will be possible to deconvolve the types and intensity of geologic processes that have operated through time on Mars. While similar analyses have been undertaken from Mariner and Viking orbiter data, our understanding of the geologic history recorded in modified impact craters is, at best, crude. High-resolution imagery from Mars Global Surveyor (MGS), Mars Odyssey (MO), and Mars Express (MEX) provides unprecedented details of not only the fluvial processes that extensively modified impact craters early in martian history, but also information about the processes that have operated on Mars to the present day. Mars Orbiter Laser Altimetry (MOLA) topographic data can also provide detailed morphometric information about modified crater shape, which is necessary for estimating the amount of erosion that has occurred. Through analyses of available imagery, remote sensing, and topographic data we will:

• Provide a better understanding of the processes that modified impact craters in the Martian highlands during the Noachian and Hesperian. These analyses will be important for determining the types of geologic processes that operated during the early history of Mars, which has important implications for climatic conditions.
• For the first time, assess the processes that continued to modify impact craters through the Amazonian, which may include eolian mantling and striping, mass wasting, such as small landslides and gullying, and the effects of small impact cratering. While analyses of high-resolution imagery and remote sensing data will help place these processes into a spatial context, analysis of superposing relations of related deposits and populations of affected craters will place our observations into a temporal context. These analyses will provide a better understanding of the types of geologic processes that have affected the Martian surface over the last few billions years.
• Derive first-order estimates of the amount of material eroded from individual impact craters by analyzing the geometric shapes of modified impact craters using Mars Orbiter Laser Altimeter (MOLA) topographic data. This information will provide a better understanding of the intensity of geologic processes responsible for crater modification as well as the early Mars erosion rates.

This is one of the first photographs of Mars taken by the Mariner spacecraft in the late 1960s. At the time scientists were both surprised and disappointed that, like the Moon, the surface of Mars was scarred by impact craters. However, closer examination reveals that many craters lack an obvious ejecta deposit. Often crater rims have also been eroded, and the interiors are shallow. We are investigating the timing and processes responsible for modifying these craters.

An algebraic method we developed for estimating the amount of erosion that took place during crater modification. A profile of a modified impact crater (red) is compared to a slightly smaller profile of a fresh crater (blue). The diameter of the fresh impact crater (as well as the corresponding parameters) is adjusted until the volume of eroded material (yellow) equals the volume of infilling (green). Volumes are estimated using the Thereom of Pappus. The results are calculated in cylindrical coordinates, which is why the area of infilling (green) looks larger than the area of erosion (yellow).