Martian impact craters provide insights into formation ages of terrain units, near-surface properties, and the planet's degradational history. Crater size-frequency distribution analyses allow determination of terrain unit relative ages, and extrapolation of the lunar crater chronology can provide estimates of absolute ages for these units. Such analyses indicate that Mars has been geologically active throughout its history, but it also retains extensive regions dating back to ca. 4 Ga. Higher cratering rates during the Late Heavy Bombardment (LHB) helped to create the Martian regolith, and formation of large impact basins helped to erode the initially denser Martian atmosphere and may have created the hemispheric dichotomy. Low global degradation rates since the end of LHB (ca. 3.8 Ga) resulted in well-preserved impact crater morphologies. The presence of layered ejecta blankets and central pits associated with fresh to moderately eroded craters suggests a volatile-rich crust. Impact crater depth-diameter relationships allow estimation of the vertical extent of crustal volatiles to depths greater than those probed by current spacecraft instrumentation. Temporal changes in subsurface ice reservoir depths are revealed by differences in layered ejecta onset diameter as a function of crater age. An understanding of the morphology and morphometry of pristine impact craters facilitates efforts to determine the amount and types of degradation and the periods over which they operated. Impact craters therefore play a major role in unraveling the climatic and geologic history of Mars.
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