Impact craters, especially those at high latitudes where groundwater or ground ice is likely, often show differences from those nearer the equator. What sets them apart is a raised platform, or pedestal, surrounding the rim. Scientists think a pedestal is born when the force of impact throws an apron of ejecta around a new crater. As the debris comes to rest, it contains mostly rock, mixed with some water from the subsurface. Extending outward to about one crater-radius, the apron of debris makes a semi-durable cap that armors the ground against erosion. As years pass, scientists suggest, the topmost layers of the plains erode due to winds or climate change or other factors. This lowers the ground — except for the area covered by the ejecta. This armored ring, or pedestal, gradually becomes more prominent as its surroundings erode.