Curious craters on Mars said result of impacts on ancient ice

U.S. geologists say they've come up with a new explanation for a mysterious double-layer type of crater observed on Mars, and it's all about ice and impacts. Impact craters on Mars are surrounded by what's known as ejecta, debris excavated by an impacting object. But astronomers have long noted a curious type of crater they've dubbed double-layer ejecta craters, or DLEs, where the debris forms two distinct layers -- a large outer layer with a smaller inner layer sitting on top. Researchers at Brown University suggest DLEs are the result of impacts onto a surface that was covered by a layer of glacial ice up to 30 feet thick. The impact would blast through the ice layer, spitting rock and other ejecta out onto the surrounding ice, but because that ejected material sits on a slippery ice surface it doesn't all stay put, they said. The layering occurs, they suggest, when material near the top of an upraised crater rim begins to slide down the slippery ice later and overtops material on the lower slopes. The findings could explain why DLEs are found at Mars' middle or high latitudes, they said, areas where scientists believe there may once have been glacial ice on Mars. Understanding how DLEs and other crater types are formed could lead to a better understanding of Mars' past and its climate, the researchers said. "I think for the first time since DLEs were discovered in the 1970s we have a model for their formation that appears to be consistent with a very wide range of known data," Brown graduate student David Kutai Weiss said. "There are over 600 DLEs on the Martian surface, so reconciling how they formed with our knowledge of the climate of Mars is pretty important," he said. "It could tell us a lot about the history of the martian climate on a global scale."