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This oblique view shows geological layers of rock exposed on a mound inside Gale Crater on Mars. Image credit: NASA\/JPL-Caltech\/University of Arizona\/USGS<\/em> Gale Crater is about the size of Connecticut, and within the crater is a huge mound, several thousand meters high, which expose a time-deep sequence of layers demonstrating dramatic changes in Mars history.<\/p>\n Clay minerals indicating wet conditions are near the bottom of the “Gale stack.” Evaporates indicating wet but drying conditions sit on top of the clays, and higher up are increasingly dry and eventually sediments with no evidence of water involved in their formation.<\/p>\n A sequence like that can happen anywhere on a planet and not indicate a global change. This could reflect local conditions. But this sequence matches well what has been postulated for the planet’s overall history, and it matches other evidence.<\/p>\n Rock exposures with compositions like various layers of the Gale stack have been mapped elsewhere on Mars, and researchers, including Jean-Pierre Bibring of the University of Paris, have proposed a Martian planetary chronology of clay-producing conditions followed by sulfate-producing conditions followed by dry conditions. However, Gale is the first location where a single series of layers has been found to contain these clues in a clearly defined sequence from older rocks to younger rocks.<\/p>\n “If you could stand there, you would see this beautiful formation of Martian sediments laid down in the past, a stratigraphic section that’s more than twice the height of the Grand Canyon, though not as steep,” said Bradley Thomson of the Johns Hopkins University Applied Physics Laboratory, Laurel, Md. He and John Grotzinger of the California Institute of Technology in Pasadena are Milliken’s co-authors.<\/p><\/blockquote>\n It is possible but not yet decided that NASA will explore this feature with the planned rover Curiosity, to launch next year.<\/p>\n
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\nIt is easy to forget how important impact craters are on planetary history on a planet like Earth with dynamic continental movement, wind and water erosion, and a short memory. But we are reminded when we look at the research coming out of Mars.<\/p>\n