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Scientists had considered the dunes to be fairly static, shaped long ago when winds on the planet's surface were much stronger than those seen today, said HiRISE Deputy Principal Investigator Candice Hansen of the Planetary Science Institute, Tucson, Ariz.
Several sets of before-and-after images from HiRISE over a period covering two Martian years -four Earth years -tell a different story.
"The numbers and scale of the changes have been really surprising," said Hansen.
Three images of the same location taken at different times on Mars show seasonal activity causing sand avalanches and ripple changes on a Martian dune. Time sequence of the images progresses from top to bottom. Each image covers an area 285 meters (312 yards) by 140 meters (153 yards). The crest of a dune curves across the upper and left portions of the image.
This time-series of black and white and false-color images, from left to right at three sites in a field of transverse sand dunes on Mars, shows that extensive erosion has taken place in one Mars year. Credit: Science/AAAS
Now, before-and-after pictures taken by NASA's Mars Reconnaissance Orbiter taken over two Martian years — the equivalent of four Earth years — reveal a different story.