Scientists have provided new evidence that liquid carbon dioxide, not running water, may have been the primary cause of erosional features such as gullies, valley networks, and channels that cover the surface of Mars. Research suggesting that condensed carbon dioxide found in Martian crust carved these features is reported by Kenneth L. Tanaka and colleagues at the U.S. Geological Survey in Flagstaff, Arizona, and the University of Melbourne, Australia, will appear this month in Geophysical Research Letters, published by the American Geophysical Union.