Dunes on Mars (IMAGE) University of Arizona Caption The retreat of Mars' polar cap of frozen carbon dioxide during the spring and summer generates winds that drive the largest movements of sand dunes observed on the red planet. Credit NASA/JPL/University of Arizona/USGS) Usage Restrictions This visual may only be used in conjunction with reporting on or posting of this news release. Credit must be given as indicated. License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.