Ebbing ice on Mars

"This Mars Orbiter Camera image shows the outer edge of the south polar residual cap during southern summer. The variability in brightness across the image would not be as apparent had the data been acquired during late winter or spring, owing to the presence of seasonally deposited, carbon dioxide frost.

"During the spring and into the early summer, seasonal carbon dioxide deposits are removed through sublimation, and then the thicker, older accumulations of carbon dioxide -- deposited hundreds or thousands of years ago -- erode. As this occurs, some surfaces become darker, either because they are roughened by erosion, contain dark material such as mineral dust, or both."