Changes in the surface of comet Tempel 1, observed by NASA's Deep Impact Mission in 2005 (top) and NASA's Stardust-NExT mission on Feb. 14, 2011 (bottom). Between the two visits, the comet made one trip around the sun. The smooth terrain is at a higher elevation than the more textured surface around it. Scientists think that cliffs, illustrated with yellow lines to the right, have eroded as much as 20 to 30 meters (66 to 100 feet) in some places. The box shows depressions that have merged together over time, also from erosion. This erosion is caused by volatile substances evaporating away from the comet. Credit: NASA/JPL-Caltech/University of Maryland/Cornell