Susitna Glacier, Alaska. Like rivers of liquid water, glaciers flow downhill, with tributaries joining to form larger rivers. But where water rushes, ice crawls. As a result, glaciers gather dust and dirt, and bear long-lasting evidence of past movements. Alaska’s Susitna Glacier reveals some of its long, grinding journey in this image, taken from space on August 27, 2009. The satellite image combines infrared, red and green wavelengths to form a false-color picture. Vegetation is red and the glacier’s surface is marbled with dirt-free blue ice and dirt-coated brown ice. Infusions of relatively clean ice push in from tributaries in the north. The glacier surface appears especially complicated near the center of the image, where a tributary has pushed the ice in the main glacier slightly southward. In the lower left corner of this image, meltwater lakes can be seen on top of the ice.