[Illustration: 3/5/79 425,000 km (264,000 mi)] [Illustration: 7/9/79 560,000 km (350,000 mi) Amalthea was observed end-on in the Voyager 2 picture, which has been computer-processed to enhance the image.] [Illustration: 3/4/79 377,000 km (234,000 mi) Io, Jupiter’s innermost Galilean satellite, displays great diversity in color and brightness. This Voyager 1 four-picture mosaic shows Io’s complex coloration of red-orange, black, and white regions, and the two major topographic features: volcanic regions, the most prominent of which is the “hoofprint” (volcanic deposition feature) in the center-right, and the intervolcanic plains that are relatively featureless. Io’s vivid coloring is probably due to its composition of sulfur-rich materials that have been brought to the surface by volcanic activity.] [Illustration: 3/5/79 129,600 km (80,500 mi) The bright area at the upper right in this Voyager 1 picture of Io appears to be a caldera (collapsed volcano) that is venting clouds of gases. The clouds may condense to form extremely fine particles that scatter light and appear blue. Because the infrared spectrometer discovered sulfur dioxide on Io, scientists believe this gas may be the main component of the clouds. Sulfur dioxide clouds would rapidly freeze and snow back to the surface. It is also possible that dark areas in the floors of the calderas are pools of encrusted liquid sulfur.] [Illustration: 3/5/79 66,000 km (41,000 mi) Evidence of erosion in Io’s southern polar region is visible in this Voyager 1 high-resolution image. The picture has been computer-enhanced to bring out surface detail while suppressing bright markings. A depressed segment of the crust, bounded by faults, is seen near the terminator in the upper right portion of the image. At the lower center are complicated scarps (slopes) and portions of isolated elevated terrain that geologists interpret as “islands” left behind as the scarps eroded. Scientists speculate that sulfur dioxide (as a subsurface liquid) may be a determinant in the creation of these features.] [Illustration: 3/4/79 862,000 km (540,000 mi) Io’s surface, less than ten million years old, is quite young compared to the other Galilean satellites and to other terrestrial bodies, such as Mercury and the Moon. The surface is composed of large amounts of sulfur and sulfur dioxide frost, both of which account for most of the surface color. This picture was taken by Voyager 1. Material deposited by the volcano (see following pictures) can be seen as a white ring near the center of Io.]