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This photo of Io, innermost of the four large Galilean satellites of Jupiter, was taken through an ultraviolet filter by the narrow angle camera of Voyager 1. The photo was taken at 2 a.m. (PST) Feb. 27, when Voyager 1 was 4.3 million miles (7 million kilometers) from Io, seen against the background of a part of Jupiter's disk. North is at the top, and the central longitude of Io is 180 degrees.
Io shows a contrasting surface, with generally very dark polar areas and numerous light and dark regions around the equator. At this resolution (about 100 miles or 160 kilometers), no topographic features, such as craters, can be seen. The brighter regions are believed to contain sulfur and various salts, making Io highly reflective (about six times brighter than Earth's Moon) in visible and enhanced light. This satellite of Jupiter has almost exactly the same size and density as our own Moon, but it apparently has followed a very different evolutionary path, influenced by its proximity to Jupiter and the intense bombardment it receives from the Jovian radiation belts of energetic charged particles.
The Voyager Project is managed for NASA by the Jet Propulsion Laboratory, Pasadena, Calif.