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Released December 20, 2000
Jupiter's 4 largest satellites have fascinated Earthlings ever since Galileo Galilei discovered them in 1610 in one of his first astronomical uses of the telescope. The images that will be released over the next several days capture each of the 4 Galilean satellites in their orbits around the giant planet.
This true color composite frame, made from narrow angle images taken on December 12, 2000 around 15 hours UTC (spacecraft event time) captures the innermost Galilean satellite, Io, and its shadow in transit again the disk of Jupiter. The distance of the spacecraft from Jupiter was 19.5 million km; the spacecraft latitude was 3 degrees above Jupiter's equator plane. The image scale is 117 km/pixel.
The entire body of Moon-sized Io is periodically flexed as it speeds around Jupiter and feels, as a result of its non-circular orbit, the periodically changing gravitational pull of the planet. The heat arising in Io's interior from this continual flexure makes it the most volcanically active body in the solar system, with more than 100 active volcanoes and plumes, like giant mushroom clouds, extending 100's of kilometers high. The white and reddish colors on its surface are due to the presence of different sulfurous materials; the black areas are silicate rocks.
Image Credit: NASA/JPL/University of Arizona
http://photojournal.jpl.nasa.gov/cgi-bin/PIAGenCatalogPage.pl?PIA02860
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