Io over Jupiter

Photojournal: PIA02860

December 16, 2004

Jupiter's four largest satellites, including Io, the golden ornament in
front of Jupiter in this image from NASA's Cassini spacecraft, have
fascinated Earthlings ever since Galileo Galilei discovered them in 1610 in
one of his first astronomical uses of the telescope.

Images from Cassini that will be released over the next several days capture
each of the four Galilean satellites in their orbits around the giant planet.

This true-color composite frame, made from narrow angle images taken on
Dec. 12, 2000, captures Io and its shadow in transit against the disk of
Jupiter. The distance of the spacecraft from Jupiter was 19.5 million
kilometers (12.1 million miles). The image scale is 117 kilometers (73 miles)
per pixel.

The entire body of Io, about the size of Earth's Moon, 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. The
white and reddish colors on its surface are due to the presence of different
sulfurous materials. The black areas are silicate rocks.

Cassini is a cooperative project of NASA, the European Space Agency and the
Italian Space Agency. The Jet Propulsion Laboratory, a division of the
California Institute of Technology in Pasadena, manages the Cassini mission
for NASA's Office of Space Science, Washington, D.C.

Credit: NASA/JPL/University of Arizona

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