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This movie clip of 48 frames shows Jupiter's moon Io in the darkness of the giant planet's shadow. The sequence was recorded over a two hour interval that spanned nearly an entire eclipse on January 1, 2001. By the end of the clip, Io is emerging from shadow. The original images were taken through the clear filter of the narrow angle camera from a distance of over 10 million kilometers (6.3 million miles), with a spatial resolution of 61 kilometers/pixel. They have been cropped and processed to remove scattered light and artifacts.
Although no sunlight shines on the moon during an eclipse, two types of glows can be seen in this movie. The bright points of light are the glows of hot lava from actively erupting volcanoes. The brightest is the volcano Pele, which appears to be erupting steadily despite its great intensity.
To the right of Pele and slightly above it is a pair of bright spots associated with the volcano Pillan, the source of a major eruption in 1997 that was observed, both by the Galileo spacecraft and the Earth-based telescopes, to dwarf that of its energetic neighbor Pele. Pillan's eruption has waned over the past 30 months to the pair of small hot spots seen here. Another, newly discovered volcano, seen below Pillan and below and to the right of Pele, varies on a time scale of days. This sequence of images illustrates the great variations in intensity and longevity of Io's volcanic eruptions. The movement of these eruptions towards the right limb of Io during the course of the movie is due to the Io's rotation.
The second type of glow seen on Io during eclipse is a set of faint, diffuse emissions due to atmospheric aurorae. Similar to the Aurora Borealis on Earth, Io's aurorae are caused by the collisions of charged particles with gases in Io's tenuous atmosphere. A faint ring encircles the satellite, while brighter glows are concentrated on both sides of the moon's equator. These equatorial glows are seen gradually shifting in location as the eclipse progresses, due to the changing orientation of Jupiter's magnetic field. This is a new detection which confirms that these visible aurorae, like their counterparts seen at ultraviolet wavelengths, are caused by electrical currents that flow between Io and Jupiter.
The Cassini-Huygens mission 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-Huygens mission for NASA's Office of Space Science, Washington, D.C. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.