Probes find a new plume on Io

Two spacecraft jointly eyeing Jupiter’s moon Io, the most volcanically active body in the solar system, have spotted a towering new plume.

Images of Io show two volcanic plumes (arrows). One spews from the equatorial volcano Pele (left). A new plume emerges from Tvashtar Catena near Io’s north pole (right). NASA/JPL

Both the height and location of the eruption, imaged last December and January by the Cassini and Galileo craft, make it a standout. It’s the first active volcano found near one of Io’s poles. Moreover, this sulfurous inferno reaches an altitude of nearly 400 kilometers. Only one other Io plume is as tall–a long-lived one spewing from the prominent volcano Pele.

The new eruption, seen in images NASA released on March 29, emanates from the Tvashtar Catena volcano near Io’s north pole. If the Tvashtar plume persists at its present height, Galileo will pass directly through it in August at an altitude of 360 km.

Material in the plume is too tenuous to harm the craft, and the flyby will provide an ideal opportunity to analyze the composition of the outflow, says Galileo project scientist Torrence V. Johnson of NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

The joint imaging venture by Cassini and Galileo is a case of fortuitous timing. Cassini’s sojourn at Jupiter is brief: It arrived in late December and will leave in June, after getting enough of a gravity kick from the giant planet to reach Saturn in July 2004. Galileo, which has toured Jupiter and some of its moons since 1995, was originally scheduled to end its mission 2 years ago.

Impressed by Galileo’s ability to withstand the harsh radiation near Jupiter, NASA already had extended Galileo’s mission twice. In January, the agency gave the craft a third and final reprieve. If all goes according to plan, on May 25 Galileo will pass within 123 km of Callisto, the second largest of Jupiter’s moons. Callisto’s gravity will enable the space probe to swing past Io in both August and October.

Galileo’s picture-taking days will end this winter, but if its health endures, the craft will continue studying Jupiter for another 18 months. In November 2002, Galileo will venture closer to Jupiter than ever before, coming within 500 km of its tiny moon Amalthea, which is less than one-tenth Io’s size. After looping away from Jupiter, Galileo will return one last time, crashing into the planet in August 2003.

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