Moon plume breaks the record

The Galileo spacecraft has found the tallest plume of material seen so far on Jupiter’s moon Io, the only volcanically active moon known in the solar system. Towering 500 kilometers above Io–more than 50 times the height of Mount Everest and at least 10 percent higher than any previously detected plume–the vented gas emanates from a previously unknown hot spot near the moon’s north pole.

Volcanic hot spots pepper an infrared image of Jupiter’s moon Io taken Aug. 6 by the Galileo probe. Arrow marks source of the tallest volcanic plume yet seen on the moon. NASA/JPL/U. of Ariz.

An earlier, visible-light image shows the same face of Io. NASA/JPL/U. of Ariz.

The craft detected the plume on Aug. 6, when it passed within 194 km of Io. Because the aging craft can transmit data only through a low-gain antenna, it took several weeks for scientists to receive the information. NASA released the findings Oct. 4.

Although the craft didn’t fly through the plume, it passed near enough for a detector to record particles freshly released from a volcanic eruption. Scientists say that the microscopic particles are frigid clumps of a few sulfur dioxide molecules each.

“We’ve had wonderful images and other remote sensing of the volcanoes on Io before, but we’ve never caught the hot breath from one of them until now,” says Galileo scientist Louis Frank of the University of Iowa in Iowa City.

The discovery of the record-setting plume proved especially surprising because Galileo researchers had expected the flyby to take the craft directly through gases rising from a volcano called Tvashtar, which lies near Io’s north pole. The Tvashtar plume, detected last January by both the Galileo and Cassini probes, isn’t even visible in the new Galileo images.

However, the August encounter revealed the new hot spot and its even taller plume 600 km southwest of Tvashtar.

“The plume we knew about might have settled down before we got there, but this new one sprang up suddenly,” says Galileo scientist Rosaly Lopes of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. Infrared images reveal that Tvashtar remains active, she adds.

Galileo took a second look at the new northern hot spot just before the craft passed close to Io’s south pole on Oct. 16. Data from that flyby may also provide a closer look at yet another new hot spot that Galileo recently found near the south pole. As a bonus, both the Aug. 6 and Oct. 16 polar passes may also reveal whether Io generates its own magnetic field, as does its sister moon Ganymede.

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