Hot spot. Galileo's snapshot of Prometheus, and a temperature map (inset).

Fireworks on Jupiter's Io

WASHINGTON, D.C.--Planetary scientists today revealed stunning new pictures of a volcanic moon run wild. Jupiter's moon Io, about the size of Earth's now inert moon, is gushing lavas so hot and voluminous that nothing like it has been seen on Earth for more than 2 billion years, they announced at a NASA press conference here. Indeed, Io's prolific volcanism could serve as a guide to how Earth lost its youthful heat before settling down to a cool middle age.

The new images were snapped by the Galileo spacecraft last October as it swooped within 611 kilometers of Io's surface, the first close look since the Voyager spacecraft discovered Io's low-lying volcanic vents, lava flows, and towering plumes in 1979. Galileo observations now confirm that the wispy plumes, rising hundreds of kilometers above the surface, are indeed sulfur dioxide particles, ejected from the surface as high-speed gas. In the Galileo images of one volcano, Prometheus, researchers can see how lavas thought to start out at temperatures above 1500oC--hundreds of degrees hotter than lavas on Earth--travel from a caldera through lava tubes to break out on the surface and produce a plume 100 kilometers distant from where Voyager last saw one. Presumably, the lava is heating and vaporizing the thick layers of sulfur dioxide "snow" that have accumulated from earlier eruptions.

Massive lavas haven't flowed 100 kilometers across Earth in the last 17 million years, notes Galileo imaging team member Alfred McEwen of the University of Arizona, Tucson, and it has been billions of years since such superhot lavas spilled onto the surface. In those early days, lavas rich in magnesium and iron poured forth, leaving rocks called komatiites. "On Io, we may be seeing this kind of eruption today," says McEwen. Unlike Earth, Io is being continually heated by Jupiter's gravitational kneading. That tremendous energy source, he notes, provides planetary geologists with a look at what Earth might have been like when its inner fires were still fiercely hot.

Galileo's next, and possibly last, look at Io comes during another flyby on Thanksgiving day. After that, the already extended mission could soon end in one of two ways, says project scientist Torrence Johnson of the Jet Propulsion Laboratory in Pasadena. Either NASA won't come up with the money to continue operations, or the intense Jovian radiation near Io, which triggered problems during the last passage, will kill or cripple Galileo. But Johnson hopes the spacecraft can make yet another Io flyby. "We've taken a few hits," Johnson says, "and Galileo is running a little bit beyond its warranty, but we're ready to go again."