Galileo spacecraft glimpses changes on Io.

En route to its first rendezvous with Jupiter's moon Ganymede, the Galileo spacecraft late last month took a quick look at the mottled face of another large Jovian moon, volcanically active Io. The relatively low-resolution images, one of which NASA released last week, can't distinguish mountains from valleys, but they do show that the color and brightness of the surface have changed significantly since the Voyager spacecraft viewed it 17 years earlier.

Several of these alterations stem from the numerous sulfur-spewing volcanoes that continually erupt, notes Michael J.S. Belton, head of the Galileo imaging team at Kitt Peak National Observatory near Tucson. Particularly striking is a fresh sulfur dioxide frost surrounding the volcano Masubi in Io's southern hemisphere. "The sulfur dioxide gas that drives the volcano makes a big plume, condenses, then paints the surface white," he says.

However, says John R. Spencer of Lowell Observatory in Flagstaff, Ariz., some hot spots on Io, seen in the infrared from Earth, have not noticeably modified the moon's surface. He speculates that these hot spots represent long-term eruptions that do not throw out enough high-temperature material at any one time to alter the surface.

Combining data from the Hubble Space Telescope with Galileo's considerably sharper images, Spencer and Alfred S. McEwen of the U.S. Geological Survey in Flagstaff have found evidence of small amounts of rare forms of sulfur. The reddish hue of Io's poles and the color of some recent volcanic deposits hint at the presence of short-lived forms of sulfur-molecules composed of three or four sulfur atoms-even though these molecules don't typically last as long as common sulfur, composed of eight atoms.

High temperatures within volcanoes may trigger chemical reactions that produce the short-chain forms of sulfur, McEwen suggests. Continual volcanic activity would maintain the supply of these ephemeral molecules. At the poles, high-intensity radiation may decompose elemental sulfur into the short-chain forms, which can persist at the cooler temperatures there.

As intriguing as the new images are, they don't reveal detail smaller than 23 kilometers across. Galileo missed its best opportunity to view Io last Dec. 7, when the craft entered orbit around Jupiter and passed near enough to Io to discern structures 10 meters in diameter. Because of a problem with the craft's tape recorder, NASA decided not to take high-resolution images at that time.

Belton is hoping that after Galileo completes its primary mission 17 months from now, the space agency will give the go-ahead for one last hurrah. Galileo would require an additional 18 months to make a final close approach to Io, but "it's just mind-boggling what you can get," he notes.