Infrared telescope eyes new eruption on Io.

Time on big telescopes is precious, and astronomers rarely get more than a few nights of observing. But as part of the Jason project (SN: 11/12/94, p.307), which features live telecasts to schoolchildren, John R. Spencer got the chance to view Jupiter's moon Io for 2 weeks.

His team's images, taken at NASA's Infrared Telescope Facility atop Hawaii's Mauna Kea, did more than wow several hundred thousand students. They revealed a huge, freshly erupting volcano on the frigid moon. The Voyager 1 spacecraft detected volcanism on Io in 1979, and the moon remains the only place in the solar system, other than Earth, known to have volcanic activity.

The finding, Spencer says, marks the first time anyone has obtained an image sharp enough to show the location of this big a volcano on Io. The image also provides the first real-time detection of a large Io eruption, allowing other astronomers to track the ephemeral feature.

Spencer, of Lowell Observatory in Flagstaff, Ariz., Jane E. B. Spencer, and David Griep of the Infrared Telescope Facility report their discovery in a March 2 circular of the International Astronomical Union. On that date, Spencer told Science News, the new feature glowed as brightly as the entire disk of Io at an infrared wavelength of 4.8 microns.

"This is the brightest hot spot I've personally seen in 5 years of observing Io," Spencer says.

His latest calculations place the spot at about 45o south of Io's equator and 95o west longitude, just inside the face that Io always keeps turned away from Jupiter. Astronomers can more accurately gauge the position of hot spots on the Jupiter-facing side of Io because these spots stand out brightly when the moon slips into the giant planet's shadow. But the position of the new spot offers one possible advantage -- the Galileo craft may obtain close-up images during the craft's December flyby of Io.

Paul D. Feldman of Johns Hopkins University in Baltimore, an investigator with the Hopkins Ultraviolet Telescope (HUT) on the Astro 2 Observatory, learned of Io's eruption soon after Astro's launch (SN: 3/4/95, p.133).

When the spot next came into view, 2 days later, HUT obtained simultaneous spectra of the moon's atmosphere and its torus, the ring of charged particles surrounding Io's orbit. These and subsequent spectra should reveal whether the eruption spewed sulfur and oxygen into Io's atmosphere and if it added ions to the torus.