Hubble finds surprises in solar system.

Designed to image the most distant denizens of the universe, the Hubble Space Telescope rarely gazes at objects in our solar system. But Hubble observations over the past few months document acts of violence among a variety of Earth's neighbors. These include an apparently fresh volcanic eruption on Jupiter's moon Io and the ejection of a blob of material from Comet Hale-Bopp.

Astronomers presented the pictures this week at the annual meeting of the American Astronomical Society's Division for Planetary Sciences at Kohala Coast, Hawaii.

Planetary scientists planned last July's photo shoot of Io--the only volcanically active moon known in the solar system--as a scouting mission for the Galileo spacecraft. Researchers hoped that Hubble might record an intriguing feature that Galileo could examine in detail during its upcoming tour of Jupiter and its moons. But Io investigators John R. Spencer of Lowell Observatory in Flagstaff, Ariz., and his colleagues got more than they bargained for.

The images reveal a huge yellow-white spot that may represent the freshest volcanic deposit ever imaged on Io. Some 320 kilometers in diameter, the spot surrounds the volcano Ra Patera. Spencer suggests that the spot's location and color, yellower than previously recorded bright features, indicate that the blemish is frozen sulfur dioxide from a recent eruption of Ra Patera.

The yellow-white feature does not appear in Hubble images taken 16 months earlier, suggesting that it formed recently, he adds. Indeed, the emergence of the spot represents the most dramatic change recorded on Io since 1979, when the moon was last viewed close-up, by Voyager 2.

Spencer rejects the notion that Io is suffering volcanic eruptions more frequently now than it did in the past. "It's hard to believe that [nothing much] has happened in the 15-year gap between Voyager and Hubble," says Spencer. The Hubble images "suggest that eruptions happen from time to time and then fade without a trace."

Visible-light telescopes on Earth could not have detected the bright spot. But ground-based infrared telescopes--if they were focused on Io at just the right time--probably would have recorded the "hot spot" that must have accompanied the eruption, Spencer speculates.

Because Hubble's wide-field and planetary camera took images at 11 visible-light wavelengths, researchers have a chance of identifying the chemical composition of the spot. Sulfur dioxide is a likely candidate, says Spencer, since it's a known resident of Io's atmosphere and associated with volcanic activity on the moon. Detailed infrared spectra taken by Galileo once it reaches Jupiter in December may unveil the composition--if the spot hasn't already vanished.

Imaging a tiny object beyond Jupiter's orbit, Hubble recently recorded another type of outburst. Observations of newly discovered Comet Hale-Bopp taken Sept. 26 brought both good news and bad news. Harold A. Weaver of Applied Research Corp. in Landover, Md., and his colleagues had hoped Hubble would take a high-resolution image of the comet, which may dazzle skywatchers when it nears the sun in April 1997 (SN: 9/23/95, p.200). But an error in positioning Hubble prevented it from taking spectra of the comet or imaging the body at a resolution sharp enough to estimate the size of its nucleus.

The comet pictures do reveal a blob of debris flung out of the nucleus, however. Weaver says the debris probably represents a chunk of the comet's icy crust hurled into space by a combination of solar heating and the comet's rapid rotation. The spiral path of the debris, he adds, reflects the rotation of the nucleus, which completes one revolution in a week.

Piecing together data from Hubble and images taken hours to days later by a telescope at Teide Observatory in the Canary Islands, Spain, Weaver estimates that the debris moves at 109 km an hour and probably separated from the comet a mere 2 days before Hubble took the picture. Hubble will try to achieve its originalgoal--obtaining a high-resolution image and spectra--at the end of the month.