Keeping an eye on Io.

Until the Voyager 1 flyby in 1979, many astronomers thought of Jupiter's moon Io as pretty commonplace. But Io's proximity to Jupiter endows this moon with several intriguing features. Stretched and compressed by the gravitational tug-of-war between Jupiter and the planet's other satellites, Io is the only moon in the solar system known to undergo volcanic eruptions. Particles spewed out by Io can interact with the inner part of Jupiter's magnetic field and replenish the doughnut-shaped cloud of ions that encircles both the planet and its satellite.

Now, 13 years after Voyager's close-up view, the Hubble Space Telescope has cast its eye on Io. From its Earth-orbiting vantage point, Hubble can only resolve features larger than 250 kilometers across -- one-third the sharpness of Voyager's camera. But the spectra and images taken earlier this year and released last week shed new light on Io and its environs.

By comparing the brightness of Hubble images taken in visible light and in the ultraviolet, astronomers confirmed that a frost of sulfur dioxide coats large areas of Io. Detecting the signature of sulfur dioxide -- bright in visible light but dark in ultraviolet -- clinched the finding, says Francesco Paresce of the Space Telescope Science Institute in Baltimore.

He adds that Io's overall appearance seems surprisingly unchanged since the Voyager flyby. Although volcanic eruptions on Io might dramatically alter the face of the satellite, Paresce suggests that other, unknown processes may remove or cover volcanic debris, preserving large-scale features.

Using a Hubble spectrograph to study sulfur and oxygen emissions, other researchers found that Io has a far shallower atmosphere than previous measurements had suggested. Melissa McGrath of the Space Telescope Science Institute and her colleagues report that Io's atmosphere extends about 900 kilometers above the surface, about one-eighth the maximum height inferred by another orbiting instrument, the International Ultraviolet Explorer. Accurately measuring the extent of the atmosphere, notes McGrath, will help researchers model how Io influences its Jovian surroundings.

Another team, using Hubble to analyze ultraviolet emissions, found evidence that Io's tenuous atmosphere is extremely patchy. Some areas may have 1,000 times the density of other regions, says John T. Clarke of the University of Michigan in Ann Arbor. This patchy atmosphere may result from volcanic "hot spots" that release more gas than other areas or from selective evaporation of surface frost due to sunlight.

Clarke also reports that the ultraviolet glow from Io's atmosphere vanishes 15 minutes after the moon passes out of reach of the sun's warming rays and into Jupiter's shadow. The finding suggests that the atmosphere condenses onto the moon's surface during the shadow period. Alternatively, he notes, the atmosphere may remain unchanged, but the supply of ions striking the upper atmosphere -- and possibly triggering the glow -- may dramatically decline in shadow.