http://science.nasa.gov/headlines/y2001/ast16oct_1.htm

Dashing through the Snows of Io
NASA Science News
October 16, 2001

Today, NASA's Galileo spacecraft is heading for its closest-ever flyby of Jupiter's moon Io -- an alien world where fiery volcanoes belch sulfurous snow.

Oct. 16, 2001: A little more than two months ago NASA's Galileo spacecraft flew past the north pole of Jupiter's moon Io -- and right through a swarm of sulfurous snowflakes hurled into space by a previously unknown volcano. Scientists were delighted ... and surprised. They hadn't expected the spacecraft to encounter volcanic "ash" so fresh and strange. But on restless Io, the most volcanic world in the solar system, the unexpected is ordinary.

Today Galileo is heading back to Io for another encounter -- its closest ever. At 0123 UT on Oct. 16, the spacecraft will skim just 181 kilometers above Io's surface near the moon's south pole. The close encounter will mark the sixth time Galileo has flown by Io since the spacecraft arrived in Jupiter's neighborhood in 1995.

"Io is always changing, so we're eager to learn what Galileo might show us this time," said Eilene Theilig, project manager for Galileo at NASA's Jet Propulsion Laboratory (JPL). "Maybe it will surprise us as much as it did in the last flyby."

When Galileo sped past Io's north pole on August 6, scientists were watching for activity from a polar volcano named Tvashtar, which had been spewing a plume several hundred km high only seven months earlier. But Tvashtar was quiet. Instead, the spacecraft spotted a new eruption from a previously-unknown volcano 600 km away. The plume, the tallest on record, soared approximately 500 km above Io's surface as Galileo glided through the outskirts of the billowing ejecta.

It proved a fortuitous encounter for scientists who have long sought a fresh sample of Io's volcanic material. Galileo's onboard plasma science instrument detected particles that had rushed out of a vent on the ground no more than a few minutes earlier. "This was totally unexpected," said the leader of that experiment, Louis Frank of the University of Iowa. "We've had wonderful images and other remote sensing of the volcanoes on Io before, but we've never caught the breath from one of them until now."

The particles Galileo caught weren't hot embers, but rather snowflakes. Snow on Io is made of sulfur-dioxide that condenses within high-flying plumes. Although Io's volcanic vents are very hot, much of the moon's surface is frigid (150 degrees or more below 0 C) and Io's thin atmosphere is space-cold. As soon as volcanic gases rise into the air they quickly begin to freeze. Snow forms in the plumes and frosts collect on the surface. Researchers think Galileo detected sulfur-dioxide snowflakes, each consisting of 15 to 20 molecules clumped together.

As Galileo heads for today's flyby, mission planners don't expect to sample another volcanic plume -- although on Io anything is possible. Instead, the primary goal of the encounter, like August's north polar flyby, is to collect magnetic data. Magnetic readings above Io's poles might reveal whether the satellite generates a magnetic field of its own.

Before August, "all of our previous magnetic measurements at Io had been on equatorial passes," noted Galileo project scientist Torrence Johnson in a recent Science@NASA article. "From those measurements we can't tell whether the field at Io is induced by Jupiter's strong magnetic field or produced by Io itself." Polar measurements may give enough additional information to distinguish between those two possibilities. If Io proves to have its own global magnetic field, it could mean that the moon harbors a self-sustaining magnetic dynamo deep within its core -- just as Earth does.

Discovering Io's magnetic field (if it exists) and finding its form could reveal much about the moon's internal structure. Is Io solid throughout? Or mushy and molten? Does Io's crust float above a global ocean of magma? Planetary scientists who wonder why Io's lava is so hot and how Io's non-volcanic mountains rise (and fall) would love to know the answers.

Galileo's southern flight path is not only good for magnetic readings. It will also provide opportunities to study several of Io's interesting volcanic features -- including a recently discovered hot spot in the far south and Loki, the most powerful volcano in the solar system.

The trajectory will also carry Galileo back inside the hazardous environment of Jupiter's intense radiation belts. Electronic components in Galileo's camera and elsewhere in the spacecraft have been degraded by repeated exposure to energetic-particle radiation near the giant planet. Indeed, Galileo has endured more than three times the cumulative dose of radiation it was designed to tolerate. Mission planners marvel at the craft's resiliency.

Galileo's mission was originally scheduled to end in 1997, but has been extended three times to take advantage of the spacecraft's durability. Even Galileo can't last forever, though. The craft is running low on the propellant it uses both for tweaking its trajectory and for adjusting its orientation to point its antenna. And each pass near Jupiter peppers Galileo's electronics with radiation that could eventually disable the spacecraft's sensors.

After today's encounter, say mission planners, Galileo is slated for a final flyby of Io next January -- one last chance to dash through alien snow -- followed by a visit to Jupiter's inner moon Amalthea in November 2002. Galileo's trajectory will carry it past the tiny satellite and away from the giant planet -- but not for long. Galileo's looping orbit will bring it back to Jupiter for a headlong plunge into the crushing pressure of that planet's atmosphere in September 2003. All good things come to an end ... and Galileo is no exception!

If you would like to learn more about Jupiter, Io, and the ongoing Galileo mission, please visit the Jet propulsion Laboratory's Galileo home page:
http://galileo.jpl.nasa.gov. JPL, a division of the California Institute of Technology in Pasadena, manages Galileo for NASA's Office of Space Science in Washington, D.C.