Aug 3, 2001

Another Daring Adventure for Galileo




NASA's durable Galileo space probe is heading for a close encounter with an alien volcano on August 6, 2001.


Marshall Space Flight Center


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August 3, 2001: NASA's Galileo spacecraft is about to buzz the north pole of Jupiter's sulfurous moon Io to get unprecedented magnetic measurements and to examine the site of a recent volcanic eruption.


The durable robot will skim about 200 kilometers above Io's surface at 12:59 a.m. EDT on August 6th. A few seconds later, Galileo will speed over an area called Tvashtar that was belching a giant plume of volcanic gases only seven months ago. The volcano might still be active, say scientists. If so, Galileo could fly right through a volcanic plume for the first time.

Right: This December 2000 photo shows Tvashtar Catena, a north polar volcanic region that Galileo will skim on Monday, August 6. See the movie "Io: A Volcanic Inferno" for more information.

Io, the innermost of Jupiter's four largest moons, is the most volcanic world in our solar system. It's peppered with hundreds of volcanoes -- of which half a dozen or so might be active at any given moment.

What makes Io so fiery? Tides. But the tides on Io are not like ocean tides we're familiar with on Earth. The gravitational fields of Jupiter and its other large moons raise tidal bulges in the solid crust of Io that are as high as a 30-story building. As Io orbits the giant planet the bulge moves, flexes the crust, and heats Io's interior like a paper clip bent rapidly back and forth. This is the source of energy for volcanoes that spew lava almost constantly.




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"Plumes in the polar regions of Io appear to be infrequent and short-lived, so we don't know whether [the one in Tvashtar] will still be there or not," says Eilene Theilig, Galileo project manager at NASA's Jet Propulsion Laboratory (JPL).

The Galileo flight team at JPL chose to fly over Io's north pole, not so much to spy volcanoes, but rather because magnetic readings above the pole might reveal whether Io generates a magnetic field of its own.

"All of our previous magnetic measurements at Io have been on equatorial passes," explains Torrence Johnson, Galileo project scientist at JPL. "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," Johnson said. 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.

Galileo's flight plan, designed to search for the moon's magnetic north pole, will carry it directly over the dynamic Tvashtar volcano, where Galileo has spotted activity many times before. In November 1999, for instance, Galileo imaged an active "curtain of fire" eruption at Tvashtar. The volcano was hurling magma more than 1.5 kilometers high. The eruption had become much less violent and had shifted positions by the time Galileo returned three months later.


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Above: This pair of images taken by NASA's Galileo spacecraft captures a dynamic eruption at Tvashtar Catena, a chain of volcanic bowls on Jupiter's moon Io. They show a change in the location of hot lava over a period of a few months in 1999 and early 2000. [more]

Then, in late December 2000, NASA's Cassini spacecraft passed by Jupiter en route to Saturn. Both Cassini and Galileo saw a tenuous plume rising from Tvashtar -- the volcano was still active! The plume rose as much as 385 kilometers high. Sulfurous material falling back to Io's surface created a red ring about 1,400 kilometers in diameter. (You can see the red ring around Tvashtar clearly in the image at the beginning of this story.)

If the plume is still active now, and the same size, Galileo will fly through about the top quarter of it. The project's scientists and engineers believe the plume material at that altitude is gaseous and very thin, without particles large enough to penetrate the spacecraft.

Whether the plume is active or not, Galileo will look for changes in the Tvashtar region. "We'll be trying to figure out just where the plume erupted from," Johnson said.


Right: A computer-generated animation by Digital Radiance, Inc. shows what a volcano erupting on Io might look like. [ ]

As Galileo passes Io, it will be out of communication with Earth. NASA's Deep Space Network, which provides the communication link for interplanetary spacecraft, has a large antenna temporarily out of service in Spain, the only one of the network's three sites that will have Jupiter in view above it during the flyby. The antenna is being upgraded to help handle an increased number of missions that will need communications in 2003 and 2004. Confirmation of Galileo's status during the flyby will not be received until several hours afterwards. Images and other data from the flyby will be returned gradually over the following two months.

Galileo has been orbiting Jupiter in elongated loops since 1995. Four of its previous 31 close flybys of Jupiter's moons have been by Io. It will swing near Io twice more after next week, once in October and again in January, then near the small inner moon Amalthea once before a mission-ending plunge into the crushing pressure of Jupiter's atmosphere in 2003. Galileo's mission was originally scheduled to end in 1997, but has been extended three times to take advantage of the spacecraft's extraordinary durability.



The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages Galileo for NASA's Office of Space Science, Washington, D.C.




Alien Volcanoes
August 3, 2001

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Thursday's Classroom

These stories and lessons are based on the Science@NASA article
"Another Daring Adventure for Galileo"


KID'S STORIES: 3rd and 4th grade -- 5th to 8th grade -- Glossary


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Web Links

Io's Alien Volcanoes -- This 1999 Science@NASA story explores what we know about Io's strange volcanoes.

Io, A Volcanic Inferno -- a movie (with soundtrack) from JPL explains the basics of Io, its volcanoes, and the moon's complex interaction with Jupiter.


The Galileo Mission - read all about NASA's intrepid Galileo spacecraft at the mission's home page from JPL


High Tide on Io - This web site from JPL's "The Space Place" explains why Io is so volcanic.

Terrible Tides -- This March 2000 Science@NASA story describes how icy Europa and fiery Io are shaped by similar tidal forces.

Io, A Continuing Story of Discovery -- a substantial collection of links to resources about Io and its dynamic volcanoes

The Prometheus Plume -- a stunning picture of one of Io's largest volcanoes, from Goddard's Astronomy Picture of the Day

JPL's Jupiter Photojournal -- This wonderful site offers a comprehensive collection of Io imagery.


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