The final
images are in, and the resulting portrait of Jupiter’s moon
Io, after a challenging series of observations by NASA’s
Galileo spacecraft, is a peppery world of even more
plentiful and diverse volcanoes than scientists imagined
before Galileo began orbiting Jupiter in 1995.

Now that Galileo’s observations of Io have ended,
scientists are focusing on trying to understand the big
picture of how Io works by examining details.

Thirteen previously unknown active volcanoes dot infrared
images from Galileo’s final successful flyby of Io,
volcanologist Dr. Rosaly Lopes of NASA’s Jet Propulsion
Laboratory reported today at the spring meeting of the
American Geophysical Union in Washington, D.C.

That brings the total number of known Ionian hot spots to
120. Galileo images revealed 74 of them.

“We expected maybe a dozen or two,” said Dr. Torrence
Johnson, Galileo project scientist at JPL in Pasadena,
Calif. That expectation was based on discoveries by NASA’s
Voyager spacecraft in 1979 and 1980, and subsequent
ground-based observations.

“The volcanoes on Io have displayed an assortment of
eruption styles, but recent observations have surprised us
with the frequency of both giant plumes and crusted-over
lakes of molten lava,” said planetary scientist Dr. Alfred
McEwen of the University of Arizona, Tucson.

Galileo’s latest images, which also show tall slopes
crumbling and surface deposits from two eruptions’ recent
giant plumes, are available online from JPL at
http://www.jpl.nasa.gov/images/io and from the University of
Arizona Lunar and Planetary Laboratory at
http://pirlwww.lpl.arizona.edu/Galileo/Releases .

Some high-resolution views taken as Galileo skimmed past
Io on Oct. 16, 2001, are aiding analysis of the connection
between volcanism and the rise and fall of mountains on
Io. Few of Io’s volcanoes resemble the crater-topped
volcanic peaks seen on Earth and Mars, said planetary
scientist Dr. Elizabeth Turtle of the University of
Arizona. Most of Io’s volcanic craters are in relatively
flat regions, not near mountains, but nearly half of the
mountains Io does have sit right beside volcanic craters.

“It appears that the process that drives
mountain-building — perhaps the tilting of blocks of crust
— also makes it easier for magma to get to the surface,”
Turtle said. She showed a new image revealing that material
slumping off a mountain named Tohil Mons has not piled up in
a crater below, suggesting that the crater floor has been
molten more recently than any landslides have
occurred. Galileo’s infrared-mapping instrument has detected
heat from the crater, indicating an active or very recent
eruption.

From the analysis of Galileo’s observations, scientists
are developing an understanding of how that distant world
resurfaces itself differently than our world does.

“On Earth, we have large-scale lateral transport of the
crust by plate tectonics,” McEwen said. “Io appears to have
a very different tectonic style dominated by vertical
motions. Lava rises from the deep interior and spreads out
over the surface. Older lavas are continuously buried and
compressed until they must break, with thrust faults raising
the tall mountains. These faults also open new pathways to
the surface for lava to follow, so we see complex relations
between mountains and volcanoes, like at Tohil.”

“Io is a weird place,” Johnson said. “We’ve known that
since even before Voyager, and each time Galileo has given
us a close look, we get more surprises. Galileo has vastly
increased our understanding of Io even though the mission
was not originally slated to study Io.”

Extensions to Galileo’s original two-year orbital mission
included six swings close to Io, where exposure to Jupiter’s
intense radiation belts stresses electronic equipment on
board the spacecraft. Researchers presented some results
today from two Io encounters in the second half of
2001. Observations were not made successfully during
Galileo’s final Io flyby, in January 2002, because effects
of the radiation belts put the spacecraft into a
precautionary standby mode during the crucial hours of the
encounter.

Galileo will make its last flyby of a moon when it passes
close to Amalthea, a small inner satellite of Jupiter, on
November 5. No imaging is planned for that flyby. With fuel
for altering its course and pointing its antenna nearly
depleted, the long-lived spacecraft will then loop one last
time away from Jupiter and perish in a final plunge into
Jupiter’s atmosphere in September 2003.

Additional information about Galileo, Jupiter and
Jupiter’s moons is available online at
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, Washington, D.C.