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Detailed analysis of Jupiter’s moon Io reveals a colorful,
active world full of surprises, according to five reports
published in the May 19 issue of Science, and based on new
results from NASA’s Galileo spacecraft and Hubble Space

The reports describe giant, erupting plumes migrating with
lava flows, red and green deposits that change as unstable sulfur
compounds condense from huge plumes, and mountains that may split
and slide sideways for hundreds of kilometers, or miles.

Galileo observations of Prometheus reveal a volcanic field
similar to Hawaii’s volcanoes, but more active and much larger.
Prometheus features an 80-kilometer (50-mile) tall plume of gas
and particles erupting from near the end of the lava flows, like
where Hawaiian flows enter the ocean. This is Io’s most
consistently active plume. Its size and shape have remained
constant since at least 1979, but the plume location wandered
about 85 kilometers (53 miles) to the west between 1979 and 1996.

“The main vent of the volcano didn’t move, but the plume
did,” said Dr. Rosaly Lopes-Gautier of NASA’s Jet Propulsion
Laboratory, Pasadena, Calif., lead author of one of the reports.

“This type of behavior has never been seen on Earth,” said
Dr. Susan Kieffer of Kieffer Science Consulting, Inc., Ontario,
Canada, lead author of a Science report. Kieffer and her
colleagues suggest that the Prometheus plume is fed when a
“snowfield” of sulfur dioxide and/or sulfur vaporizes under the
lava flow and material erupts through a rootless conduit in the

Scientists had speculated that bright red material on Io
came from unstable forms of sulfur condensing from sulfur gas.
By combining Galileo and Hubble Space Telescope results,
scientists have learned more about the role of sulfur in Io’s
volcanoes. While Galileo carried out the first of three recent
Io flybys in October 1999, Hubble scanned Io with its ultraviolet
spectrograph to measure the composition of gases escaping from
volcanoes. Hubble detected a surprise — a 350 kilometer (220
mile) high cloud of gaseous sulfur in the plume ejected by the
volcano Pele. The sulfur gas is a specific type, with sulfur
atoms joined in pairs, that had never before been seen on Io; it
is stable only at the very high temperatures found in the throats
of Io’s volcanoes. When these molecules fall onto Io’s frigid
surface (about -160 Celsius or -250 Fahrenheit) away from the
volcanoes, they probably recombine into larger molecules with
three or four sulfur atoms. The latter types of sulfur are red,
so the Hubble results explain the 1,200-kilometer (750-mile)
wide, red debris ring around Pele.

“These Hubble findings should help scientists understand the
chemistry of Io’s interior,” said Dr. John Spencer of Lowell
Observatory, Flagstaff, Ariz., lead author of two of the Science

Galileo has found many other smaller, red patches near Io’s
active volcanoes, where this sulfur conversion process probably
also occurs. The red deposits are found near calderas or shield
volcanoes where lava first reaches the surface, often distant
from plumes like Prometheus where lava flows apparently vaporize
surface materials.

The composition of bright green materials on Io has been
puzzling. In some places, it appears that when red material is
deposited onto fresh lava flows, especially on caldera floors, it
transforms into green material. It is possible that the surfaces
are still warm, which accelerates the transformation of the red
types of sulfur and the sublimation of sulfur dioxide.
Eventually both red and green materials acquire the pale yellow
color that is characteristic of ordinary yellow sulfur, made of
rings of eight sulfur atoms.

Although Io is the most volcanically active body in the
solar system, the mountains (up to 16 kilometers or 10 miles
high) are not volcanoes. They have no volcanic vents or flows;
instead, they appear to be giant tilted blocks of crust. Giant
depressions on Io are thought to be calderas formed by collapse
over empty magma chambers. Unlike Earth’s calderas, many Io
depressions have very straight margins, sharp corners, and are
located next to mountains. In new images of the Hi’iaka Patera
depression and adjacent mountains, it looks as though two
mountain blocks have split and slid apart by 145 kilometers (90
miles), forming a pull-apart basin like California’s Death Valley
or Salton Sea. This is surprising because such large-scale
lateral movements on Earth are caused by plate tectonics, but
there are no indications of a similar process on Io.

“We consider it more likely that lateral movements may be
driven by deep ‘mantle plumes’ of rising hot rock masses within
Io,” said Dr. Alfred McEwen of the University of Arizona, Tucson,
lead author of one of the papers.

New images are at

Galileo has been studying Jupiter and its moons for 4-1/2
years. It completed a two-year primary mission in December 1997
and a two-year extended mission in December 1999. Galileo is
continuing its studies under yet another extension, the Galileo
Millennium Mission. On Sat., May 20, the spacecraft will fly by
Jupiter’s moon Ganymede, the largest moon in the solar system, for
the first time since May 7, 1997. JPL, a division of the
California Institute of Technology in Pasadena, manages the Galileo
mission for NASA’s Office of Space Science, Washington, D.C.