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Jupiter’s fiery moon Io is providing scientists with a
window on volcanic activity and colossal lava flows similar to
those that raged on Earth eons ago, thanks to new pictures and
data gathered by NASA’s Galileo spacecraft.

The sharp images of Io were taken on Oct. 11 during the
closest-ever spacecraft flyby of the moon, when Galileo dipped to
just 611 kilometers (380 miles) above Io’s surface. The new
data reveal that Io, the most volcanic body in the solar system,
is even more active than previously suspected, with more than 100
erupting volcanoes.

“The latest flyby has shown us gigantic lava flows and lava
lakes, and towering, collapsing mountains,” said Dr. Alfred
McEwen of the University of Arizona, Tucson, a member of the
Galileo imaging team. “Io makes Dante’s Inferno seem like
another day in paradise.”

Ancient rocks on Earth and other rocky planets show evidence
of immense volcanic eruptions. The last comparable lava eruption
on Earth occurred 15 million years ago, and it’s been more than 2
billion years since lava as hot as that found on Io, reaching
1,482 degrees Celsius or 2,700 degrees Fahrenheit, flowed on Earth.

“No people were around to observe and document these past
events,” said Dr. Torrence Johnson, Galileo project scientist at
NASA’s Jet Propulsion Laboratory (JPL), Pasadena, CA. “Io is the
next best thing to traveling back in time to Earth’s earlier
years. It gives us an opportunity to watch, in action, phenomena
long dead in the rest of the solar system.”

The new data focus on three of Io’s most active volcanoes —
Pele, Loki and Prometheus. The vent region of Pele has an
intense high-temperature hot spot that is remarkably steady,
unlike lava flows that erupt in pulses, spread out over large
areas, and then cool over time. This leads scientists to
hypothesize that there must be an extremely active lava lake at
Pele that constantly exposes fresh lava. Galileo’s camera
snapped a close-up picture showing part of the volcano glowing in
the dark. Hot lava, at most a few minutes old, forms a thin,
curving line more than 10 kilometers (6 miles) long and up to 50
meters (150 feet) wide. Scientists believe this line is glowing
liquid lava exposed as the solidifying crust breaks up along the
caldera’s walls. This is similar to the behavior of active lava
lakes in Hawaii, although Pele’s lava lake is a hundred times

Loki, the most powerful volcano in the solar system,
consistently puts out more heat than all of Earth’s active
volcanoes combined. Two of Galileo’s instruments — the
photopolarimeter radiometer and near-infrared mapping
spectrometer — have provided detailed temperature maps of Loki.
“Unlike the active lava lake at Pele, Loki has an enormous
caldera that is repeatedly flooded by lava, over an area larger
than the state of Maryland,” said Dr. Rosaly Lopes-Gautier of
JPL, a member of the spectrometer team.

Observations of Prometheus made early in the Galileo
mission showed a new lava flow and a plume erupting from a
location about 100 kilometers (60 miles) west of the area where
the plume was observed in 1979 by NASA’s Voyager spacecraft. New
Galileo data clarify where lava is erupting, advancing, and
producing plumes. The most unexpected result is that the 75
kilometer- (50 mile-) tall plume erupts from under a lava flow,
far from the main volcano. The plume is fed by vaporized sulfur
dioxide-rich snow under the lava flow.

Mountains on Io are much taller than Earth’s largest
mountains, towering up to 16 kilometers (52,000 feet) high.
Paradoxically, they do not appear to be volcanoes. Scientists
are not sure how the mountains form, but new Galileo images
provide a fascinating picture of how they die. Concentric ridges
covering the mountains and surrounding plateaus offer evidence
that the mountains generate huge landslides as they collapse
under the force of gravity. The ridges bear a striking resemblance
to the rugged terrain surrounding giant Olympus Mons on Mars.

Scientists hope to learn more about dynamic Io when Galileo
swoops down for an even closer look on Nov. 25 from an altitude
of only 300 kilometers (186 miles). Because Io’s orbit is bathed
in intense radiation from Jupiter’s radiation belts, there is a
risk of radiation damage to spacecraft components. In fact,
several spacecraft systems sustained damage during the October
flyby. Given these radiation risks, the Io flybys were scheduled
near the end of the spacecraft’s two-year extended mission.

New Io images taken by the spacecraft are available at: or

Galileo entered orbit around Jupiter and its moons on
Dec. 7, 1995, for a two-year prime mission. JPL manages the
Galileo mission for NASA’s Office of Space Science, Washington, DC.
JPL is operated for NASA by the California Institute of
Technology, Pasadena, CA.