Contact: Susan Mitgang or Mary Beth Murrill

Abundant volcanoes, including some that turn on and off, and
puzzling surface textures are some of the new findings from
NASA’s Galileo spacecraft during the closest flyby ever of
Jupiter’s moon Io.

Images and other data gathered by Galileo during a flyby of
Io on February 22 will be released today at the American
Geophysical Union’s spring meeting in Washington, D.C.

The high-resolution observations made by one of the
instruments on Galileo, the near-infrared mapping spectrometer,
revealed 14 volcanoes in a region that was previously known to
have only four. The region covers about five percent of Io’s
surface and is about three times larger than Texas. Before the
flybys, Io was known to have only about 81 active volcanoes.

“Since the distribution of active volcanoes on Io appears to
be uniform, we can expect Io to have some 300 active volcanoes,
most of which have not been discovered,” said Dr. Rosaly Lopes-
Gautier, research scientist at NASA’s Jet Propulsion Laboratory,
Pasadena, Calif.

Scientists have detected changes in the months between the
three different flybys in October 1999, November 1999 and
February 2000. Some of the smaller, fainter volcanoes appear to
turn on and off, changing from hot and glowing to cool and dim
within a few weeks. The larger or brighter volcanoes tend to
remain active for years or even decades, based on previous
observations by Galileo and the Voyager spacecraft that flew
through the Jovian system in 1979.

Loki, the most powerful volcano in the solar system, was
scanned by Galileo’s photopolarimeter/radiometer during the
October 1999 flyby and again this past February. These two close-
up views showed Loki near the beginning and end of one of its
periodic major eruptions.

“Most of the surface of its caldera, a region of more than
10,000 square kilometers (about 4,000 square miles) or half the
size of Massachusetts, seems to have been covered by hot lava in
the intervening four and a half months,” said Dr. John
Spencer of Lowell Observatory, Flagstaff, Ariz., a co-
investigator for the radiometer instrument.

New images show that Chaac Patera has a caldera wall that is
about 2.8 kilometers (1.7 miles) high with a 70-degree slope —
about twice as high and steep as the typical slopes of the Grand

“The wall rocks must be very strong to support this
topography,” said Dr. Alfred S. McEwen of the University of
Arizona, Tucson, a member of the Galileo imaging team. “Each
volcanic center on Io is proving to have unique characteristics.”

Observations also show a smaller caldera filled with bright
white deposits containing sulfur dioxide that is purer than at
any other place observed on Io. Scientists believe it may be a
frozen layer of sulfur dioxide ice.

One of the most spectacular volcanic eruptions observed by
Galileo was from Tvashtar Catena in November 1999. The February
images show that the eruption seen in November has waned, but
there is a new eruption with extremely hot lava nearby.

The February Io flyby has yielded more images with higher
resolution than previous flybys. They show unprecedented views
of small surface areas that give new clues about the volcanic
terrain but also reveal landforms that are perplexing to
geologists. There are views of one surface area that appears
eroded, showing thin, alternating bright and dark layers.
Scientists don’t yet understand how the layers formed and were
eroded or how other plains textures on Io have formed.

“There are processes on Io for which we have no terrestrial
experience,” said McEwen. “Strange new observations like these
will provide fodder to current and future scientists for
understanding the processes that have shaped this fascinating

The new images of Io are available at . More images of Io
can be found at . Additional
information and images taken by Galileo are available at

Galileo has been studying Jupiter and its moons for four and
a half 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.. 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.