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FOR IMMEDIATE RELEASEOctober 26, 2000
JUPITER'S VOLCANIC MOON IO: STRANGE SHAPES IN A SIZZLING WORLD
The volcanoes on Jupiter's moon Io are like exotic dishes:
they're hot, spicy, and have unfamiliar ingredients, according to
new data from NASA's Galileo spacecraft.
Galileo's near infrared mapping spectrometer instrument has
found extremely high temperatures inside the volcanoes, which are
more abundant than previously believed and contain surprising
substances. The spectrometer detects heat from lava and shows
the location of different materials on Io's surface.
The volcano Pele, named for the mythological Polynesian fire
goddess, showed much higher temperatures inside its volcano than
any found now on Earth -- about 1,500 Celsius (more than 2,700
Fahrenheit). "One of the most interesting questions about Io
is: do all Io's volcanoes erupt such hot lavas, or are most
volcanoes similar to basaltic volcanoes on Earth that erupt
lavas with lower temperatures, about 1,200 Celsius (2,192 F)?"
said Dr. Rosaly Lopes-Gautier, the instrument's science coordinator
for Io at NASA's Jet Propulsion Laboratory (JPL), Pasadena,
California. Billions of years ago on Earth, such hot volcanoes
may have been common.
Before the three Io flybys by Galileo in late 1999 and early
2000, scientists knew Io had two volcanoes with very high
temperatures. As the Galileo spacecraft moved close to Io during
its flybys, it revealed more high-temperature areas than could be
detected by distant observations. This means that Io could
contain many smaller volcanoes with very hot lava.
A small, inactive volcano in the Chaac region was found to
have a bright white floor covered in sulfur dioxide. The fact
that this deposit is confined within the caldera walls indicates
that it could have originally been a liquid, rising from lower
layers. Because Io's atmosphere is so thin that it is almost a
vacuum, the liquid would normally boil off. "Our calculations
indicate that given sufficiently large quantities, some of the
liquid could freeze to form a layer of sulfur dioxide ice inside
the caldera," said Dr. Bill Smythe, a JPL research scientist.
The infrared instrument can measure different substances by
the light that they absorb or reflect. When it showed an unusual
light pattern in a part of Io, scientists knew they had found a
mysterious and unexpected substance. "It was possible that it
could be a mineral containing iron, such as pyrite, present in
silicate lavas. If this was true, we would expect the band to be
stronger in areas of the surface where lava deposits are new,"
said JPL's Dr. Robert Carlson, principal investigator for the
spectrometer team. However, the higher spatial resolution
observations taken during the flybys showed that the opposite is
true: the band is weaker in the dark volcanic areas. "This means
that, whatever the compound is, it probably doesn't come to the
surface in the lava, but instead could be ejected in the volcanic
plumes," Carlson said. Identifying this mysterious compound will
probably require experiments in the laboratory as well as using
the spacecraft observations.
The spectrometer team from JPL is joined in their studies by
Dr. Sylvain Doute of the University of California, Los Angeles (UCLA).
Io may be giving off so much total heat, the best
explanation would be that virtually the whole sphere is covered
with lava spewed so recently it is still cooling, new
calculations suggest. Earlier estimates of Io's heat output
totaled the amounts from active volcanoes and other localized
areas warm enough to be measured. That approach sets a lower
limit to the total output, but excludes about nine-tenths of Io's
surface, said Dr. Dennis Matson, a JPL planetary scientist. He
and four JPL colleagues calculated an upper limit to the estimate
of Io's total heat output.
Their result, about 13.5 watts of energy per square meter
(about 1.3 watt per square foot), is about five times as much as
the heat output from the ground in the Yellowstone hot springs
area of Wyoming, Matson said. Heat output does not include energy
absorbed from the Sun, which is much less on Io than on Earth.
Recent measurements by Galileo of nighttime temperatures on Io's
surface, averaging 90 to 95 degrees Kelvin (-297 to -288 degrees
Fahrenheit), correspond closely with the team's new upper-limit
estimate of the moon's heat flow. The measurements do not vary
much by latitude or time of night, implying that most of the heat
comes from Io itself, rather than absorbed sunshine. That
suggests Io's actual total heat output is close to the new upper-
limit calculation, Matson said. The JPL researchers say that for
Io to be putting out that much heat, most of its surface would
have to be covered with lava in various stages of cooling.
Knowing the moon's heat output helps scientists check theories
about the interiors of both Io and Jupiter, Matson said.
Not all of Io is hot. It has a solid metal core, surrounded
by a rocky mantle, much like Earth. But Earth is only distorted
slightly when the moon's gravity pulls the surface water into high
tides. Jupiter pulls Io's crust into a permanent oval shape, due
to its rotation and the tidal influence of Jupiter. Io has no
long-term strength to resist these forces, behaving as though it
were a fluid. Galileo measured Io's polar gravity when it flew by
this large moon in May 1999. From the gravitational field, says
Dr. Jerry Schubert of the UCLA, it's possible to determine Io's
internal structure. The relationship between the polar and
equatorial gravity shows that Io has a large metallic core, which
is mostly iron. Measuring Io's gravity at the poles confirms an
earlier idea, derived from measurements of Io's gravity at its
equator, that Io's core is made of iron. On Earth, the metallic
core generates Earth's magnetic field. It is not yet known if
Io's metallic core also generates a magnetic field.
Additional information on Galileo is available at
http://galileo.jpl.nasa.gov. The Galileo mission is managed for
NASA's Office of Space Science, Washington, D.C. by JPL, a
division of the California Institute of Technology in Pasadena.