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Contact: Mary Beth Murrill, (818) 354-6478
FOR IMMEDIATE RELEASE
October 23, 1996
GALILEO FINDS HIGH-ALTITUDE IONOSPHERE AT JUPITER'S MOON IO
Scientists participating in NASA's Galileo mission have
discovered that the Galileo spacecraft may have flown though a
dense, high-altitude ionosphere during its encounter with
Jupiter's volcanic moon Io last December. The discovery suggests
that Io's atmosphere is time variable and is made of volcanic gas
lofted to very high altitudes.
An ionosphere is a region of electrically charged gas that
exists at the top of some planetary atmospheres. The surprising
discovery is being reported by Galileo scientists this week at a
meeting of the American Astronomical Society's Division of
Planetary Sciences being held in Tuscon, AZ, along with other
Galileo results, including remarkable new images of the planet
and its moons.
Sensors on the spacecraft found a very dense region of
ionized oxygen, sulfur and sulfur dioxide at 900 kilometers (550
miles) above Io that must be pumped into that region by Io's
relentless volcanic activity, said Dr. Louis A. Frank of the
University of Iowa and principal investigator on Galileo's plasma
science experiment. Instead of being swept away by Jupiter's
rotating magnetosphere as anticipated, the ionized gases
surprisingly remain with Io, he said.
"Passage of the Galileo spacecraft through an ionosphere was
not expected because images of the volcanic plumes previously
taken with the Voyager spacecraft indicated that the plume
heights extended only to a few hundred kilometers or less," said
Frank. A radio occultation by NASA's Pioneer 10 spacecraft in
1973 indicated ionospheric heights only 50 to 100 kilometers
(about 30 to 60 miles) above the surface, he added. "No one
expected to see to see this at 900-kilometer altitude." The
difference between what Pioneer saw and what Galileo has observed
indicates that Io's atmosphere and ionosphere are variable and
may grow and shrink with more or less volcanic activity, Frank
said.
The results may lend credence to previous theories proposed
by Galileo Project Scientist Dr. Torrence Johnson of NASA's Jet
Propulsion Laboratory, Pasadena, CA, that invisible "stealth
plumes" deliver volcanic gases to great heights above Io. Io's
weak gravity field apparently permits the invisible gases
emanating from the volcanoes to reach extraordinary heights far
beyond the lower altitudes achieved by the dust and other
volcanic ejecta that reflects sunlight and can be seen in images,
Frank said.
Io Electron Beams
In a related finding, the energetic particle detector on the
spacecraft measured intense, bi-directional electron beams that
are aligned with Galileo's magnetic field in Io's vicinity. The
beams are similar to those that impinge on Earth's atmosphere to
produce aurora and, also, positive ions and electrons in Earth's
atmosphere.
Dr. Donald J. Williams of the Applied Physics Laboratory of
Johns Hopkins University and principal investigator on the
energetic particle experiment, said the electron beams span the
energy range of 15 kiloelectron volts to 190 kiloelectron volts
and represent an energy deposition into Jupiter's atmosphere of
up to 1 billion watts.
"This is sufficient energy input into the Jovian atmosphere
to produce visible auroral emissions," Williams said. "These
beams are a signature of remarkable particle acceleration
processes that occur in the vicinity of Io – processes that are
thought to be linked to Io's motion through Jupiter's plasma and
magnetic field environment." Additional work is required to
determine whether the beams play a role in producing some of the
auroral emissions observed in Jupiter or if they are related to
radio emissions that have been correlated with Io's orbital
motion.
The electron beams must have a role in maintaining the Io
torus, the doughnut-shaped cloud of ionized gases that flows
between Jupiter and Io, Galileo scientists said. Auroral
emissions in Io's atmosphere are one likely result of the
electron beams, they reported, and the two-way electron highway
that the beams produce between Jupiter and Io must contribute to
some of the aurora observed in Jupiter's atmosphere as well.
Io Volcano Shifting?
Several images recently returned by Galileo show new details
of surface features on the moon Ganymede and Io. One new image
of the active volcano Prometheus on Io has been compared to one
of the same features taken by NASA's Voyager spacecraft 17 years
ago, and shows that the plume is now erupting from a position
about 75 kilometers (about 46 miles) west from where the hot
spot resided in 1979. It is not known if the plume source is the
same or if the plume is now emanating from a new source.
Overall, scientists studying Galileo images of Io are observing
that a wide variety of surface changes have occurred in the
nearly two decades since a spacecraft last visited Jupiter's
system.
Frosted Rims on Ganymede
Bright white areas seen around the circular rims of high-
latitude impact craters on Ganymede in new Galileo images of that
moon are likely water-ice frosts, Galileo scientists reported.
Even though the Sun is shining from the south, the north-facing
walls of the ridges and craters are brighter than the walls
facing the Sun. Images of regions elsewhere on Ganymede show
more details of the remarkable juxtaposition of newer and older
fractured and faulted terrain that characterizes so much of this
big moon's surface. A stereoscopic view of Ganymede has also
been produced with two images of the Galileo Reggae region (one
was taken during the first Ganymede flyby in June and the second
was acquired in the September flyby). The image, which was
computer-reconstructed by imaging scientists at JPL, shows new
topographic information about the moon.
Galileo science team members are reporting on numerous other
new findings about Jupiter and its moons:
The photopolarimeter radiometer experiment produced heat
maps of Jupiter's Great Red Spot, the day side of the moon
Europa, the night side of Io, and both the day and night sides of
Ganymede during the spacecraft's flyby of Ganymede in June. The
images of the Great Red Spot show temperatures of the atmosphere
at the 250 and 500 millibar pressure levels, much like
terrestrial weather maps. The Great Red Spot is colder than its
surroundings, consistent with earlier Voyager and Earth-based
observations in which the spot is modeled as an anticyclonic
vortex with central up-welling balanced by subsidence at its
edges.
The radiometer also produced temperature data for Io
indicating a nighttime temperature about 80 Kelvin to 85 K, or -
375 degrees Fahrenheit to -380 degrees F.
The first midday temperature for Europa, 128 Kelvin (-229
Fahrenheit), has allowed the radiometer instrument team to
determine that that moon has a more porous or "fluffy" ice
surface than the other moons. Researchers said that such
porosity indicates Europa's surface is covered with finely
powdered ice grains.
The near-infrared mapping spectrometer instrument and
Galileo's solid state imaging camera measured hot regions on Io
including erupting volcanoes and individual volcanoes, finding
temperatures off 420 Kelvin to 620 K (296 degrees Fahrenheit to
656 F).
On Callisto and Ganymede, the near-infrared mapping
spectrometer found surface features indicating the presence of
hydrated materials, or possibly carbon dioxide frost.
The Jet Propulsion Laboratory, Pasadena, CA, manages the
Galileo mission for NASA's Office of Space Science, Washington,
DC. The Galileo mission home page is on the World Wide Web at
http://www.jpl.nasa.gov/galileo. Images from the mission are
also posted on NASA's Planetary Photojournal at
http://photojournal.jpl.nasa.gov.
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