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JET PROPULSION LABORATORY
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FOR RELEASE WEDNESDAY, JULY 10, 1996 AT 11 A.M. PDT
GALILEO SPACECRAFT MAKES NEW DISCOVERIES AT GANYMEDE
NASA's Galileo spacecraft has returned stunning close-ups of
Jupiter's moon Ganymede revealing that the face of the huge
satellite has been extensively bombed by comets and asteroids and
dramatically wrinkled and torn by the same forces that make
mountains and move continents on Earth.
"These images have exceeded our wildest expectations," said
Dr. Michael Belton, leader of Galileo’s imaging team.
At the same time, scientists studying data from space
physics instruments on the spacecraft have made the major
discovery that planet-size Ganymede possesses its own
magnetosphere -- a bubble-shaped region of charged particles that
surrounds many of the planets but has never been found to exist
around a moon. The finding indicates that Ganymede, which is
three-quarters the size of Mars, very likely creates its own
magnetic field. Possible sources of a magnetic field include a
molten iron core or even a thin layer of conducting salty water
underneath its icy crust.
"What we've found is a magnetosphere within a
magnetosphere," said Galileo Project Scientist Dr. Torrence V.
Johnson at NASA's Jet Propulsion Laboratory, Pasadena, CA.
"While we expected some degree of interaction between Ganymede
and Jupiter's magnetic environment, the size and the effect at
Ganymede was completely unexpected," he said.
The crisp new images and magnetospheric findings were
revealed in data returned by Galileo in the days since its first
flyby of Ganymede on June 27, when the spacecraft came within
just 835 kilometers (519 miles) of the big moon. Ganymede is the
largest moon in the solar system. It is made of about equal
proportions of rock and water ice. It is one of Jupiter's four
large satellites that will be repeatedly visited by the Galileo
spacecraft over the course of its two-year mission in orbit
around the giant planet. Galileo entered orbit around Jupiter on
December 7 last year. The spacecraft was launched from Earth on
October 18, 1989.
The discoveries announced today are based on just a small
portion of the data gathered and returned from the Ganymede flyby
and mark the start of a steady stream of images and other
information to be returned from Galileo over the next 18 months.
The data were returned using new software radioed to the
spacecraft earlier this year that allows Galileo to send back its
scientific findings in shorthand form. This helps compensate for
the loss of the use of Galileo's high-gain antenna and allows
Galileo to return its findings via the smaller low-gain antenna
also on the spacecraft.
These first images show two of the regions selected for
close photographic study on Galileo's first pass of Ganymede
yielded surprising new information about its geological past.
The areas, called Galileo Regio and Uruk Sulcus, show ancient
cratered ice fields adjacent to or overlain by younger ice
volcanic plains, of ridged ice mountains, deep furrows and smooth
broad basins that are products of tectonic forces. About half of
Ganymede's older cratered surface appears to have been resurfaced
by younger volcanic and tectonic activity.
"These images reveal fundamental details about how features
seen by Voyager formed and show us age relationships and
sequences that turn our previous thinking upside down," said
imaging team member Dr. James Head of Brown University.
The discovery of Ganymede's magnetosphere was made by space
physicists using data from Galileo's plasma wave spectrometer,
which measures variations in electromagnetic waves in Jupiter's
environment and from the magnetometer, which measures the
strength and direction of magnetic fields. Both instruments were
sending data to Earth during the Ganymede flyby while recording
even more detailed information to be returned later this month.
The plasma wave spectrometer also showed that the densities
of charged particles around Ganymede increased by a factor of
more than 100 near Galileo's closest approach, "This indicates
that Ganymede is surrounded by a thin ionosphere," said Dr.
Donald A. Gurnett of the University of Iowa and principal
investigator on the plasma wave spectrometer experiment. "The
existence of an ionosphere suggests that Ganymede also probably
has a tenuous atmosphere," he said.
As the spacecraft approached Ganymede, the magnetometer
found the measured field was as expected at that position in
Jupiter's powerful field -- fairly uniform and pointed in a
southerly direction. But as the spacecraft crossed into the
region where the plasma wave spectrometer sensed signals
characteristic of a magnetosphere, the field increased in
strength by a factor of nearly five and abruptly changed
direction to "point" at Ganymede itself, Dr. Margaret Kivelson of
the University of California at Los Angeles, principal
investigator of the magnetometer experiment.
Taken together, these two measurements strongly suggest that
Ganymede is the first known moon with its own magnetosphere and
the first example ever seen of a "magnetosphere within a
"We knew Ganymede was an interesting place," said Johnson.
"What we have just found makes it even more exciting."
The new discoveries will be quickly followed up by other
data to being returned by the spacecraft this summer. All the
experiments on Galileo that measure magnetic fields and particle
recorded detailed data during the close approach and these data
will be played back from the tape recorder in the next two
months. "With all the data in hand, we will gain better insight
into what is causing the strange environment around this moon,"
said Johnson. The Galileo science and engineering teams are
planning three more close flybys of Ganymede over the next 18
months, which will take the spacecraft to different regions of
the big moon's magnetosphere and allow close study of other
regions of its surface.
The Galileo mission is managed by JPL.