PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
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PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
Contact: Jane Platt
FOR IMMEDIATE RELEASE
May 23, 1997
GALILEO RETURNS NEW INSIGHTS INTO CALLISTO AND EUROPA
Jupiter's icy moon Europa has a metallic core and layered
internal structure similar to the Earth's, while the heavily
cratered moon Callisto is a mixture of metallic rock and ice with
no identifiable central core, according to new results from
NASA's Galileo mission.
In addition, recent plasma wave observations from Galileo
show no evidence of a magnetic field or magnetosphere around
Callisto, but do hint at the prospect of a tenuous atmosphere.
These peer-reviewed findings, reported in today's issue of
Science magazine and the May 16 issue of Nature magazine, are
based on data gathered during Galileo's Nov. 4, 1996 flyby of
Callisto and its Europa encounters on Dec. 19, 1996, and Feb. 20,
"Before Galileo, we could only make educated guesses
about the structure of the Jovian moons," said Dr. John Anderson,
a planetary scientist at NASA's Jet Propulsion Laboratory (JPL),
Pasadena, CA. "Now, with the help of the spacecraft, we can
measure the gravitational fields of the satellites and determine
their interior structure and density. We can determine how the
matter is distributed inside."
While scientists use seismic waves to study Earth's
interior, Galileo performs remote studies of Jupiter's moons by
measuring small changes in the spacecraft's trajectory as it
passes each body.
"These new results from the gravity data are very
consistent with the idea of subsurface oceans on Europa,"
Anderson said. "We know that Europa has a very deep layer of
water in some form, but we don't yet know whether that water is
liquid or frozen."
In an article appearing in the May 23 edition of Science,
Dr. Margaret Kivelson, principal investigator for Galileo's
magnetometer, reports that during its December 1996 pass by
Europa, the magnetometer detected what she described as "a
substantial magnetic signature," and also found that Europa's
north magnetic pole is pointed in an odd direction. Based on
these observations, Kivelson, a professor at the University of
California at Los Angeles, said Europa may have a magnetic field
about one-quarter the strength of Ganymede's magnetic field.
Although the magnetometer was malfunctioning during
Galileo's Europa flyby in February 1997, Kivelson said the
problem is corrected and the device is expected to return
valuable data during its upcoming Europa flybys. The next Europa
encounter is scheduled for November, with a series of flybys
planned during a two-year Galileo extended mission.
Galileo's findings on the Jovian moon Callisto revealed a
much different structure than Europa. Scientists believe that
because Callisto is the Galilean moon located farthest from
Jupiter, it was never subjected to the same gravitational pull as
the inner moons and, therefore, never experienced enough heating
to form different layers.
"Callisto had a much more sedate, predictable and
peaceful history than the other Galilean moons, "Anderson
explained, "and, therefore, it is a more typical solar system
object." The findings indicate Callisto has no core, but instead
has a homogeneous structure, with 60 percent of its ingredients
being rock, including iron and iron sulfide, and 40 percent made
of compressed ice.
Dr. Donald Gurnett, principal investigator for the
Galileo spacecraft's plasma wave instrument, said the instrument
displayed a very minor response from Callisto and, consequently,
showed no evidence of a magnetic field or magnetosphere. The
latest issue of Nature magazine contains these findings, as well
as supportive data from magnetometer
studies of Callisto, as reported by Dr. Krishan Khurana of UCLA.
However, Gurnett added, "There is some evidence of a
plasma source on Callisto, which might indicate a very tenuous
atmosphere." Gurnett is a professor at the University of Iowa at
The Galileo spacecraft was launched in October 1989 and
entered orbit around Jupiter on Dec. 7, 1995. The Galileo
mission is managed by JPL for NASA's Office of Space Science,
Images and other data from Galileo are posted on the
Galileo mission home page on the World Wide Web at