Galileo Returns New Insights into Callisto and Europa

    Douglas Isbell
    Headquarters, Washington, DC              May 23, 1997
    (Phone: 202/358-1753)

    Jane Platt
    Jet Propulsion Laboratory, Pasadena, CA
    (Phone: 818/354-5011)

    RELEASE: 97-110

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, 1997.

"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 Iowa City.

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, Washington, DC.

Jupiter Galileo

##### Calvin J. Hamilton.