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Contact: Mary Beth Murrill
FOR IMMEDIATE RELEASEDecember 12, 1996
GALILEO TEAM HEARS 'VOICE' OF GANYMEDE; EUROPA FLYBY IS NEXT
Jupiter's big moon Ganymede is not only the size of a planet
-- it sounds like one too, as heard in audio recordings made from
data returned by NASA's Galileo spacecraft released today.
Characterized by a soaring whistle and hissing static,
Ganymede's song reveals that the solar system's largest moon is
also the only one known to possess a planet-like, self-generated
magnetic cocoon called a magnetosphere, which shields the moon
from the magnetic influence of its giant parent body, Jupiter.
Published in the scientific journal Nature this week, these
new Galileo findings and other measurements from several Galileo
sensors were presented today in a news briefing held at NASA's
Jet Propulsion Laboratory. Taken together, scientists say the new
Ganymede findings are painting a portrait of a body that from the
inside out closely resembles a planet like Earth rather than
other moons in the solar system.
"The data we get back is in the form of a spectrogram, and
reading it is kind of like looking at a musical score," said Dr.
Donald Gurnett, University of Iowa physicist and principal
investigator on Galileo's plasma wave instrument. It was his
experiment that first detected the telltale signals of a
magnetosphere during the spacecraft's close flybys of that moon
on June 27 and Sept. 6.
Gurnett said the unique pattern of frequencies his
instrument detected is characteristic of a magnetosphere and
closely matches his previous studies of the magnetospheres of
Earth, Saturn and Jupiter. "The instant I saw the spectrogram, I
could tell we had passed through a magnetosphere at Ganymede,"
Describing the electromagnetic wave activity that his
experiment detected at Ganymede, Gurnett said the approach to the
large moon was relatively quiet, "until all of a sudden, there's
a big burst of noise that signals the entry into Ganymede's
magnetosphere. Then, for about 50 minutes, we detected the kinds
of noises that are typical of a passage through a magnetosphere.
As we exited the magnetosphere, there was another big burst of
Gurnett checked with Dr. Margaret Kivelson of UCLA,
principal investigator for the magnetometer experiment on the
spacecraft. Kivelson quickly confirmed the detection of a large
increase in magnetic field strength near Ganymede. Related data
from the two close flybys have confirmed that Ganymede has a
magnetic field of its own.
Using extremely precise data from tracking the spacecraft,
investigators on Galileo's celestial mechanics team also have
been able to confirm that Ganymede's interior is differentiated,
probably having a three-layer structure. "These data show clearly
that Ganymede has differentiated into a core and mantle which is,
in turn, enclosed by an ice shell," said JPL planetary scientist
Dr. John Anderson, team leader on the Galileo radio science
"Combined with the discovery of an intrinsic magnetic field,
our gravity results indicate that Ganymede has a metallic core
about 400 to 1,300 kilometers (250 to 800 miles) in," said
Anderson. "This is surrounded by a rocky silicate mantle which
is, in turn, enclosed by an ice shell about 800 kilometers (500
miles) thick. Depending on whether the core is pure iron or an
alloy of iron and iron sulfide, it could account for as little as
1.4 percent or as much as one-third of the total mass of
This differentiated structure is the most likely cause of
Ganymede's newly discovered magnetic field which, in turn, gives
rise to the magnetosphere, reported UCLA geophysicist Dr. Gerald
Schubert, an interdisciplinary investigator on Galileo's science
team. Scientists suspect Ganymede's magnetic field is generated
the same way as Earth's, through the dynamo action of the fluid
mantle rotating above a metallic core. The only other solid
bodies in the solar system known to have magnetic fields are
Mercury, Earth and possibly Jupiter's volcanic moon Io.
"At Ganymede, the magnetic field is strong enough to carve
out a magnetosphere with clearly defined boundaries within
Jupiter's magnetosphere, making it the only 'magnetosphere within
a magnetosphere' known in the solar system," Kivelson said.
The strong magnetic characteristics of Ganymede, in
combination with its residence within Jupiter's powerful
magnetosphere, makes the Jovian environment even more intriguing,
said Galileo Project Scientist Dr. Torrence V. Johnson. "The
physics taking place within Jupiter's magnetosphere are of great
interest to scientists attempting to understand the complex
interplay of magnetic forces and matter throughout the universe,"
Data from the plasma wave instrument can be heard and seen
on the Galileo home page at the following URL:
Newly received Galileo images of Jupiter's moon Callisto and
one of Europa also were released at the briefing. Scientists were
surprised by the lack of small craters visible in the images.
Some small craters appear to have been softened or modfied by
downslope movement of debris, revealing ice-rich surfaces.
Galileo's next moon encounter occurs Dec.18-19, when the
spacecraft makes its first close approach to Europa, the moon
thought to harbor a liquid ocean beneath its icy surface. Results
from that flyby will be radioed to Earth starting in the last
half of December and continuing through early February 1997.
The Europa image received from the spacecraft earlier this
week shows the cracked surface of this moon in greater detail
than it has been seen before. The new image shows an area 238 by
225 kilometers (150 by 140 miles) that has been highly disrupted
by fractures and ridges.
Arizona State University planetary scientist Kelly Bender of
Galileo's imaging team said that symmetric ridges in the dark
bands suggest that Europa's surface crust was separated and
filled with darker material, somewhat analogous to spreading
centers in the ocean basins of Earth. Although some impact
craters are visible, their general absence indicates a youthful
surface, she said.
The youngest ridges, such as the two features that cross the
center of the picture, have central fractures, aligned knobs and
irregular dark patches. These and other features could indicate
cryovolcanism, or processes related to the eruption of ice and
Galileo was launched in 1989 and entered orbit around
Jupiter on December 7, 1995. The Galileo mission is managed by
JPL for NASA's Office of Space Science, Washington, DC.
Images and other data received from Galileo are posted on
the Galileo mission home page on the World Wide Web at