ARE THERE HABITABLE MOONS AROUND EXTRASOLAR JOVIAN PLANETS?
December 12
Source: Larry Claes
Douglas Isbell
Headquarters, Washington, DC December 12, 1996
(Phone: 202/358-1753)
Mary Beth Murrill
Jet Propulsion Laboratory, Pasadena, CA
(Phone: 818/354-5011)
RELEASE: 96-255
BIG ICY MOON OF JUPITER FOUND TO HAVE
A 'VOICE' AFTER ALL; EUROPA FLYBY NEXT FOR GALILEO
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 (JPL),
Pasadena, CA. 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," Gurnett said.
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 noise."
Gurnett checked with Dr. Margaret Kivelson of the
University of California at Los Angeles (UCLA), principal
investigator for the magnetometer experiment on the
spacecraft. Kivelson 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 experiment.
"Combined with the discovery of an intrinsic magnetic
field, our gravity results indicate that Ganymede has a
metallic core about 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 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
Ganymede."
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, make the Jovian environment even more
intriguing, said Galileo project scientists 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," he said.
Data from the plasma wave instrument can be heard and
seen on the Galileo home page at the following URL:
http://www.jpl.nasa.gov/galileo
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 modified 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 through early February.
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 150 miles by 140 miles that has been highly disrupted by
fractures and ridges.
Arizona State University planetary scientist Dr. 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
eruption of ice and gases.
Galileo was launched in 1989 and entered orbit around
Jupiter Dec. 7, 1995. The Galileo mission is managed by JPL
for NASA's Office of Space Science, Washington, DC.
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