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Contact: Mary Beth MurrillMay 3, 1996
NASA'S GALILEO FINDS GIANT IRON CORE IN JUPITER'S MOON IO
Jupiter's volcano-pocked moon Io has been found by NASA's
Galileo spacecraft to have a giant iron core that takes up half
its diameter, scientists report in today's issue of Science
The spacecraft also has detected a large "hole" in Jupiter's
magnetic field near Io, leading to speculation about whether Io
possesses its own magnetic field. If so, it would be the first
planetary moon known to have one.
These newly identified characteristics of Io may be related
to the intense heating of the moon caused by the constant
squeezing and distortion of Io in Jupiter's powerful
gravitational grip, according to Galileo Project Scientist Dr.
Torrence Johnson of NASA's Jet Propulsion Laboratory. Io is the
most geologically active body in the Solar System, and though it
is less than a third of Earth's size, it generates twice as much
heat as the Earth.
"Jupiter's massive gravity field distorts the shape of Io in
the same way that tides are raised in Earth's oceans by the
gravitational tugs of the Sun and Moon," said Johnson. As Io
orbits Jupiter, these so-called "body tides" rise and fall due to
subtle changes in Io's orbit which, in turn, are caused by the
gravitational nudges from Europa and Ganymede, other moons of
As a result, Io is squeezed like a rubber ball. Friction
created by this action heats and melts rock within Io to produce
the volcanoes and lava flows seen all over its surface and huge
geysers that spew sulfur dioxide onto Io's landscape.
The large, dense core Galileo found within Io was deduced
from data taken during the spacecraft's flyby within 899
kilometers (559 miles) of the moon last Dec. 7, as Galileo passed
by Io on its way to enter orbit around Jupiter. Precise
measurements of the spacecraft's radio signal revealed small
deviations in Galileo's trajectory caused by the effects of Io's
own gravity field.
From these data, Galileo scientists have determined that Io
has a two-layer structure. At the center is a metallic core,
probably made of iron and iron sulfide, about 900 kilometers (560
miles) in radius, which is overlain by a mantle of partially
molten rock and crust, according to JPL's Dr. John Anderson, team
leader of Galileo's celestial mechanics experiment and principal
author of the paper published in Science today. The core was
probably formed from heating in the interior of the moon, either
when it originally formed or as a result of the perpetual tidal
heating driving its volcanoes.
Galileo scientists are also trying to determine the cause of
the hole they found in Jupiter's magnetic field when the
spacecraft was closest to Io. "Instead of increasing continuously
as the spacecraft neared Jupiter, the magnetic field strength
took a sudden drop of about 30 percent," said Johnson.
"It's an astonishing result and completely unexpected," said
Dr. Margaret Kivelson of the University of California at Los
Angeles, who heads Galileo's magnetic fields investigation team.
Preliminary analyses of these data are currently being prepared
for formal publication.
"The data suggest that something around Io -- possibly a
magnetic field generated by Io itself -- is creating a bubble or
hole in Jupiter's own powerful magnetic field," Kivelson said.
"But it's not clear to us just how Io can dig such a deep and
wide magnetic hole."
Possible explanations for this signature can only be sorted
out using data from all the other space physics instruments
onboard Galileo, said Johnson. "We're eagerly awaiting the return
of data from the magnetospheric measurements taken during the Io
flyby to see if we can resolve this mystery," he said. This
data, recorded onboard the spacecraft, will be transmitted back
to Earth in June or July.
If analysis of this data eventually proves that Io indeed
has a magnetic field of its own, it would be the first moon shown
to have one. Io would join the Earth, planet Mercury and the
outer giant planets as bodies in the Solar System that generate
their own magnetic fields.
Other studies conducted by Galileo during its December flyby
of Io have provided new evidence that Io is most likely the
source of high velocity dust streams littering millions of miles
of space around Jupiter.
In July 1994, Galileo's dust detector began sensing dust
streams more powerful than those previously discovered by the
Ulysses spacecraft. Dust detectors on Galileo sensed more and
more particles during its approach to Jupiter, reaching a peak of
20,000 impacts per day during the longest and most intense
interplanetary dust storm ever observed.
These fast-moving particles travel at speeds of 50 to 100
kilometers per second (30 to 60 miles per second) away from
Jupiter -- fast enough to escape the Solar System. These dust
impacts continued up to the time of Galileo's Io flyby and then
ceased, said Dr. Eberhard Grun of Germany's Max Planck Institute
in Heidelberg, who is principal investigator for Galileo's dust
"My preliminary interpretation of these observations is that
they support the idea that Io is in some way the source of the
Jupiter dust streams," Grun said.
One theory proposed after the NASA Voyager spacecraft flybys
in the late 1970s is that dust particles emitted from Io's
volcanoes could become electrically charged and then swept away
by Jupiter's rotating magnetic field. Recent modifications to
this theory suggest that the dust is subsequently accelerated in
the magnetosphere and flung outward from Jupiter at high
velocity, creating dust streams.
Galileo's next close encounter with a moon of Jupiter will
occur June 27, when the spacecraft will pass about 850 kilometers
(530 miles) above the surface of Ganymede. Larger than Mercury,
Ganymede is the largest moon in the Solar System. Galileo will
make repeated close flybys of Ganymede, Callisto and Europa
during its two-year mission in orbit around Jupiter.
Galileo was launched aboard Space Shuttle Atlantis on Oct.
18, 1989. The mission is managed by JPL for NASA's Office of
Space Science, Washington, DC.
Additional information on the Galileo mission and its
results can be found on the World Wide Web at:
[Note to Editors: To obtain a copy of the Science magazine
article by Dr. John Anderson, et al, contact the AAAS Office of
Communications, (202) 326-6421.