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Fiery volcanoes on Jupiter’s moon Io are the main source of
dust streams that flow from the Jupiter system into the rest of
the solar system, according to new findings from NASA’s Galileo
spacecraft analyzed by an international team of scientists.

The scientists, led by Amara Graps of the Max Planck
Institute of Nuclear Physics in Heidelberg, Germany, analyzed the
frequency of dust impacts on Galileo’s dust detector subsystem.
They found peaks that coincided with the periods of Io’s orbit
(approximately 42 hours) and of Jupiter’s rotation (approximately
10 hours).

Although dust scientists had suspected Io as the source of
the dust streams, it was difficult to prove. They ruled out
several possible sources, including Jupiter’s main ring and Comet
Shoemaker-Levy 9, but Jupiter’s gossamer ring and Io remained as
candidates. The dust scientists studied several years of Galileo
data to show that the motion of the dust stream particles is
strongly influenced by Jupiter’s magnetic field, with a unique
signature that could exist only if Io were the main contributor
to the dust streams.

“Now, for the first time we have direct evidence that Io is
the dominant source of the Jovian dust streams,” said Graps, lead
author of a paper on the findings that appears in the May 4 issue
of the journal Nature.

The Jovian dust streams are intense bursts of submicron-
sized particles (as small as particles of smoke) that originate
in Jupiter’s system and flow out about 290 million kilometers
(180 million miles), or twice the distance between Earth and the
Sun. They were first discovered in 1992 by the dust detector
onboard the Ulysses spacecraft during its Jupiter flyby.

“The escape of dust from the Jovian system in 1992 was a
total surprise,” said Dr. Mihaly Horanyi, a dust plasma physicist
at the Laboratory for Atmospheric and Space Physics, Boulder, CO,
and co-author of the paper. Since 1995, the Galileo dust
detector, a twin to the Ulysses instrument, has observed the
streams, both while the spacecraft was en route to Jupiter and
within the Jupiter system.

Very, very early in the history of our solar system, before
and during the formation of the planets, small dust grains were
much more abundant. These charged grains were influenced by
magnetic fields from the early Sun, much as the dust on Io is
affected by Jupiter’s magnetic field today. Thus, studies of the
behavior of these dust grains may provide insight into processes
that led to the formation of the moons and planets in our solar
system.

“The dust from the Jovian dust streams is clearly
magnetically-controlled dust,” said Dr. Eberhard Gruen of the Max
Planck Institute. “Dust particles carry information about
charging processes in regions of the Jovian magnetosphere, where
information is otherwise sparse or unknown.” Gruen built the dust
detectors for several spacecraft, including Galileo, Ulysses and
Cassini.

These new results provide a useful window on Io. In-situ
dust measurements can monitor Io’s volcanic plume activity,
complementing observations made by Galileo and from Earth-based
telescopes.

The Jovian dust streams, with their Io source, are minor
when compared to the huge amounts of dust created in the solar
system by comet activity and asteroid collisions. Nonetheless,
they add to the variety of dust sources in the solar system. In
fact, the Jovian dust streams travel so fast that some particles
can actually leave the solar system to join the local
interstellar medium — the gas and dust that fill the space
between stars.

In December 2000, during a joint observation of Jupiter by
Galileo and Cassini, scientists will have a unique opportunity to
study the Jovian dust streams using dust instruments on both
spacecraft.

In addition to Graps, Gruen and Horanyi, authors on this
paper are Dr. Harald Krueger, Andreas Heck and Sven Lammers of
Max Planck, and Dr. Hakan Svedhem of the European Space Research
and Technology Centre in Noordwijk, the Netherlands. This work
was supported by the German space agency, Deutsches Zentrum fuer
Luft-und Raumfahrt E.V. (DLR).

More information on the Galileo mission is available at
http://galileo.jpl.nasa.gov .

The Max Planck, Heidelberg Dust Group web site is at
http://galileo.mpi-hd.mpg.de/ .

The Galileo, Cassini and Ulysses missions are managed by the
Jet Propulsion Laboratory, Pasadena, CA, for NASA’s Office of
Space Science, Washington, DC. JPL is managed by the California
Institute of Technology in Pasadena.

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