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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