The Jovian dust streams: Io's volcanoes drizzle dust into the solar system

Figure 1: Sketch of one of Galileo's orbital trajectories from late-1997 overlaid with trajectory results from a dust stream particles model by Horanyi and co-workers. The submicron dust particles from Io's volcanoes are strongly attracted to Jupiter's magnetic field, and they move in a warped sheet around Jupiter, as Jupiter rotates on its axis. The Galileo orbital trajectory is indicated with a solid blue line and the dust streams' impact direction is indicated with green arrows. Full size image available through contact

An international team of scientists led by the Max-Planck-Institut fuer Kernphysik (Max Planck Institute of Nuclear Physics) in Heidelberg, Germany, has identified Io as the dominant source of the Jovian dust streams. The scientists: Amara Graps, Eberhard Gruen, Harald Krueger, Andreas Heck, Sven Lammers (MPI-K, Heidelberg, Germany), Hakan Svedhem (ESTEC, Noordwijk, The Netherlands), and Mihaly Horanyi (LASP, Boulder, USA) analyzed the periodicities in the dust impacts signal from the Galileo spacecraft, and found Io's fingerprint plainly visible (Nature 10 June 2000).

The Jovian dust streams are high rate bursts of submicron sized particles travelling in the same direction from a source in the Jovian system and observed in interplanetary space out to 2AU. The Jovian dust streams were first discovered in 1992 when the Ulysses spacecraft dust detector observed the collimated streams during its Jupiter flyby. Since 1995, the Galileo spacecraft dust detector, which is a twin to the Ulysses' dust instrument, has observed the Jovian dust streams, both on its way to Jupiter, and while the spacecraft has been in the Jupiter system. "The escape of dust from the Jovian system in 1992 was a total surprise," said Mihaly Horanyi, a dust plasma physicist in Colorado, involved in Jovian dust streams research since the streams' discovery.

The conclusion that Io could be a source of the Jovian dust streams was logical, but finding hard evidence for this connection proved difficult and elusive for the dust scientists. Earlier investigations of the source of the Jovian dust streams considered Jupiter's rings (main, halo, or gossamer ring), comet Shoemaker-Levy 9, and the volcanoes on Io. After several sources were ruled out, the investigators couldn't absolutely distinguish between a Jupiter gossamer ring source and an Io source, even though an Io model fit the data. "Now, for the first time we have direct evidence that Io is the dominant source of the Jovian dust streams," said Amara Graps (MPI-K, Heidelberg, Germany), who is the first author of the Nature paper. Graps, and her coauthors in Heidelberg, Noordwijk and Boulder, applied frequency analysis to several years of Galileo dust detector data to show that the dust stream particles' motion is strongly influenced by Jupiter's magnetic field with a unique signature that could only be present if Io were the dominant source of the dust streams. The dynamics and charging of dust in planetary magnetospheres is a special laboratory to study the small-scale physical processes that may have shaped our solar system. Submicron particles are dominated by electromagnetic forces. Therefore, small, charged particles can be considered as astrophysical tracers of the magnetic field conditions. "The dust from the Jovian dust streams is clearly magnetically-controlled dust," said Eberhard Gruen, who is also involved in the research. "Dust particles carry information about charging processes in regions of the Jovian magnetosphere where information is otherwise sparse or unknown." Gruen, who has been leading interplanetary dust research for several decades, built the dust detectors that are currently flying on the Galileo, Ulysses, Cassini, and the Gorid spacecraft.

Figure 2: An image of Io from Galileo's orbit 9, taken by the Solid State Imaging (CCD) system in June 1997, where we see the dusty plume of one of Io's volcanoes: Pillan erupting against the limb of Io. The 2 km (1.2 mi) resolution image shows a plume 140 kilometers (86 miles) high, and was also detected by the Hubble Space Telescope. This explosion left a "black eye" of dark silicates that covered an area larger than Germany. Image Catalog # PIA0108, Courtesy of NASA-JPL. ( http://photojournal.jpl.nasa.gov) Full size image available through contact

These new results provide a useful window on Io. Now dust in-situ measurements can be an effective way to monitor Io's volcanoes plume activity, providing more complete coverage and a complement to the Galileo and Earth-based image observations.

These new results also provide more pieces to the story of our solar system evolution. The lifetimes of dust particles in our solar system are very short compared to the lifetime of the Sun. The source of the Jovian dust streams is a minor dust source compared to collisions of the main belt asteroids and comet activity, nevertheless, it adds to the variety of dust sources in the solar system. At the dust streams' high velocity, the Jovian dust stream particles can also leave the solar system to slightly populate the local interstellar medium.

The scientists will have more visions of the Jovian dust streams in December 2000, when the streams are observed in a rare and unique opportunity by two spacecraft simultaneously. Cassini's Cosmic Dust Analyzer (CDA) will observe the streams at the same time as the Galileo dust instrument, during the Cassini-Jupiter flyby.

Note:
The authors of this work are Amara Graps, Eberhard Gruen (MPI-K, Heidelberg, Germany), Hakan Svedhem (ESTEC, Noordwijk, The Netherlands), Mihaly Horanyi (LASP, Boulder, USA), and Harald Krueger, Andreas Heck, Sven Lammers (MPI-K, Heidelberg, Germany). This work was supported by Deutsches Zentrum fuer Luft-und Raumfahrt E.V. (DLR).

More information is available at the following links:

This Jovian Dust Streams May 4 Nature Press Release: http://galileo.mpi-hd.mpg.de/~graps/nature2000/press.html
Some Jovian dust stream history: http://galileo.mpi-hd.mpg.de/~graps/nature2000/history.html
Technical Links and References: http://galileo.mpi-hd.mpg.de/~graps/nature2000/references.html
The MPI-K, Heidelberg Dust Group Website: http://galileo.mpi-hd.mpg.de/

For further information you may contact:

Amara Graps
Max Planck Institute of Nuclear Physics
Heidelberg, Germany
Phone: 49-622-151-6543
Fax: 49-622-151-6324
e-mail: Amara.Graps@mpi-hd.mpg.de http://galileo.mpi-hd.mpg.de/~graps

Eberhard Gruen
Max Planck Institute of Nuclear Physics
Heidelberg, Germany
Phone: 49-622-151-6478
Fax: 49-622-151-6324
e-mail: Eberhard.Gruen@mpi-hd.mpg.de http://galileo.mpi-hd.mpg.de/~gruen

Mihaly Horanyi
Laboratory for Atmospheric and Space Physics
University of Colorado
Boulder, CO, USA
Phone: 303-492-6903
Fax: 303-492-6946
horanyi@styx.colorado.edu http://styx.colorado.edu/~horanyi/

Harald Krueger
Max Planck Institute of Nuclear Physics
Heidelberg, Germany
Phone: 49-622-151-6563
Fax: 49-622-151-6324
Harald.Krueger@mpi-hd.mpg.de Harald.Krueger@mpi-hd.mpg.de