Max Planck Institute of Nuclear Physics
Heidelberg, Germany

Contacts:
Harald Kr=FCger
Max Planck Institute of Nuclear Physics
Heidelberg, Germany
Phone: (+49 62 21) 516 - 563
Fax: (+49 62 21) 516 - 324
krueger@galileo.mpi-hd.mpg.de

Eberhard Gr=FCn
Max Planck Institute of Nuclear Physics
Heidelberg, Germany
Phone: (+49 62 21) 516 - 478
Fax: (+49 62 21) 516 - 324
Eberhard.Gruen@mpi-hd.mpg.de

Douglas P. Hamilton
Department of Astronomy
University of Maryland, USA
Phone: (+01 301) 405 - 1548
Fax: (+01 301) 314 - 9067
hamilton@astro.umd.edu

June 2, 1999

Jupiter's moon Ganymede surrounded by an impact-generated dust cloud

Dust pervades the solar system, being especially concentrated in the ring systems surrounding the giant planets and along the plane of the planetary orbits (the Zodiacal cloud). Individual dust grains are thought to be generated when impacts of interplanetary meteoroids kick up material from larger bodies, such as satellites. In these impacts the meteoroids hit the surface so fast that they evaporate and explode, causing puffs of debris to be ejected at such high speed that they can leave the satellite's gravitational field. This is the first time that in situ measurements of this important physical process have been made.

''For the first time we can investigate this important process in situ which is an important mechanism for dust production in space," said Dr. Harald Kr=FCger, Heidelberg, lead author of the paper in Nature. "With the dust instrument we measure impact directions, speeds and masses of the grains. Our in situ measurements go one step further than optical investigations made on the smaller moons because we can directly study various physical parameters of the ejected particles. We have detected similar dust clouds at two others of Jupiter's Galilean satellites, Callisto and Europa, suggesting that they too are significant sources of dust debris.''

The Dust Detector System (DDS) on board Galileo detected the dust cloud when the spacecraft flew by Ganymede within a few thousand kilometers. The DDS instrument was built under the leadership of Prof. Dr. Eberhard Gr=FCn -- who is also co-author of the Nature paper -- at the Max-Planck- Institut f=FCr Kernphysik in Heidelberg, Germany, with financial support from the German National Space Agency (DLR). ''The Galileo measurements are like a large impact experiment offered by Nature," said Gr=FCn. ''They provide important improvements over laboratory experiments because projectile and target materials and projectile speeds are of astrophysical relevance."

''Our modelling indicates that the dust cloud is formed by hypervelocity impacts of interplanetary dust onto Ganymede's surface," explains Dr Alexander V. Krivov from St Petersburg University, Russia, who is also involved in the research. ''A fraction of dust in the cloud leaves the environment of the moon and is distributed into circumjovian space to form a tenuous ring around Jupiter."

Recently, another team of scientists, analyzing data from Galileo's cameras, have revealed structures in the ring system. ''The structures can most naturally be explained by impact-generated particles lofted from Jupiter's small moons -- the same mechanism which we see here," says Dr Douglas P. Hamilton, College Park, MD, U.S.A., who is a co-author of the Nature paper. Larger moons however, like Ganymede, are much weaker sources of dust because of their stronger gravitational field and the Ganymede dust cloud is by far too thin to be detectable with Galileo's cameras.

Only with the high-sensitive Dust Detector System on board Galileo, which measures dust grains hitting a 1000 cm2 gold target, could these dust clouds be detected. In the case of the Ganymede cloud, the dust concentration is so low that only one grain can be found in a cube with 20 meters on a side. Although the cloud is very interesting scientifically, it does not cause a danger for the Galileo spacecraft.

Galileo has been orbiting Jupiter since December 1995, and has currently completed three-forth of its two-year extension, known as the Galileo Europa Mission. During its orbital tour around the planet the spacecraft performs close fly-bys at Jupiter's Galilean satellites. Galileo is operated for NASA by the Jet Propulsion Laboratory which is a division of Caltech, Pasadena, CA.