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Jupiter's Moon Ganymede Surrounded By An Impact-Generated Dust Cloud

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

2-6-99

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

An international team of scientists lead by the Max-Planck-Institut f=FCr
Kernphysik in Heidelberg (Max Planck Institute of Nuclear Physics), Germany,
has found a cloud of dust grains surrounding Jupiter's moon Ganymede which
is the largest of the planet's four Galilean satellites. Grains are kicked up
from the moon's surface by impacts of interplanetary meteoroids. These
measurements, obtained with the Heidelberg dust detector on board NASA's
Galileo spacecraft, are published in Nature on 10 June 1999. The authors
present the first in situ study of impact-generated dust in the vicinity of
a source moon. The research will lead to a better understanding of the
processes that form the ring systems surrounding all giant planets in our
solar system.

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.

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