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Dusty Jupiter moon gives clues to interplanetary rings

Image of Ganymede taken by Galileo in October 1996   

June 16, 1999
Web posted at: 3:17 p.m. EDT (1917 GMT)

By CNN Interactive
Senior Writer Robin Lloyd

PASADENA (CNN) -- Jupiter's moon Ganymede is covered by thin clouds of fine dust knocked from its surface by interplanetary meteors, scientists with NASA's Galileo spacecraft say.

The finding could explain the formation of dust rings throughout the solar system, including some of the more obscure rings around Saturn, as well as dust structures visible around Jupiter and its moons.

"Each of the satellites in the solar system are pummeled by interplanetary dust particles, or meteoroids which are marble-sized or millimeter-sized objects," said Galileo scientist Douglas Hamilton, a University of Maryland astronomer. "When those hit a satellite, or moon, at high speeds, they slam into the surface and blast a crater, and all sorts of particles fly off. That's what is happening at Ganymede, and we think that's what's happening at all the satellites in the solar system."

Galileo spacecraft
The Galileo spacecraft is deployed from the space shuttle Atlantis in October 1989   

The results were published in the latest issue of Nature.

A highly sensitive instrument on the Galileo spacecraft, which has been orbiting the gigantic gaseous planet and its moons since December 1995, had found similar dust clouds around two of Jupiter's other major moons - Europa and Callisto. The Galileo mission, now mostly finished with a two-year extended study of Europa, is managed by NASA's Jet Propulsion Laboratory in Pasadena.

The dust detector allowed the team of German, Russian and American scientists to measure dust grains directly as they hit a gold target inside a tin can about a foot in diameter. The detector allowed scientists to learn the weight and directions of grains as they whizzed through the jovian planetary system, said Harald Krueger, a Galileo scientist from the Max Planck Institute of Nuclear Physics.

The dust cloud around Ganymede, Jupiter's largest moon and one of 15 others around the planet, is too faint to be photographed by Galileo's cameras and is so thin that only one grain can be found, on average, in the space of a cube with 25 yards on a side.


Dust pervades the solar system. Dust clouds form around planets and moons when meteoroids hit them and evaporate and explode, causing puffs of debris to be ejected so fast that they leave the satellite's gravitational field, said Krueger.

Smaller moons have gravitational fields that are weak enough to release ejected dust but strong enough to then hold it at bay so it forms a ring, Hamilton said. "By hitting a satellite, what you are doing is taking debris that would usually zip by Jupiter and keep going -- now it hits the satellite and stays around.

"That's how we think the jovian rings are formed," he said, "and we see evidence of it around Ganymede." Ganymede's gravitational field is stronger than any of the other jovian moons, due to its size. It is larger than Mercury, so this new finding supports the meteoroid theory of dust rings with an extreme case, Hamilton said.

The new finding does not explain how most of Saturn's rings formed as they are comprised mainly of house-sized chunks of ice, rather than tiny dust particles, Hamilton said. There is no strong agreement on how Saturn's most prominent rings formed, he said.

Oh, Io! Hubble views volatile moon
April 20, 1999
Watch the sky: Venus, Jupiter to 'pair up' Tuesday
February 23, 1999

List of Max Planck Institutes
NASA: Galileo Project Information
NASA Ames Research Center: Galileo Probe
Galileo (JPL)
Galileo - Countdown to Europa 19
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