New dust around Jupiter
The Galileo spacecraft has discovered a novel ring of dust around the planet Jupiter. With a diameter of about 1,000,000 kilometers, it is larger than other rings in our solar system, and the dust does not come from Jupiter's immediate vicinity, but from interplanetary and interstellar space. In 1979, Voyager 2 had discovered an irregular ring of dust around Jupiter, the strength of which varied between 320 and 4,800 km. According to Joshua Colwell of the University of Colorado at Boulder, this ring was formed by the collisions of small moons with micrometeorites in the Jovian system. The newly discovered dust ring, on the other hand, appears to have originated far from the planet. It is also obviously much larger, more sparse and - at least according to current knowledge - unique in the solar system.
Surprisingly, most interstellar and interplanetary dust particles appear to be in a "retrograde" orbit: traveling in the opposite direction to the rotating planet and its moons, Colwell explained. However, the reason for the reverse movement of the tiny particles is not yet clear.
Proof of the existence of the new ring was provided by computer simulations, which gave the same results as the dust detector on board the Galileo spacecraft. The results were published on April 3, 1998 by Colwell and Mihaly Horanyi of the University of Colorado and planetary scientist Eberhard Grun of the Max Planck Institute for Astronomy in Science.
According to the study, interstellar and interplanetary dust grains ranging in size from 0.6 microns to 1.4 microns are being trapped by Jupiter's magnetosphere. Such particles are thinner than a human hair and about the size of smoke particles."Once these particles are the right size, they lose energy to the magnetosphere and end up trapped in the ring," Colwell said. Smaller grains, on the other hand, bounce off the magnetosphere while larger particles retain enough energy to escape.
The bowl-shaped metal dust detector on Galileo has a charged grid on its top. As the detector moves through space, small dust particles that hit the dish evaporate. They create a small cloud, which is then perceived through the grid. The direction and movement of the detector tells scientists the direction and speed of the interplanetary and interstellar dust rings.
Dust is constantly entering and leaving the solar system. In 1993, Horanyi made measurements with the Ulysses spacecraft that indicated that some of the microscopic dust particles from the volcanoes on Jupiter's moon Io were streaming through space at speeds of over 100 miles per second. With that much energy, they can outrun not only Jupiter's magnetic field, but the entire solar system, Colwell said.
In the newly discovered ring type, the dust grains are so sparse that a photon would only hit a dust grain with a probability of one in a billion. "I suppose we'll eventually find something similar on Saturn," Colwell believes. In 2004 the Cassini probe will reach the ringed planet Saturn.
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