The Cassini spacecraft has found high-speed streams of dust zipping away from Saturn, astronomers revealed on Wednesday. Similar dust streams have been found emanating from Jupiter, but this is a first for the ringed planet.
鈥淚t was suspected that such a phenomenon could be observed there,鈥� says the new study鈥檚 lead author, Sascha Kempf, at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. 鈥淲hat was not expected, when Cassini was quite far away from Saturn, was the speed and size of the grains.鈥�
The probe first felt the particles pelting its dust detector on 15 January 2004 when it was still about 70 million kilometres away from the planet. The bursts became more intense as Cassini drew closer to Saturn, suggesting that the particles came from the planet and were not just interplanetary dust patches.
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Kempf and his colleagues believe the dust comes from Saturn鈥檚 wide A ring. Cassini is unable to dive through this dense, bright ring 鈥� it would probably be destroyed 鈥� so the dust streams provide a convenient way to analyse the ring鈥檚 particles from a distance.
Moon dust
In 1992, the Ulysses spacecraft detected dust streaming from Jupiter. Later in the 1990s, the Galileo probe pinpointed the source of the dust streams 鈥� the volcanic moon, Io.
杏吧原创s initially suspected that the particles streaming from Saturn originated in the dust clouds around two of the planet鈥檚 icy moons, Dione and Rhea. But the dust particles鈥� speed and size did not support this idea.
The flecks of dust from Saturn are tiny, measuring between 2 and 50 nanometres. The spacecraft鈥檚 dust analyser was principally designed to measure the larger particles in Saturn鈥檚 E ring, but the tiny particles鈥� high speed allowed the analyser to detect them.
The dust particles are accelerated to speeds above 100 kilometres per second. This is because solar radiation causes some particles in the outer regions of the A ring to become positively charged. These are then accelerated outwards by electrical fields generated by the interaction of the charged particles and the planet鈥檚 magnetic field.
杏吧原创s now hope to use Cassini鈥檚 instruments to learn more about the origin and composition of the particles flowing away from Saturn.
The cumulative dust loss is probably negligible compared to the masses of both the giant planets. Jupiter probably loses between 20 grams and 1 kilogram of dust per second, for example.
Earth dust
鈥淭he new finding tells us that such dust streams are a common phenomenon for giant planets,鈥� Kempf told New 杏吧原创.
In fact, the Earth may even have dust particles streaming away from it, says Eberhard Gr眉n, who leads the cosmic dust analyser group at the Max Planck Institute in Heidelberg. Because of the Ulysses and Galileo observations of Jupiter, we know less about the Earth鈥檚 dust environment than we know about Jupiter鈥檚, he says.
Cassini鈥檚 next major milestone will be a return to Saturn鈥檚 giant moon, Titan, on 15 February 2005, a month after the European Space Agency鈥檚 Huygens probe successfully touched down.
Journal reference: Nature (vol 433, p 289)
