Pill power

日期:2017-09-18 07:45:48 作者:牧抨椹 阅读:

By Jeff Hecht IMAGES from the Galileo spacecraft show conclusively that the three faint rings around Jupiter come from moon dust spiralling down into the planet’s atmosphere. The fact that Jupiter has rings was not discovered until Voyager flew by in 1979. The dusty rings are only visible when light hits them at certain angles, just as house dust reflects light slanting though a window, so scientists have had a hard time getting good photos. The latest pictures, released this week at a NASA-sponsored press conference at Cornell University in Ithaca, New York, are the best ones yet, and finally clear up the mystery of how the rings formed. Unlike the icy, snowball-sized chunks that make up Saturn’s rings, the dust particles surrounding Jupiter are so small that the mere act of reflecting light gradually slows their orbit, so that after 1000 to 100 000 years they drop into the planet’s atmosphere. Scientists knew that something had to replace the lost particles, and they were quick to suspect that micrometeorites were knocking dust off of Jupiter’s four small innermost moons, because the rings lay close to their lunar orbits. The Galileo images, together with pictures taken with the 10-metre Keck telescope in Hawaii last year when the Earth lay in the plane of the rings, firmly link the moons to the rings. Most impressive were new pictures of the outermost and faintest ring, which was only discovered in 1985. Galileo’s photos show that this “gossamer” ring contains two cloudy lobes of different thicknesses that look like dust plumes coming down from Amalthea and Thebe, the outermost of the four inner moons. The moons orbit at the outer edge of each cloudy lobe, and the shape and thickness of the lobes matches what would be expected if dust is moving from the slightly tilted moons towards Jupiter’s equator. “The story looks a lot more convincing now with the gossamer ring,” says Phil Nicholson, a Cornell astronomer who was on the team that worked on the Keck images. Jupiter’s inner and denser ring has been linked to the innermost satellites, Adrastea and Metis. The photos show that most of the material comes from Adrastea, the smallest of the inner moons. It produces the most dust because it has the weakest gravity and debris can escape more easily, says astronomer Joe Veverka of Cornell, who analysed the Galileo images. “The Galileo data are just terrific in demonstrating that the phenomenon is creating dusty rings,” says Larry Esposito of the University of Colorado in Boulder, who has been studying the origins of planetary rings for a decade. The same process that made Jupiter’s rings out of moon dust may explain the rings around Neptune, says Nicholson. But it could not generate the narrow rings of Uranus, which are denser and made of dark particles which can measure a centimetre across, or the bright, icy rings of Saturn. Composition of the dust and the inner Jovian satellites remains a mystery,