Science: Halley's Comet has 'unique' signature

时间:2019-03-04 10:19:01166网络整理admin

HALLEY’S COMET is made of material unlike anything ever found in the Solar System, according to astronomers in the US, France and Australia. When Halley’s Comet passed near the Earth in 1986, astronomers gathered at Mt Stromlo Observatory, near Canberra in Australia, to measure the spectrum of light from the comet using the observatory’s 74-inch (190-centimetre) reflecting telescope. The comet contained an unusual ratio of the isotopes carbon-12 and carbon-13. The ratio varied between 40:1 and 65:1. Susan Wyckoff, a physicist who participated in the research at Arizona State University in Tempe, says: ‘The carbon in Halley’s Comet differs from that found in all other Solar-System objects examined.’ The ratio of carbon-12 to carbon-13 in rocks from the Earth and the Moon, in meteorites, the atmospheres of the planets and the Sun is 89:1. One explanation for this difference is that a nearby supernova ‘splattered’ our proto-Sun and the nebular disc around it before the disc had coalesced into planets, perhaps within 10 million years of the Sun’s creation. Material from the supernova did not mix thoroughly throughout the gaseous disc, according to this theory, and Halley’s Comet eventually formed from an uncontaminated region of the disc. The proposed existence of the seedbed of most comets, believed to be a belt beyond Neptune and Pluto, supports this hypothesis. This faraway corner of the inchoate Solar System could have escaped contamination from the supernova. The second explanation holds that the comet came from interstellar space. This could explain why its carbon ratio matches interstellar gases but differs from objects in our Solar System. Supposedly, the comet approached the Sun and was caught in its gravitational field. ‘The fact that Halley’s Comet orbits in the opposite direction from that of almost all other objects in the Solar System makes this explanation plausible,’ says Wyckoff. But she herself favours the first explanation. Interstellar distances are so great that the chance of a close encounter with the Sun is very slight, she says. Astronomers hope to get another chance to test the two explanations when Comet Brorsen-Metcalf comes within view from Earth in the northern hemisphere next August. If the carbon ratio of this comet matches Halley’s, it would support the idea of ‘splatter’ from the supernova because, says Wyckoff,