Science: Neptune's new moon baffles the astronomers

 作者:孟撮     |      日期:2019-03-04 05:12:05
By JOHN MASON THE discovery by the Voyager spaceprobe of a new Neptunian moon is causing planetary scientists to rethink their theories about the origin of the planet’s largest satellite, Triton. The new moon, 1989 N1, orbits Neptune every 1 day 3 hours in a nearly circular orbit at the planet’s equator. The orbit is 92 700 kilometres above the planet’s cloud tops. The orderly nature of its orbit contrasts sharply with the other two known Neptunian moons, Triton and Nereid, which follow very unusual paths around the planet. Triton’s orbit, which is also circular, is inclined at about 20 degrees to the planet’s equator, but the moon moves in a retrograde, or backwards, direction. It is the only large planetary moon to do so. Nereid has a direct, or forwards motion, but its orbit is tilted at 30 degrees to Neptune’s equator. It is the most elliptical of any known planetary satellite. Before the recent discovery, scientists thought that because of Triton’s peculiar retrograde and tilted orbit, it must have been a body wandering the Solar System alone when it was captured by Neptune. Its initial path around Neptune would have been non-circular, so the theory goes, but, over a period of hundreds of millions of years, it became more circular as tidal forces siphoned energy from its orbital motion. Now, with the discovery of 1989 N1, this theory seems not to fit the available evidence. ‘The difficulty is that we have found a moon in a place we didn’t think one should have existed,’ said Voyager’s assistant project scientist, Ellis Miner of NASA’s Jet Propulsion Laboratory in Pasadena. ‘If Triton were a relative newcomer to the Neptune system,’ he said, ‘it would have passed near enough to the low orbit of any pre-existing moon, such as 1989 N1, either to collide with it or sweep it up through gravitational attraction.’ The existence of 1989 N1 in the orbit it now occupies suggests that Triton may not be a captured object after all. Instead, it may be a native to Neptune. Indeed, the fact that Triton is so much larger than all other known planetary satellites which have retrograde orbits had already led some scientists to question the view that Triton is a captured object. Theoreticians are now struggling to see if the ‘Triton-as-a-captured-object’ theory can be salvaged or if alternative explanations for its unusual retrograde orbit can be found. According to Archie Roy of Glasgow University, ‘The whole of our handed down beliefs on the origin of Triton will have to be scrapped,