Spaceprobe reveals mystery moon's secrets

日期:2019-03-03 01:03:05 作者:杞迟 阅读:

By David L Chandler, Pasadena (Image: NASA/JPL/Space Science Institute) Saturn’s intriguing moon Phoebe, inspected at close range by the Cassini spacecraft on June 11, has proved to be unlike any object examined closely before. After initial analysis of the data collected during the fl-by, researchers have concluded it is almost certainly a captured object from the Kuiper Belt, out beyond Pluto. Such objects are thought to be primordial remnants from the formation of the Solar System – frozen time capsules that may preserve detailed information about the materials from which the planets were made 4.6 billion years ago. Phoebe is about 214 kilometres in diameter and orbits Saturn in the opposite direction from most of its 31 moons. It turns out to be made of a comet-like mixture of water ice, rock, hydrated minerals and carbon-rich material, according to Peter Thomas of Cornell University, a member of the Cassini imaging team. And the whole surface is more complex and “chewed up” by impacts than had been expected, he told a briefing at NASA’s Jet Propulsion Laboratory, in Pasadena, California, on Wednesday. Cassini’s infrared spectrometer also identified carbon dioxide ice, previously detected on Pluto but never before on small moons, comets or asteroids. The ice is unevenly distributed on Phoebe’s surface and seems to be associated with a very dark material, possibly hydrocarbons, seen on some parts of the heavily cratered surface. Detailed temperature mapping of the surface using another infrared instrument shows evidence that the visible surface may be covered with a loose, fluffy material. And a precise determination of Phoebe’s density shows that it must be a mix of rock and ice. The density of 1.6 grams per cubic centimetre is too low to be rock, but too high for pure ice. The new compositional information shows “it may have a lot in common with things like Pluto and the Kuiper Belt Objects,” Johnson said. If so, it would be the first such object studied in detail and could provide important clues to the nature of the whole outer Solar System. Perhaps most surprising for such a small object is evidence from some of the crater walls that there seem to be different layers of material at the surface. Such layering is completely unexpected, said Torrence Johnson of JPL,