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Althoughwe know the diameters of Pluto and Charon individually, theirorbital characteristics tell us only the total mass of the system. In practice,this is not too much of a handicap. If the two objects have the same density,we can find this by dividing the total mass by the combined volume. Even ifthey do not have the same density, the answer must be close to the actualdensity of Pluto, because it contributes seven-eighths of the volume of thesystem, and presumably the major proportion of the mass.
Buieand Tholen found that Plutoturns out to be close to twiceas dense as water. This came as a surprise,for two reasons. Spectra of Pluto had shown that its surface is coated with methane, and solidmethane has a low density.If Pluto were made entirely of frozen methane it would have a density littlemore than half that of water.
Astronomershave also found that, in general, the density of the solid worlds in the SolarSystem decreases with distancefrom the Sun. The fourinner planets (including the Earth) are made of rock and iron, and arefour or five times as dense as water. The outer planets themselves are madelargely of gases, so are not a fair comparison. But the Voyager spacecraft havemeasured the densities of their satellites, several of which are larger thanPluto. The main satellites of Jupiter are between two and three times as dense as water.These worlds contain less rock than the inner planets, with much of their bulkconsisting of frozen water: some of Saturn’s moons are virtually pure ice.
Soastronomers had placed their bets on a Pluto composed mainly of either water ice or methane ice: but it mustbe made largely of rock.William McKinnon, of Washington University in Saint Louis, Missouri, andSteve Mueller of the Southern Methodist University in Dallas, Texas, haveinvestigated in more detail. They had already built theoretical models of thesatellites of Jupiter and Saturn, tailoring the inner structure to fit thedensities and other measurements made by the Voyager probes. In the case ofPluto, they find that three-quarters of its material must be rock. Most of the rest iswater ice, withabout one-third ofthe ice turning to water. As a result, the rock sank to the centre. Surrounding this rockycore today is a mantle of waterice, with some methanetrapped in the surface layers.
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发表于 2019-8-30 09:03:49
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