[英国留学之利物浦租房Europa解析]这座城市被誉为创新之城，你知道为什么吗？因为它孕育了许多世界闻名的艺术家、表演者、喜剧演员。这座城还是运动之城，你了解吗？利物浦、埃弗顿足球俱乐部享誉世界，这些知名的球员...+阅读
Europa is the sixth of Jupiter's known satellites and the fourth largest; it is the second of the Galilean moons. Europa is slightly smaller than the Earth's Moon.
orbit: 670,900 km from Jupiter
diameter: 3138 km
mass: 4.80e22 kg
Europa was a Phoenician princess abducted to Crete by Zeus, who had assumed the form of a white bull, and by him the mother of Minos.
Discovered by Galileo and Marius in 1610.
Europa and Io are somewhat similar in bulk position to the terrestrial plas: primarily posed of silicate rock. Unlike Io, however, Europa has a thin outer layer of ice. Recent data from Galileo indicate that Europa has a layered internal structure perhaps with a small metallic core.
But Europa's surface is not at all like anything in the inner solar system. It is exceedingly smooth: few features more than a few hundred meters high he been seen. The prominent markings seem to be only albedo features with very low relief.
There are very few craters on Europa; only three craters larger than 5 km in diameter he been found. This would seem to indicate a young and active surface. However, the Voyagers mapped only a fraction of the surface at high resolution. The precise age of Europa's surface is an open question.
The images of Europa's surface strongly resemble images of sea ice on Earth. It is possible that beneath Europa's surface ice there is a layer of liquid water, perhaps as much as 50 km deep, kept liquid by tidally generated heat. If so, it would be the only place in the solar system besides Earth where liquid water exists in significant quantities.
Europa's most striking aspect is a series of dark strea
ks crisscrossing the entire globe. The larger ones are roughly 20 km across with diffuse outer edges and a central band of lighter material. The latest theory of their origin is that they are produced by a series of volcanic eruptions or geysers.
Recent observations with HST reveal that Europa has a very tenuous atmosphere (1e-11 bar) posed of oxygen. Of the many moons in the solar system only five others (Io, Ganymede, Callisto, Titan and Triton) are known to he atmospheres. Unlike the oxygen in Earth's atmosphere, Europa's is almost certainly not of biologic origin. It is most likely generated by sunlight and charged particles hitting Europa's icy surface producing water vapor which is subsequently split into hydrogen and oxygen. The hydrogen escapes leing the oxygen.
The Voyagers didn't get a very good look at Europa. But it is a principal focus of the Galileo mission. Images from Galileo's first two close encounters with Europa seem to confirm earlier theories that Europa's surface is very young: very few craters are seen, some sort of activity is obviously occurring. There are regions that look very much like pack-ice on polar seas during spring thaws on Earth. The exact nature of Europa's surface and interior is not yet clear but the evidence is now strong for a subsurface 'ocean'.
Galileo has found that Europa has a weak magic field (perhaps 1/4 of the strength of Ganymede's). And most interestingly, it varies periodically as it passes thru Jupiter's massive magic field. This is very strong evidence that there is a conducting material beneath Europa's surface, most likely a salty ocean.
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