Europa 木卫二(英)

　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 abducted¹ 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 composition to the terrestrial planets: primarily composed of silicate² 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 metallic³ 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 have been seen. The prominent markings seem to be only albedo features with very low relief.

　　There are very few craters⁴ on Europa; only three craters larger than 5 km in diameter have 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 streaks⁵ crisscrossing the entire globe. The larger ones are roughly 20 km across with diffuse⁶ outer edges and a central band of lighter⁷ material. The latest theory of their origin is that they are produced by a series of volcanic⁸ eruptions⁹ or geysers.

　　Recent observations with HST reveal that Europa has a very tenuous¹⁰ atmosphere (1e-11 bar) composed of oxygen. Of the many moons in the solar system only five others (Io, Ganymede, Callisto, Titan and Triton) are known to have 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 vapor¹¹ which is subsequently split into hydrogen and oxygen. The hydrogen escapes leaving 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 thaws¹² 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 magnetic field (perhaps 1/4 of the strength of Ganymede's). And most interestingly, it varies periodically as it passes thru Jupiter's massive magnetic field. This is very strong evidence that there is a conducting material beneath Europa's surface, most likely a salty ocean.