Jupiter II

   Europa ("yoo ROH puh") 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 composition to the terrestrial planets: primarily composed of silicate rock. However, Europa's density (2.97 g/cm3, a little less than the Moon's and substantially less than Earth's) indicates the lack of a dense iron 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.

   There seem to be very few craters 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. 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 streaks 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) composed of oxygen. Of the 61 moons in the solar system only three others (Io, Ganymede, 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 vapor 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. Galileo's first close encounter with Europa will come in December 1996.

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Bill Arnett; last updated: 1996 Nov 7