Europa is somewhat similar in bulk composition to the terrestrial planets, being primarily composed of silicate? rock. It has an outer layer of ice, and recent [magnetic field]? data from the [Galileo probe]? indicate that Europa has a layered internal structure perhaps with a small metallic core.
Europa's surface is extremely smooth; few features more than a few hundred meters high have been seen. The prominent markings crisscrossing Europa's surface seem to be only albedo features with very low vertical relief. There are very few craters on Europa, with only three craters larger than 5 km in diameter, and its albedo is one of the highest of all moons. This would seem to indicate a young and active surface. The smoothness and markings visible on Europa's surface strongly resemble that of sea ice on Earth, and it isthought that under Europa's surface ice there is a layer of liquid water perhaps as much as 50 km deep kept liquid by tidally generated heat.
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 by the [Hubble Space Telescope]? reveal that Europa has a very tenuous atmosphere (10-11 bar surface pressure) composed of oxygen. Of the 61 moons in the solar system only four 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 Europa's gravity due to its low atomic mass, leaving the oxygen behind.
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 through 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.
It has been suggested that life may exist in this under-ice ocean. There is no evidence to support this hypothesis at this time.
Data for Europa:
http://www.jpl.nasa.gov/galileo/europa/
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