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|Discovered by||S. Marius
|Mean radius||670,900 km|
|Revolution period||3d 13h 14.6m|
|Is a satellite of||Jupiter|
|Equatorial diameter||3,138 km|
|Surface area||3.1 × 107 km2|
|Mean density||3.01 g/cm3|
|Surface gravity||1.42 m/s2|
|Rotation period||3d 13h 14.6m|
|Atmospheric pressure||10-6 Pa|
Europa is somewhat similar in bulk composition to the terrestrial planets, being primarily composed of silicate rock. It has an outer layer of water ice thought to be around 100 km thick, and recent magnetic field data from the Galileo probe indicate that Europa has a layered internal structure perhaps with a small metallic iron 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; based on estimates of the frequency of cometary bombardment Europa probably endures, Europa's surface must be no more than 30 million years old. The smoothness and markings visible on Europa's surface strongly resemble that of sea ice on Earth, and it is thought that under Europa's surface there is a layer of liquid water kept warm by tidally generated heat. The surface of Europa is 110 K at the equator and only 50 K at the poles, however, so water ice on the moon's surface is as hard as rock. The largest craters on Europa appear to be filled with flat, fresh ice; based on this and on the calculated amount of heat generated by Europa's tides it is predicted that the outer crust of solid ice is approximately 10-30 km thick, which could mean that the liquid ocean may be as much as 90 km deep underneath.
Europa's most striking surface feature is a series of dark streaks crisscrossing the entire globe. These streaks strongly resemble the cracks that form in sea ice on Earth, and close examination shows that the edges of Europa's crust on either side of the cracks have moved relative to each other. The larger bands are roughly 20 km across with diffuse outer edges, regular striations, and a central band of lighter material which is thought to have been produced by a series of volcanic water eruptions or geysers as the Europan crust spread open to expose warmer layers beneath. The effect is similar to that seen in the Earth's oceanic ridges. These various fractures are thought to have been caused in large part by the tidal stresses exerted by Jupiter; Europa's surface is thought to rise and fall up to 30 meters between high and low tides. Since Europa is tidally locked with Jupiter and therefore always maintains the same orientation towards the planet the stress patterns should form a distinctive and predictable pattern. However, only the youngest of Europa's fractures appear to conform to this predicted pattern; other fractures appear to have occurred at increasingly different orientations the older they are. This pattern can be explained if Europa's surface has rotated slightly faster than its interior, an effect which is possible due to the subsurface ocean mechanically decoupling the moon's surface from its rocky mantle and to the effects of Jupiter's gravity tugging on the moon's outer crust. Comparisons of Voyager and Galileo probe photos suggest that Europa's crust rotates no faster than once every 10,000 years relative to its interior.
Another type of feature present on Europa's surface are circular and elliptical lenticulae, Latin for "freckles." Many are domes, some are pits and some are smooth dark spots. Others have a jumbled or rough texture. The dome tops look like pieces of the older plains around them, suggesting that the domes formed when the plains were pushed up from below. It is thought that these lenticulae were formed by diapirs of warm ice rising up through the colder ice of the outer crust, much like magma chambers in the Earth's crust. The smooth dark spots could be formed by meltwater released when the warm ice breaks the surface, and the rough, jumbled lenticulae (called regions of "chaos," for example the Conamara Chaos) appear to be formed from many small fragments of crust embedded in smooth dark surface like iceburgs in a frozen sea.
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 five others (Io, Callisto, 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.
The Galileo spaceprobe has found that Europa has a weak magnetic field (about 1/4 of the strength of Ganymede's and similar to Callisto's), and, most interestingly, it varies periodically as it passes through Jupiter's massive magnetic field. On March 2, 1998 NASA announced that Galileo had uncovered very strong evidence that there is a conducting material beneath Europa's surface, most likely a salty ocean. Spectrographic evidence suggests that the dark reddish streaks and features on Europa's surface appear to be rich in salts such as magnesium sulfate (Epsom salt), deposited by evaporating water that emerged from within. However, since the salts are generally colorless or white, some other material must also be present to account for the reddish color. Sulfur or iron compounds are suspected.
It has been suggested that life may exist in this under-ice ocean, perhaps subsisting in an environment similar to Earth's deep-ocean hydrothermal vents or the Antarctic Lake Vostok. There is no evidence to support this hypothesis at this time, however.
Data for Europa: