Europa (IPA: [juˈroʊpə] listen (help·info); Greek Ευρώπη) is the sixth nearest and fourth largest natural satellite of the planet Jupiter. Europa was discovered in 1610 by Galileo Galilei (and independently by Simon Marius shortly thereafter) and is the smallest of the four Galilean moons named in Galileo's honor.
Europa is primarily composed of silicate rock, has an outer layer of water, and likely has an iron core. At just over 3000 kilometers in diameter, it is slightly smaller than the Earth's moon and the sixth largest moon in the solar system. The satellite has a very tenuous oxygen atmosphere and one of the smoothest surfaces in the solar system. The young surface of the planet is straited by cracks and streaks, while craters are relatively infrequent. Due to an hypothesized water ocean beneath its icy surface, and an energy source provided by tidal heating, Europa has been cited as a possible host of extraterrestrial life.

Discovery and naming
Europa orbits Jupiter in just over three and a half days, with an orbital radius of about 670,900 km (416,900 mi). The satellite follows a very nearly circular orbit, with an eccentricity of only 0.009. Europa's orbital inclination relative to the Jovian equatorial plane is also slight, at 0.470°.
Because of its orbit's slight eccentricity, maintained by the gravitational disturbances from the other Galilean satellites of the planet, the sub-jovian point oscillates about a mean position. Europa strives to assume a slightly elongated shape pointing towards Jupiter in response to the tidal force of the giant planet; because different parts of Europa end up being on different points of this departure from sphericity at varying times, the crust flexes up and down. This motion dissipates energy from Jupiter's rotation into Europa (tidal heating), giving the moon a source of heat and energy, allowing the subsurface ocean to stay liquefied and driving subsurface geological processes.

Orbit

Physical characteristics
Europa is somewhat similar in bulk composition to the terrestrial planets, being primarily composed of silicate rock. It has an outer layer of water thought to be around 100 km thick (some, as frozen ice upper crust; some, as liquid ocean underneath the ice), and recent magnetic field data from the Galileo orbiter probe, which orbited Jupiter and studied Europa between 1995 and 2003, shows that Europa generates an induced magnetic field by interacting with Jupiter's field, which suggests the presence of a subsurface conductive layer which is likely a salty liquid-water ocean. Europa probably also contains a metallic iron core.

Internal structure
The Europan surface is relatively smooth; few features more than a few hundred metres high have been observed, but topographic relief in places approaches a kilometre (0.62 mi)). Cynthia Phillips, a member of SETI and an expert on Europa states there is currently no consensus among the often contradictory explanations for the surface features of Europa.[2]

Surface features

Main article: List of lineae on Europa Lineae

Main article: List of geological features on Europa Other geological features
It is thought that under the surface there is a layer of liquid water kept warm by tidally generated heat. The temperature on the surface of Europa averages about 110 K (-163 °C) at the equator and only 50 K (-223 °C) at the poles, and so the surface water ice is permanently frozen. The first hints of a subsurface ocean came from theoretical considerations of the tidal heating (a consequence of Europa's slightly eccentric orbit and orbital resonance with the other Galilean moons). Galileo imaging team members have analyzed Voyager and Galileo images of Europa to argue that Europa's geological features also demonstrate the existence of a subsurface ocean. Spectrographic evidence suggests that the dark reddish streaks and features on Europa's surface may be rich in salts such as magnesium sulfate, deposited by evaporating water that emerged from within. Sulfuric acid hydrate is another possible explanation for the contaminant observed spectroscopically. In either case, since these materials are colorless or white when pure, some other material must also be present to account for the reddish color. Sulfur compounds are suspected.

Atmosphere
Most human knowledge of Europa has been derived from a series of flybys since the 1970s. The sister craft Pioneer 10 and Pioneer 11 were the first to visit Jupiter, in 1973 and 1974, respectively; the first photos of Jupiter's largest moons produced by the Pioneers were fuzzy and dim.

Exploration
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.

Spacecraft proposals and cancellations

Jupiter's moons in fiction
List of craters on Europa
List of lineae on Europa
List of geological features on Europa
List of Jupiter's moons
Colonization of Europa