Europa

Europa (Jupiter II) is one of Jupiter's first four discovered moons, and of them, the smallest and the second outward from the planet. It is one of the four Galilean Moons discovered centuries ago and is the seventh outward according to current knowledge. Its radius is 1564 km (a quarter of Earth's), its mass is 4.5×10²² kg (about a hundredth of Earth's), with a mean density about three times that of water (about 1/2 Earth's) and oceans are suspected. Europa's orbital period is 3.5 days and it is tidally locked. Its orbit is in sync with Ganymede (1/2) and Io (2×). Flyby observations of Europa have been made by the Pioneer, Voyager, Galileo, and Juno missions.

Current theory presumes Europa has a subsurface ocean of water, which is liquid due to tidal heating (Europa is beyond the snow line, where bodies can cool to the point where water would be frozen). Evidence is that the surface is apparently water ice with a smooth surface and high albedo, gravity measurements suggest the right mass distribution, and response to Jupiter's magnetic field suggest an internal conductor such as salt water. Its surface shows both characteristics of the Earth's crust (e.g., few visible impact craters) and of the Earth's ice shelves. The surface's markings are cracks, pits, and domes. Patterns in the cracks suggest an analog to Earth's crustal plates, showing subsumption bands, regions where the pattern where surface features show a discontinuity suggesting some surface was forced downward, analogous to Earth subduction zones. The surface also has regions with some color and there is research and theories as to the cause. Given the ocean, there is debate whether the ice constitutes a thin shell (e.g., less than 10 km, making leaks and plumes plausible) or a thick shell (e.g., tens of km or more), with some thought that it has some of each. The presumed oceans make Europa a candidate for life. If the ocean is salty, a question of interest is what type of salt, and whether it is in contact with a rocky core, which would present the opportunity for some chemical reactions and increase the likelihood of generation of molecules associated with life.