Europa is considered by many planetary scientists to be the most likely place in our Solar System to harbour life, besides Earth. It is very cold on the surface, between 50 K and 110 K (-220 C to -160 C), but it's abundant in water. Our understanding of Europa's inner structure is based mostly on photographs taken by spacecraft, in particular the Galileo probe during its many flybys of the moon.
Europa is covered with a crust of ice, estimated to be 10-30 km thick, but planetary models indicate that underneath it there should be liquid ocean, as deep as 100 km. As Europa's eccentric orbit moves it closer or farther from Jupiter, the planet's tidal forces change in strength causing the moon to elongate slightly and then relax to its rounder shape. This constant squeezing and pulling is thought to generate enough heat to keep the ocean from freezing completely.
Europa has an atmosphere that's made mostly of oxygen. It is quite thin, with the surface pressure a trillion times lower than Earth's. The oxygen is not thought to be of biological origin. It's likely a result of molecules of water being split into oxygen and hydrogen by solar ultra-violet radiation and charged particles from Jupiter's magnetosphere.
A mission to Europa to examine it up close and to look for signs of life is being proposed by NASA's Jet Propulsion Laboratory together with Johns Hopkins University, Maryland. The spacecraft named Clipper would be launched in 2021 and enter an orbit of Jupiter some 3 years later to focus on flybys of Europa. The mission hasn't yet been funded so its future is uncertain, but exploring the moon is high on the list of priorities for future planetary exploration. ESA is planning to launch its own spacecraft JUICE (Jupiter Icy Moon Explorer) around the same time (2022) that would target also Ganymede and Callisto.
The image taken by Galileo in 1998 shows the surface of Europa with its characteristic lines and freckles, thought to be a result of liquid water or warmer ice erupting through to the surface of the moon.
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Image credit: NASA/JPL/University of Arizona/University of Colorado