Data sent from the Galileo spacecraft indicates that Jupiter's moon Europa has a liquid ocean under a thick crust of ice.

Galileo was an American robotic space probe that studied the planet Jupiter and its moons, as well as the asteroids Gaspra and Ida. Named after the Italian astronomer Galileo Galilei, it consisted of an orbiter and an entry probe. It was delivered into Earth orbit on October 18, 1989, by Space Shuttle Atlantis, during STS-34. Galileo arrived at Jupiter on December 7, 1995, after gravitational assist flybys of Venus and Earth, and became the first spacecraft to orbit an outer planet.

Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than 2.5 times that of all the other planets in the Solar System combined and slightly less than one-thousandth the mass of the Sun. Its diameter is 11 times that of Earth and a tenth that of the Sun. Jupiter orbits the Sun at a distance of 5.20 AU (778.5 Gm), with an orbital period of 11.86 years. It is the third-brightest natural object in the Earth's night sky, after the Moon and Venus, and has been observed since prehistoric times. Its name derives from that of Jupiter, the chief deity of ancient Roman religion.

Europa is a natural satellite (moon) of Jupiter. Being observable from Earth with common binoculars, it is one of the four Galilean moons. As such it is a planetary-mass moon; the smallest and least massive orbiting Jupiter, and slightly smaller and less massive than Earth's. Europa is an icy moon, and, of the three icy Galilean moons, the closest orbiting Jupiter. As a result, it exhibits a relatively young surface, driven by tidal heating.