Europa, the brightest of Jupiter's satellites, is about 1,950 mi (3,160 km) in diameter or about the size of Earth's Moon. Its density is about three times that of water. The moon is covered with a thin ice crust and is crisscrossed with an amazingly complex network of ridges. Some of the fractures on its crust are more than 1,850 mi (3,000 km) long. Very few impact craters are visible on the surface. In fact, Europa is the smoothest object in the solar system. Its mostly flat surface doesn't exceed 0.62 mi (1 km) in height.
Galileo spacecraft photos taken at its closest flyby on Feb. 20, 1997, at a distance of 363 mi (586 km), showed the existence of ice flows on the surface that strongly suggest that the moon has a hidden subsurface ocean of water or ice-slush. The photos revealed chunky ice rafts that appear to be floating, comparable to icebergs on Earth. The presence of water and enough heat to keep water in a liquid state on Europa enhances the possibility that it could provide an environment for some form of extraterrestrial ocean life.
New evidence that a liquid ocean lies beneath Europa's crust was found when Galileo visited the moon in Jan. 2000. The spacecraft detected changes in Europa's magnetic field that are best explained by an electrically conducting (salty) body of water.
Definitive answers may not be possible for another decade, however. NASA scientists had proposed sending a spacecraft called the Europa Ice Clipper to the moon in 2001, but the mission was scrapped due to lack of funding.