Europa, a satellite of the planet Jupiter, has a diameter of 3,126 km (1,942 mi). It is the smallest of that planet's four Galilean satellites, so called because they were discovered by Galileo Galilei in 1610. In relation to its size, Europa is as smooth as a billiard ball. Its surface, about five times brighter than the Moon's surface, is composed primarily of water ice.Europa's density, however, is 3.01 times that of water. The satellite probably consists mostly of rock, with a substantial metallic core, and an outer mantle of water about 100 km thick, based on gravity measurements by the Galileo spacecraft. The body of Europa is continually flexed by the tidal effects of Jupiter, which vary periodically because of a resonant coupling among the orbits of satellites Io, Europa, and Ganymede. The tidal working may produce enough heat to keep much of the water in a liquid state, as a global ocean covered with a thin crust of ice. Scientists speculate that if watery depths do remain on Europa, and if organic molecules were deposited there by past cometary and asteroidal encounters, life could have evolved there.The surface is densely crisscrossed by linear ridge systems, usually consisting of pairs of ridges up to about 100 m (330 ft) high. Some complexes of these ridges form long, continuous features of global scale. They were visible even in the low-resolution images made by the Voyager spacecraft in 1979, because of their great size and their dark borders. These borders are composed of reddish material that appears to contain salts and perhaps organic compounds, possibly released from the ocean through cracks along the ridge systems. Galileo spacecraft images at much higher resolution showed ridges to be as densely packed as the lines in a ball of string. The ridges are likely associated with cracks in the icy crust that may penetrate through to the liquid ocean. A ridge seems to have grown along both sides of such cracks, yielding the characteristic double ridges.
Numerous dark bands on the surface separate regions of terrain that seem to have been pulled apart, or drifted apart, evidently sliding over the ocean or some other viscous layer. One crack on Europa is a fault comparable in size and in shear offset to the San Andreas Fault.Another common type of terrain found on Europa by the Galileo mission is typified by the Conamara Chaos region, in which older ridged crust appears to have been replaced by a lumpy matrix, reminiscent of frozen slush. Within this matrix, rafts whose surfaces are part of the older surrounding terrain seem to have floated to new positions. The region has the appearance that the crust has melted through, perhaps exposing the ocean temporarily, but long enough for chunks of crust to float around. Alternative explanations might involve solid-state convection in the ice.Europa has few craters, indicating that the current surface is very young, perhaps even as young as 10 million years. Thus the other geological features were formed recently in Europa's age. Some of the craters appear to have impacted sites where the crust was very thin, while others seem to have interacted with a thicker solid crust.Europa has an extremely thin atmosphere of molecular oxygen, with a surface pressure a hundred millionth the pressure of Earth's atmosphere at sea level. Moreover, interference of radio signals from the Galileo spacecraft revealed a tenuous ionosphere of charged particles around Europa. The particles are probably continually removed from the icy surface of the satellite by energetic particles in Jupiter's magnetosphere.