At just over 3,100 kilometres (1,900 mi) in diameter, Europa is slightly smaller than Earth 's Moon and is the sixth-largest moon in the Solar System . Though by a wide margin the least massive of the Galilean satellites , its mass nonetheless significantly exceeds the combined mass of all moons in the Solar System smaller than itself.It is primarily made of silicate rock and likely has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This young surface is striated by cracks and streaks, while craters are relatively infrequent. The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath it, which could conceivably serve as an abode for extraterrestrial life. Heat energy from tidal flexing ensures that the ocean remains liquid and drives geological activity.
Although only fly-by missions have visited the moon, the intriguing characteristics of Europa have led to several ambitious exploration proposals. The Galileo mission provided the bulk of current data on Europa. A new mission to Jupiter's icy moons, the Europa Jupiter System Mission (EJSM) is proposed for a launch in 2020. Conjecture on extraterrestrial life has ensured a high profile for the moon and has led to steady lobbying for future missions.
Discovery and naming
Europa, along with Jupiter's three other largest moons, Io, Ganymede, and Callisto, was discovered by Galileo Galilei in January 1610. The first reported observation of Io was made by Galileo Galilei on January 7, 1610 using a 20x-power, refracting telescope at the University of Padua. However, in that observation, Galileo could not separate Io and Europa due to the low power of his telescope, so the two were recorded as a single point of light. Io and Europa were seen for the first time as separate bodies during Galileo's observations of the Jupiter system the following day, January 8, 1610 (used as the discovery date for Europa by the Astronomical Union IAU).[1] Like all the Galilean satellites, Europa is named after a lover of Zeus, the Greek counterpart of Jupiter, in this case Europa, daughter of the king of Tyre. The naming scheme was suggested by Simon Marius, who apparently discovered the four satellites independently, though Galileo alleged that Marius had plagiarized him. Marius attributed the proposal to Johannes Kepler.[18] The names fell out of favor for a considerable time and were not revived in general use until the mid-20th century. In much of the earlier astronomical literature, Europa is simply referred to by its Roman numeral designation as Jupiter II (a system introduced by Galileo) or as the "second satellite of Jupiter". In 1892, the discovery of Amalthea, whose orbit lay closer to Jupiter than those of the Galilean moons, pushed Europa to the third position. The Voyager probes discovered three more inner satellites in 1979, so Europa is now considered Jupiter's sixth satellite, though it is still sometimes referred to as Jupiter II.
Orbit and rotation
Europa orbits Jupiter in just over three and a half days, with an orbital radius of about 670,900 km. With an eccentricity of only 0.009, the orbit itself is nearly circular, and the orbital inclination relative to the Jovian equatorial plane is small, at 0.470°. Like its fellow Galilean satellites, Europa is tidally locked to Jupiter, with one hemisphere of the satellite constantly facing the planet. Europa's prime meridian intersects the north and south poles, and the equator at the sub-Jovian point. Research suggests the tidal locking may not be full, as a non-synchronous rotationhas been proposed: Europa spins faster than it orbits, or at least did so in the past. This suggests an asymmetry in internal mass distribution and that a layer of subsurface liquid separates the icy crust from the rocky interior.[22] The slight eccentricity of Europa's orbit, maintained by the gravitational disturbances from the other Galileans, causes Europa's sub-Jovian point to oscillate about a mean position. As Europa comes slightly nearer to Jupiter, the planet's gravitational attraction increases, causing the moon to elongate towards it. As Europa moves slightly away from Jupiter, the planet's gravitational force decreases, causing the moon to relax back into a more spherical shape. The orbital eccentricityof Europa is continuously pumped by its mean-motion resonance with Io. Thus, the tidal flexing kneads Europa's interior and gives the moon a source of heat, allowing its ocean to stay liquid and driving subsurface geological processes. The ultimate source of this energy is Jupiter's rotation, which is tapped by Io through the tides it raises on Jupiter and is transferred to Europa and Ganymede by the orbital resonance.