On the 8th of January 1610, Galileo Galilei observed a faint point of light near Jupiter that would become known as Europa. He used a 20×-magnification refracting telescope at the University of Padua to see this object alongside Io and Callisto. The low resolution of his instrument could not separate the two objects on the first night he looked. On the following night, he saw Io and Europa for the first time as distinct bodies. Simon Marius also discovered the moon independently around the same time. Marius later named it after Europa, the Phoenician mother of King Minos of Crete and lover of Zeus. This naming scheme was suggested by Johannes Kepler according to historical records from 1614. 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 II or as the second satellite of Jupiter. In 1892, the discovery of Amalthea pushed Europa to the third position. The Voyager probes discovered three more inner satellites in 1979, so Europa is now counted as Jupiter's sixth satellite though it is still referred to as II.
Orbital Dynamics And Tidal Heating
Europa orbits Jupiter in roughly 3.55 days with an orbital radius of about 670,900 km. Its orbit has an eccentricity of only 0.009 making it nearly circular. The orbital inclination relative to Jupiter's equatorial plane is small at 0.470 degrees. Like its fellow Galilean satellites, Europa is tidally locked to Jupiter with one hemisphere constantly facing the planet. Because of this there is a sub-Jovian point on Europa's surface where Jupiter would appear to hang directly overhead. Research suggests that tidal locking may not be full as a non-synchronous rotation has 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. The slight eccentricity of Europa's orbit causes its sub-Jovian point to oscillate around a mean position. As Europa comes slightly nearer to Jupiter Jupiter's gravitational attraction increases causing Europa to elongate towards and away from it. As Europa moves slightly away from Jupiter Jupiter's gravitational force decreases causing Europa to relax back into a more spherical shape creating tides in its ocean. The orbital eccentricity of Europa is continuously pumped by its mean-motion resonance with Io. Thus the tidal flexing kneads Europa's interior and gives it a source of heat possibly allowing its ocean to stay liquid while driving subsurface geological processes.