Europa is the smallest of the four Galilean moons, but still one of the largest bodies in the Solar system. Europa is the fourth largest moon of Jupiter and the sixth-largest moon in the Solar System.
Discovery
Europa's discovery is credited to Galileo Galilei, who was the first to observe it on January 7, 1610.
Together with Ganymede, Callisto and Io, they are collectively known as Galilean satellites after the discover.
Naming
Europa is named after a Phoenician noblewoman in Greek mythology, Europa, who was courted by Zeus and became the queen of Crete.
Stats
Diameter: 3,122 km
Semi-major axis: 671,034 km
Orbital Period: 3.55 days
Orbit
Europa is in a 1:2 mean-motion orbital resonance with Io, completing one orbit of Jupiter for every two orbits completed by Io.
Like its fellow Galilean satellites, Europa is tidally locked to Jupiter, with one hemisphere of the satellite constantly facing the planet.
Subsurface ocean
Most planetary scientists believe that a layer of liquid water exists beneath Europa's surface, kept warm by tidally generated heat.
Europa's surface temperature averages about 110 K (−160 °C) at the equator and only 50 K (−220 °C) at the poles, keeping Europa's icy crust as hard as granite.
The first hints of a subsurface ocean came from theoretical considerations of tidal heating (a consequence of Europa's slightly eccentric orbit and orbital resonance with the other Galilean moons).
Galileo imaging team members argue for the existence of a subsurface ocean from analysis of Voyager and Galileo images. The most dramatic example is "chaos terrain", a common feature on Europa's surface that some interpret as a region where the subsurface ocean has melted through the icy crust.
Thick-ice Model
The best evidence for the thick-ice model is a study of Europa's large craters. The largest impact structures are surrounded by concentric rings and appear to be filled with relatively flat, fresh ice.
Based on this and on the calculated amount of heat generated by Europan tides, it is predicted that the outer crust of solid ice is approximately 10–30 km thick, including a ductile "warm ice" layer, which could mean that the liquid ocean underneath may be about 100 km deep. This leads to a volume of Europa's oceans of 3 × 1018 m3, slightly more than two times the volume of Earth's oceans.
Thin-ice Model
The thin-ice model suggests that Europa's ice shell may be only a few kilometers thick. However, most planetary scientists conclude that this model considers only those topmost layers of Europa's crust which behave elastically when affected by Jupiter's tides.
One example is flexure analysis, in which the moon's crust is modeled as a plane or sphere weighted and flexed by a heavy load. Models such as this suggest the outer elastic portion of the ice crust could be as thin as 200 metres.
If the ice shell of Europa is really only a few kilometers thick, this "thin ice" model would mean that regular contact of the liquid interior with the surface could occur through open ridges, causing the formation of areas of chaotic terrain.
Atmosphere
Observations with the Goddard High Resolution Spectrograph of the Hubble Space Telescope revealed that Europa has a tenuous atmosphere composed mostly of molecular oxygen (O2).
Unlike the oxygen in Earth's atmosphere, Europa's is not of biological origin. The surface-bounded atmosphere forms through radiolysis, the dissociation of molecules through radiation.
Solar ultraviolet radiation and charged particles (ions and electrons) from the Jovian magnetospheric environment collide with Europa's icy surface, splitting water into oxygen and hydrogen constituents. These chemical components are then adsorbed and "sputtered" into the atmosphere.
Molecular oxygen is the densest component of the atmosphere because it has a long lifetime; after returning to the surface, it does not freeze like water or hydrogen peroxide molecule but rather desorbs from the surface and starts another ballistic arc. Molecular hydrogen never reaches the surface, as it is light enough to escape Europa's surface gravity.
Life
Europa has emerged as one of the top Solar System locations in terms of potential habitability and possibly, hosting extraterrestrial life.
Life could exist in its under-ice ocean, perhaps subsisting in an environment similar to Earth's deep-ocean hydrothermal vents. Life in such an ocean could possibly be similar to microbial life on Earth in the deep ocean.
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