There is an asteroid by the same name - 52 Europa
Europa is the smallest and least massive of Jupiter's four galilean moons. Its planetary-sized, roughly as big and massive as Earth's Moon. Its an icy moon close to Jupiter, thanks to which has a relatively young surface driven by tidal heating.
Its probably has an iron-nickel core mainly of silicate rock, with a water-ice shell. Europa has a very thin atmosphere of mostly oxygen. Its geologically young white-beige surface is covered with cracks, but has very little craters. Its the smoothest surface of any Solar System object known, this is thanks to its subsurface ocean, possibly hosting life.
Other than telescopes on and near Earth Europa was explored by many space probes, earliest in the 1970s. The latest was Juno in September 2022, which flew 320 km away from the satellite.
Europa was discovered by Galileo Galilei and Simon Marius independently on the same day. Marius named it Europa after in greek mythology daughter of a phoenician king, mother of King Minos of Crete, most importantly, lover of Zeus - greek equivalent of roman god Jupiter, which is who the romans named the planet after. Majority of current known data on Europa was provided by Galileo mission in 1989.
Future space probes to Europa include Jupiter Icy Moons Explorer (Juice), while mainly to Ganymede will also fly past Europa twice, launched on 14 April 2023. NASA also launched Europa Clipper on 14 October 2024.
No spacecraft yet landed on the moon, although several such missions were suggested.
Discovery and naming
Europa was discovered together with Ganymede, Callisto and Io, the three other large satellites on Jupiter by Galileo Galilei in January 1610. However, on the first day (January 7) Galilei used a telescope with only 20x magnification, and Io and Europa appeared as one. He saw them as separate bodies the next day.
Simon Marius who independelty discovered Europa next day suggested the name and the names of three other galilean moons after lovers of Zeus. These names were not really used until mid-20th century, Europa was simply named Jupiter II, by the system suggested by Galileo. Discovery of Amalthea in 1892 which was closer and Europa was supposed to become Jupiter III. More objects were found later, and right now Europa is the sixth closest to Jupiter, although still refered 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. It has a nearly circular orbit with an eccentricity of only 0.009. Its orbital inclination to Jupiter's equator is also relatively small, at 0.470°. Like the other Galilean satellites, Europa is tidally locked to Jupiter, this means one half of the moon always faces the planet, and the other never does. This line is called Europa's prime meridian. Research suggests the tidal lock may not be full, as a non-synchronous rotationhas been proposed: Europa spins faster than it orbits, or at least did 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 is maintained by the gravitational disturbances from the other Galileans. It 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.
Physical characteristics
Europa is slightly smaller than the Moon at just over 3180 km in diameter. This makes it the sixth largest moon and fifteenth largest object in the Solar System. Its the least massive out of the galileans. Its density suggests its simmilar to terrestrial planets in composition, mainly made of silicate rock.
Internal structure
Europa is thought to have an outer layer of water around 100 km thick. A part of it is frozen being its crust, the rest is the subsurface ocean. Parts of Europa's crust have undergone a rotation of almost 80* (almost flipping over), which would be very unlikely to happen if its ice crust directly touched its mantle.
Magnetic-field data from the Galileo space probe shows Europa has an induced magnetic field from interaction with Jupiter's magnetic field, which suggests the presence of a subsurface conductive layer. This layer is likely a salty water ocean.
Europa probably has a metallic iron core.
Subsurface ocean
Its the scientifc consensus Europa has a layer of liquid water beneath its surface, and that it remains liquid thanks to heat from tidal flexing. Europa's surface temperatures avearage -160 *C at the equator, and -220 *C at the poles, which makes its ice crust as hard as granite.
Most scientists support the Thick Ice model. It says Europa's ocean rarely or never touches its surface, the best evidence for this is large craters and tidal warming heat. It estimates Europa's icy crust to be 10-30 km thick. Under this ice, an ocean can be up to 100 km deep. This would mean Europa holds 2x-3x more liquid water than Earth does.
Some scientists argue for the Thin Ice model, that the ice of Europa is very thin, pointing at "chaos terrain" where ice appears to float. One estimate suggests an only 200 meter thick ice. But this is a controversial idea.
Composition
Galileo spacecraft has discovered a weak magnetic field of Europa. It did not appear on its own, its created by the insanely powerful magnetic field of Jupiter. However for that to happen Europa would need a layer of electrically conductive material inside of it. The most likely candidate for this layer is a large subsurface ocean of salty liquid water.
Scientsits were curious about the reddish-brown material covering its young cracked surface ever since it was found to do so in 1979. Analysis of light from Europa suggests these areas could be covered in various salts, which could be left behind by water evaporating from below. It could also be a type of sulfuric acid. However, both salts and sulfuric acid when pure do not have a color, and something must mix with them to make it reddish. Sulfur compounds could do that.
Another idea is that the reddish-brown material are organic compounds (tholins), organic compounds formed by radiation of Sun or space itself, tholins are not signs of life tho. Shape of some features on Europa suggests water welled up from the cracks, where radiation can create them. Tholins are important for the search for life because they are the kinds of chemicals that can be the starting points for it.
New observations show Europa's underground ocean has simple table salt (sodium chloride). This was detected in recently resurfaced "chaos terrain". The ocean also turned out to contain carbon, and even carbon dioxide in a place called Tara Regio.
New telescope observations show water ice on Europa behaves differently in different areas. In the northern hemisphere its more disordered, but there is more structured ice below it. In the southern hemisphere the ice is crystalline both on surface and below surface. This is because Jupiter's strong radiation messes with it.
Radioactive decay
In addition to tidal heating, the interior of Europa could also be heated by the decay of radioactive material (radiogenic heating) within the rocky mantle. But the models and values observed are one hundred times higher than those that could be produced by radiogenic heating alone, thus implying that tidal heating has a leading role in Europa.
Trivia
- Europa's ice might glow
Gallery
Galilean satellites: Io * Europa * Ganymede * Callisto