Galilean Moons and Other Jovian Moons

Jupiter has 67 confirmed moons, more than any other planet in the Solar System. Most famous of them are four Galilean moons discovered by Galileo Galilei in 1610. By discovering these four moons, Galileo became convinced that Earth orbits the Sun and not the other way around, since it was clear to him that there are celestial bodies that orbit other planets and that Earth is not the centre of the Universe as it was believed at the time. Beside Galilean moons, only four other moons are regular satellites in orbit around Jupiter and they are closer to the planet than Galilean moons. 

All other moons have irregular orbits, with great eccentricities, both prograde and retrograde orbits and high inclinations with respect to Jupiter’s equator. Retrograde orbit is when a moon is orbiting a planet in the direction opposite of the planet’s rotation.

Eight innermost, regular Jovian moons are thought to have been created from the planet’s subnebula that existed around Jupiter after it has formed. When Jupiter was younger, this subnebula was denser, causing moons to slow down due to the friction and eventually crash into the planet. The moons we see today are probably the fifth generation of Jovian moons.

Galilean Moons

Galilean moons are Io, Europa, Ganymede and Callisto. They are very massive compared to other Jovian moons, having 99.997% of the mass of all moons and rings in Jupiter’s orbit. Io, Europa, Ganymede and Callisto are the fourth, sixth, first and third largest natural satellites, respectively. Ganymede is larger even than the planet Mercury. If they were not orbiting Jupiter, they would have been either regular or dwarf planets. Four Galilean moons are more massive than any of the dwarf planets we have discovered so far. They are named after Zeus’s lovers, Jupiter being the Roman equivalent of Zeus. Io, Europa and Ganymede revolve in a 1:2:4 resonance, meaning that for one Ganymede’s orbit, Europa makes exactly two and Io exactly four orbits around Jupiter. Io, Europa, Ganymede and Callisto have orbital period of 1.77, 3.55, 7.16 and 16.7 Earth days, respectively.

Io is the innermost Galilean moon with a diameter of 3,642 km. It is geologically the most active object in the Solar System with over 400 active volcanoes. Its proximity to Jupiter exerts tidal forces on it that are keeping Io’s interior from cooling down, hence the geological activity. It might have its own magnetic field, but this was not yet confirmed. It orbits Jupiter at the distance of 421,800 km.

Europa is the second closest Galilean moon, seen from Jupiter. It is the smallest of four with diameter of 3121.6 km. It has a smooth surface made of water ice, under which is thought to be a water ocean 100 km thick. Tidal forces keep the water liquid and this could potentially allow for extraterrestrial life similar to that in depths of Earth’s oceans. Europa is tidally locked to Jupiter i.e. it is always facing the planet with the same side. Its distance from Jupiter is 671,100 km.

Ganymede is the third distant Galilean moon and the largest natural satellite in the Solar System with the diameter of 5262.4 km. It is larger even than the planet Mercury, although Ganymede has only half of Mercury’s mass, due to being icy world. Its surface is covered with craters and it is only satellite in the Solar System with confirmed magnetic field. Between aforementioned layers of ice, at the depth 200 km below the surface, there could be a water ocean. Ganymede’s atmosphere is thin, but it has oxygen. Its orbit is 1,070,400 km from Jupiter’s surface.

Callisto is the fourth Galilean moon with the diameter of 4820.6 km. It’s surface is covered with many craters, but underneath it could be an ocean of liquid water. The moon is made of rock and ices and is the least dense of the four Galilean moons. It has a thin atmosphere made of carbon dioxide and possibly molecular oxygen. Just like Europa, it is tidally locked to Jupiter, always facing the planet with the same side. Callisto orbits the planet at the distance of 1,882,700 km.

Amalthea Group

Amalthea group is the name for four innermost Jovian moons that have regular, prograde and nearly circular orbit. They are called Metis, Adrastea, Amalthea and Thebe. Metis and Adrastea orbit Jupiter in less time than it takes the planet to rotate around its own axis. These four moons replenish and maintain Jupiter’s ring system. Amalthea is the largest of four, with the diameter of 250 km and it either formed farther from the planet or was captured by it. It was discovered in 1892, whereas other three were discovered in 1979 as Voyager 1 probe flew by Jupiter.

Irregular Satellites

The majority of 59 irregular Jovian moons was not discovered until the 21st century. This is not surprising considering that most of them have diameter of under 5 km. Pasiphae is the largest with 60 km diameter, discovered in 1908. The second largest is Carme, discovered 30 years later in 1938, with the diameter of 46 km.

All these satellites have high orbital inclination and eccentricity as well as retrograde orbits. They were probably asteroids and comets that were captured by Jupiter’s gravity.

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