Galilean satellites

It has been suggested that Medician stars be merged into this article or section. (Discuss)

Jupiter's 4 Galilean moons, in a composite image comparing their sizes and the size of Jupiter (Great Red Spot visible). From the top, they are Io, Europa, Ganymede, Callisto

The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei. They are by far the largest of the many moons of Jupiter.

Discovery

The Galilean moons were first observed by Galileo on January 7, 1610. A Chinese historian of astronomy, Xi Zezong, has claimed that Gan De, a Chinese astronomer, may have seen one of Jupiter's moons in 362 BC, nearly 2 millennia earlier^[1].

Galileo observed the moons' motion over several days and realized that they were in orbit around Jupiter. This discovery supported the heliocentric theory of Nicolaus Copernicus and showed that not everything in the heavens revolves around Earth.

Galileo also developed a method of determining longitude based on the timing of the orbits of the Galilean moons.

Name

Galileo initially called his discovery the Cosmica Sidera ("Cosimo's stars"), in honour of Cosimo II de' Medici (1590–1621), grand-duke of Tuscany from 1609, whose patronage he wanted to secure. At the grand-duke's suggestion, Galileo changed the name to Medicea Sidera ("the Medician stars"), honouring all four Medici brothers (Cosimo, Francesco, Carlo, and Lorenzo). The discovery was announced in the Sidereus Nuncius ("Starry Messenger"), published in Venice in March 1610, less than two months after the first observations.

Other names put forward include 'Principharus, Victipharus, Cosmipharus and Ferdinandipharus', for each of the four Medici brothers, proposed by Giovanni Batista Hodierna, a disciple of Galileo and author of the first ephemerides (Medicaeorum Ephemerides, 1656) Johannes Hevelius called the moons the 'Circulatores Jovis' or 'Jovis Committees', and Jacques Ozanam called them 'Gardes' or 'Satellites' (from the Latin satelles, satellitis, meaning "escorts").

The names that eventually prevailed were chosen by Simon Marius, who claimed to have discovered the moons at the same time as Galileo: he named them after lovers of the god Zeus (the Greek equivalent of Jupiter): Io, Europa, Ganymede and Callisto, in his Mundus Jovialis, published in 1614.

Galileo steadfastly refused to use Marius's names and invented as a result the numbering scheme that is still used nowadays, in parallel with proper moon names. The numbers run from Jupiter outward, thus I, II, III and IV for Io, Europa, Ganymede and Callisto respectively. Galileo used this system in his notebooks but never actually published it.

The Galilean moons are, in order from closest to Jupiter to farthest away:

Name	Image	Interior structure	Diameter (km)	Mass (kg)	Semi-major axis (km)	Orbital period (days)
Io			3643	8.93×1022	421,800	1.77
Europa			3122	4.8×1022	671,100	3.55
Ganymede			5262	1.48×1023	1,070,400	7.16
Callisto			4821	1.08×1023	1,882,700	16.69

Visibility

All four Galilean moons are bright enough that they could, if they were farther away from Jupiter, be sighted without a telescope. They have apparent magnitudes between 4.5 and 5.2 when Jupiter is in opposition with the Sun, and about one unit of magnitude higher when Jupiter is in conjunction. The main difficulty in observing them is due to the fact that they are located very close to Jupiter, and are masked by its brightness. Their maximum angular separations from Jupiter are between 2 and 8 minutes of arc, close to the limit of human visual acuity. Ganymede and Callisto, at their maximum separation, are the likeliest targets for potential naked-eye observation. The easiest way to observe them is to cover Jupiter with an object, e.g. a tree limb or a power line that is perpendicular to the plane of moons' orbits.

Io

Main article: Io (moon)

Of the four Galilean moons, Io is the closest to Jupiter and the second smallest. Io is the fourth largest moon in the solar system, is the most volcanically active body in the solar system, has had the largest recorded volcanic eruptions, and has the highest density of all the moons in the solar system. Io is estimated to be 3642.6 kilometers in diameter, and, because of its smaller size, only has 18.3% Earth’s gravity. Io got its name from Greek mythology, and although Simon Marius suggested the name soon after its discovery, Io was simply referred to as “Jupiter I,” or “The first satellite of Jupiter,” until mid-20th century. Io does not seem to have many craters in pictures because it has many active volcanoes, 9 of which were observed by Voyager I. Although not proven, recent data from the Galileo orbiter indicates that Io might have its own magnetic field. Io has an extremely thin atmosphere made up mostly of sulfur dioxide (SO₂). If a surface data or collection vessel were to land on Io in the future, it would have to be extremely tough (similar to the tank-like bodies of the Soviet Venera landers) to survive the radiation and magnetic fields that originate from Jupiter.

Europa

Main article: Europa (moon)

Europa, the second of the four Galilean moons, is the second closest to Jupiter and the smallest at 3121.6 kilometers in diameter. Europa is the smoothest object in the solar system. There is a layer of water surrounding the mantle of the planet, thought to be 100 kilometers thick. The top of the water is ice and under the ice, the water is liquid. If any life exists in the water, it may be similar to Earth’s deep-sea creatures. Several things could cause the marks on the surface of the moon. Some theories are that Jupiter’s gravity is causing these markings, as one side of Europa is constantly facing Jupiter; volcanic water eruptions splitting the surface of Europa; and even geysers have been considered as a cause. The colour of the markings, reddish-brown, is thought to be caused by sulfur, but scientists can't be sure, as no data collection devices have been sent to Europa. These markings crossing the moon emphasize a flatter, smoother surface. Some have even considered the reddish-brown markings to be bacteria under the ice on Europa. Any bacteria on Europa may be similar to the extremophile bacteria found on Earth because of the intense conditions on this moon. The name Jupiter II was used for a long time instead of Europa. Again, Europa’s name was from Greek mythology, was suggested by Simon Marius, and wasn't used widely until mid-20th century.

Ganymede

Main article: Ganymede (moon)

Ganymede, the third Galilean moon and the seventh known satellite from Jupiter, is less shrouded in mystery than Europa. Ganymede is the largest moon in the entire solar system at 5262.4 kilometers in diameter. Ganymede is the only moon in the solar system known to have a magnetic field. Ganymede is composed of silicate rock and water ice, with an ice crust floating over a warmer ice mantle. The metallic core of Ganymede suggests a greater heat at some time in Ganymede's past than had previously been proposed. The surface of Ganymede is a mix of two types of terrain – highly cratered dark regions and younger, but still ancient, regions with a large array of grooves and ridges. Ganymede has a high number of craters, but many are gone or barely visible due to Ganymede’s icy crust forming over them. A small oxygen atmosphere is present on Ganymede. Ganymede was simply called Jupiter III (three) until mid 20th-century, when Simon Marius’s name began to be used.

Callisto

Main article: Callisto (moon)

Callisto is the fourth and last Galilean moon, and is the second largest at 4820.6 kilometers in diameter. Callisto is also the least dense of the Galilean moons. Callisto is one of the most heavily cratered satellites in the solar system, and has one major feature, a basin around 3000 km wide called Valhalla, which probably dates back to the formation of Callisto’s crust. This moon’s surface lies above a layer of ice, which is 150 kilometers thick, and a layer of water, which is ten kilometers thick. Callisto’s crust is approximately four billion years old. Callisto has a small atmosphere of carbon dioxide. Callisto too was called Jupiter IV (four) until mid 20th-century, when Marius’s name came into operation.

Notes

See also

• Jupiter's natural satellites
• Jupiter's moons in fiction

External links

• Animation of Galileo's observation, march 1613

Natural satellites of the Solar System
Planetary satellites	Terrestrial · Martian · Jovian · Saturnian · Uranian · Neptunian
Other satellite systems	Plutonian · Eridian · Asteroid satellites
Largest satellites	Ganymede · Titan · Callisto · Io · Moon · Europa · Triton Titania · Rhea · Oberon · Iapetus · Charon · Umbriel · Ariel · Dione · Tethys  · Enceladus · Miranda · Proteus · Mimas
Inner satellites • Trojans • Irregulars • List • List by diameter • Timeline of discovery • Naming

Jupiter
Major Moons	Io · Europa · Ganymede · Callisto
Characteristics	Cloud pattern on Jupiter · Great Red Spot · Jupiter's magnetosphere · Oval BA · Rings of Jupiter
Exploration	Pioneer program · Voyager program · Galileo (spacecraft) · Juno (spacecraft) · Europa Orbiter · Exploration · Colonization
Other	Jupiter-crosser asteroid · Earthly Branches · Jupiter mass

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