Galilean moons
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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.
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[edit] 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 Ganymede 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.
[edit] 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 Comites', 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 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 |
[edit] Io
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 183 thousandths 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 go land on Io's in the future, they would have to be built extremely tough (similar to the tank-like bodies of the Soviet Venera landers) to survive the radiation and magnetic fields that originate from Jupiter.
[edit] Europa
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 would 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 color of the markings, reddish-brown, is thought to be caused by sulfur, but scientists cannot 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 would 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 was not used widely until mid-20th century.
[edit] Ganymede
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. Other than the Moon, Ganymede is the only planetary satellite that can be seen by the naked eye, but still can only be seen in good conditions. 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. The two terrain types are highly cratered dark regions and younger, but still ancient, regions with and 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.
[edit] Callisto
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 made of carbon dioxide. Callisto too was called Jupiter IV (four) until mid 20th-century, when Marius’s name came into operation.
[edit] Notes
- ^ Xi Zezong, The Discovery of Jupiter's Satellite Made by Gan De 2000 years Before Galileo, Chinese Physics 2 (3) (1982): 664-667.
[edit] See also
| Moons of Jupiter | |
|---|---|
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Listed in increasing distance from Jupiter. Italics indicate ungrouped moons.
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|
| Amalthea group | Metis · Adrastea · Amalthea · Thebe |
| Galilean moons | Io · Europa · Ganymede · Callisto |
| Themisto | |
| Himalia group | Leda · Himalia · Lysithea · Elara · S/2000 J 11 |
| Carpo · S/2003 J 12 | |
| Ananke group | Ananke · Praxidike · Harpalyke · Iocaste · Euanthe · Thyone (core) Euporie · S/2003 J 3 · S/2003 J 18 · Thelxinoe · Helike · Orthosie · S/2003 J 16 · Hermippe · Mneme · S/2003 J 15 (peripheral) |
| Carme group | S/2003 J 17 · S/2003 J 10 · Pasithee · Chaldene · Arche · Isonoe · Erinome · Kale · Aitne · Taygete · S/2003 J 9 · Carme · S/2003 J 5 · S/2003 J 19 · Kalyke · Eukelade · Kallichore |
| Pasiphaë group | Eurydome · S/2003 J 23 · Hegemone · Pasiphaë · Sponde · Cyllene · Megaclite · S/2003 J 4 · Callirrhoe · Sinope · Autonoe · Aoede · Kore |
| S/2003 J 2 | |
| 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 |
|
| The Solar System |
|---|
 |
| The Sun · Mercury · Venus · Earth · Mars · Ceres · Jupiter · Saturn · Uranus · Neptune · Pluto · Eris |
| Planets · Dwarf planets · Moons: Terrestrial · Martian · Asteroidal · Jovian · Saturnian · Uranian · Neptunian · Plutonian · Eridian |
| Small bodies: Meteoroids · Asteroids (Asteroid belt) · Centaurs · TNOs (Kuiper belt/Scattered disc) · Comets (Oort cloud) |
| See also astronomical objects, the solar system's list of objects, sorted by radius or mass, and the Astronomy Portal |
