thumb|Montage of [[Jupiter's four Galilean moons in a composite image depicting part of Jupiter and their relative sizes (positions are illustrative, not actual). From top to bottom: Io, Europa, Ganymede, Callisto.]]

The Galilean moons (), or Galilean satellites, are the four largest moons of Jupiter. They are, in descending-size order, Ganymede, Callisto, Io, and Europa. They are the most readily visible Solar System objects after Saturn, the dimmest of the classical planets; though their closeness to bright Jupiter makes naked-eye observation very difficult, they are readily seen with common binoculars, even under night sky conditions of high light pollution. The invention of the telescope allowed astronomers to discover the moons in 1610. Through this, they became the first Solar System objects discovered since humans have started tracking the classical planets, and the first objects to be found to orbit any planet beyond Earth.

They are planetary-mass moons and among the largest objects in the Solar System. All four, along with Titan, Triton, and Earth's Moon, are larger than any of the Solar System's dwarf planets. The largest, Ganymede, is the largest moon in the Solar System and surpasses the planet Mercury in size (though not mass). Callisto is only slightly smaller than Mercury in size; the smaller ones, Io and Europa, are about the size of the Moon. The three inner moons — Io, Europa, and Ganymede — are in a 4:2:1 orbital resonance with each other. While the Galilean moons are spherical, all of Jupiter's remaining moons have irregular forms because they are too small for their self-gravitation to pull them into spheres.

The Galilean moons are named after Galileo Galilei, who observed them in either December 1609 or January 1610, and recognized them as satellites of Jupiter in March 1610; they remained the only known moons of Jupiter until the discovery of the fifth largest moon of Jupiter Amalthea in 1892. Galileo initially named his discovery the Cosmica Sidera ("Cosimo's stars") or Medicean Stars, but the names that eventually prevailed were chosen by Simon Marius. Marius discovered the moons independently at nearly the same time as Galileo, 8 January 1610, and gave them their present individual names, after mythological characters that Zeus seduced or abducted, which were suggested by Johannes Kepler in his Mundus Jovialis, published in 1614. Their discovery showed the importance of the telescope as a tool for astronomers by proving that there were objects in space that cannot be seen by the naked eye. The discovery of celestial bodies orbiting something other than Earth dealt a serious blow to the then-accepted (among educated Europeans) Ptolemaic world system, a geocentric theory in which everything orbits around Earth.

History

Pre-discovery

thumb|Photograph of Jupiter and its Galilean moons, showing how they appear with binoculars

Chinese astronomical records report that in 365 BC, Gan De detected what might have been a moon of Jupiter, probably Ganymede, with the naked eye. However, Gan De reported the color of the companion as reddish, which is puzzling since moons are too faint for their color to be perceived with the naked eye. Shi Shen and Gan De together made fairly accurate observations of the five major planets. If the companion is indeed Ganymede, this might predate Galileo's discovery by around two millennia.

Discovery

thumb|Galileo Galilei's observations of 13 January 1610 in a manuscript noting the positions of the Jovian moons, with modern depiction of their positions

As a result of improvements that Galileo Galilei made to the telescope, with a magnifying capability of 20×, he was able to see celestial bodies more distinctly than was previously possible. This allowed Galileo to observe in January 1610 what came to be known as the Galilean moons.

On 7 January 1610, Galileo wrote a letter containing the first mention of Jupiter's moons. At the time, he saw only three of them, and he believed them to be fixed stars near Jupiter. It turned out to be Ganymede, Callisto, and the combined light from Io and Europa. The next night he noticed that they had moved. On January 13, he saw all four at once for the first time, but had seen each of the moons before this date at least once. By January 15, Galileo concluded that the stars were actually bodies orbiting Jupiter. He continued to observe these celestial orbs to 2 March 1610.

Galileo's discovery proved the importance of the telescope as a tool for astronomers by showing that there were objects in space to be discovered that until then had remained unseen by the naked eye. More importantly, the discovery of celestial bodies orbiting something other than Earth dealt a blow to the then-accepted Ptolemaic world system, which held that Earth was at the center of the universe and all other celestial bodies revolved around it. Galileo's 13 March 1610, Sidereus Nuncius (Starry Messenger), which announced celestial observations through his telescope, does not explicitly mention Copernican heliocentrism, a theory that placed the Sun at the center of the universe. Nevertheless, Galileo accepted the Copernican theory. However, because he did not publish these findings until after Galileo, there is a degree of uncertainty around his records.

Names

thumb|right|upright|The Medician stars in the [[Sidereus Nuncius (the 'starry messenger'), 1610. The moons are drawn in changing positions.]]

thumb|A [[Jovilabe: an apparatus from the mid-18th century for demonstrating the orbits of Jupiter's satellites]]

In 1605, Galileo had been employed as a mathematics tutor for Cosimo de' Medici. In 1609, Cosimo became Grand Duke Cosimo II of Tuscany. Galileo, seeking patronage from his now-wealthy former student and his powerful family, used the discovery of Jupiter's moons to gain it. – by Giovanni Battista Hodierna, a disciple of Galileo and author of the first ephemerides (Medicaeorum Ephemerides, 1656);

  • Circulatores Jovis, or Jovis Comites – by Johannes Hevelius;
  • Gardes, or Satellites (from the Latin satelles, satellitis, meaning "escorts") – by Jacques Ozanam.

The names that eventually prevailed were chosen by Simon Marius, who had discovered the moons independently at about the same time as Galileo. At the suggestion of Johannes Kepler, he named them after four lovers of the god Zeus (the Greek equivalent of Jupiter) 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.

<gallery caption="The Galilean moons' namesakes">

File:Io Argos MAN Napoli Inv9556.jpg|Io (left) watched by Argus Panoptes (right) on Hera's orders

File:Wall painting - Europa and the bull - Pompeii (IX 5 18-21) - Napoli MAN 111475 - 02.jpg|Europa on the back of Zeus turned into a bull

File:Zeus abducts Ganymede, large terracotta, before 470 BC, AM Olympia, Olym26.jpg|Ganymede (left) abducted by Zeus (right)

File:Wall painting - Artemis and Kallisto - Pompeii (VII 12 26) - Napoli MAN 111441.jpg|Callisto (leftmost) with Eros and other nymphs, with Artemis seated

</gallery>

Determination of longitude

thumb|Map of France presented in 1684, showing the outline of a previous map (Sanson, light outline) compared to the new survey by Cassini and Picard using the moons of Jupiter as timing reference (heavier, shaded outline). The King of France reportedly quipped that the astronomers had taken more territory from him than his enemies. The times of the eclipses of the moons could be precisely calculated in advance and compared with local observations on land or on ship to determine the local time and hence longitude. Galileo applied in 1616 for the Spanish prize of 6,000 gold [[ducats with a lifetime pension of 2,000 a year, and almost two decades later for the Dutch prize, but by then he was under house arrest for possible heresy.

The main problem with the Jovian moon technique was that it was difficult to observe the Galilean moons through a telescope on a moving ship, a problem that Galileo tried to solve with the invention of the celatone. Others suggested improvements, but without success.

Land mapping surveys had the same problem determining longitude, though with less severe observational conditions. The method proved practical and was used by Giovanni Domenico Cassini and Jean Picard to re-map France.

Comparative structure

thumb|600px|center|Surface features of the four members at different levels of zoom in each row

<div style="float:right; margin:2px;">

{| class=wikitable style="text-align:center; font-size:11px"

|+ Jovian radiation<!-- Could use some supporting body text. -->

! Moon !! rem/day

|-

| Io || 3600

|-

| Europa || 540 Its surface is dotted with more than 100 mountains, some of which are taller than Earth's Mount Everest. Unlike most satellites in the outer Solar System (which have a thick coating of ice), Io is primarily composed of silicate rock surrounding a molten iron or iron sulfide core.

Although not proven, 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 (SO<sub>2</sub>). 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

thumb|Closeup of Europan lineae

Europa (Jupiter II), the second of the four Galilean moons, is the second closest to Jupiter and the smallest at 3121.6 kilometers in diameter, which is slightly smaller than Earth's Moon. The name comes from a mythical Phoenician noblewoman, Europa, who was courted by Zeus and became the queen of Crete, though the name did not become widely used until the mid-20th century. with a layer of water surrounding the mantle of the planet, thought to be 100 kilometers thick. The smooth surface includes a layer of ice, while the bottom of the ice is theorized to be liquid water. The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath it, which could conceivably serve as an abode for extraterrestrial life. Heat energy from tidal flexing ensures that the ocean remains liquid and drives geological activity. Life may exist in Europa's under-ice ocean. So far, there is no evidence that life exists on Europa, but the likely presence of liquid water has spurred calls to send a probe there.

thumb|Recurring plume erupting from Europa.

The prominent markings that criss-cross the moon seem to be mainly albedo features, which emphasize low topography. There are few craters on Europa because its surface is tectonically active and young. Some theories suggest 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 also been considered as causes. The reddish-brown color of the markings is theorized to be caused by sulfur, but because no data collection devices have been sent to Europa, scientists cannot yet confirm this. Europa is primarily made of silicate rock and likely has an iron core. It has a tenuous atmosphere composed primarily of oxygen.

Ganymede

thumb|Ancient tectonic features on [[Xibalba Sulcus on Ganymede]]

Ganymede (Jupiter III), the third Galilean moon, is named after the mythological Ganymede, cupbearer of the Greek gods and Zeus's beloved. since Ganymede is an icy world. It is the only satellite in the Solar System known to possess a magnetosphere, likely created through convection within the liquid iron core.

Ganymede is composed primarily of silicate rock and water ice, and a salt-water ocean is believed to exist nearly 200&nbsp;km below Ganymede's surface, sandwiched between layers of ice. The metallic core of Ganymede suggests a greater heat at some time in its past than had previously been proposed. The surface 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 its icy crust forming over them. The satellite has a thin oxygen atmosphere that includes O, O<sub>2</sub>, and possibly O<sub>3</sub> (ozone), and some atomic hydrogen.

Callisto

thumb|Callisto's [[Valhalla (crater)|Valhalla impact crater in enhanced color as seen by Voyager]]

Callisto (Jupiter IV) is the fourth and last Galilean moon, and is the second-largest of the four, and at 4820.6 kilometers in diameter, it is the third largest moon in the Solar System, and barely smaller than Mercury, though only a third of the latter's mass. It is named after the Greek mythological nymph Callisto, a lover of Zeus who was a daughter of the Arkadian King Lykaon and a hunting companion of the goddess Artemis. The moon does not form part of the orbital resonance that affects three inner Galilean satellites and thus does not experience appreciable tidal heating. Callisto is composed of approximately equal amounts of rock and ices, which makes it the least dense of the Galilean moons. It is one of the most heavily cratered satellites in the Solar System, and one major feature is a basin around 3000&nbsp;km wide called Valhalla.

Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide and probably molecular oxygen. Investigation revealed that Callisto may possibly have a subsurface ocean of liquid water at depths less than 300 kilometres. The likely presence of an ocean within Callisto indicates that it can or could harbour life. However, this is less likely than on nearby Europa. Callisto has long been considered the most suitable place for a human base for future exploration of the Jupiter system since it is furthest from the intense radiation of Jupiter's magnetic field.

{| class="sortable wikitable"

|+

|-

! Name<br>

! class="unsortable" |Image

! class="unsortable" |Model of interior

! Diameter<br>(km)

! Mass<br>(kg)

! Density<br>(g/cm<sup>3</sup>)

! <small>Semi-major axis<br>(km)</small>

! <small>Orbital period (days) (relative to Io)</small>

! Inclination<br>(°)

! <small>Eccentricity</small>

|-

| style="text-align:center;" | Io <br>Jupiter I

| style="background:black; text-align:center;" |135px

| style="background:black; text-align:center;" |215px

| style="text-align:center;" | <br><br>

| style="text-align:center;" |

| style="text-align:center;" |

| style="text-align:center;" |

| style="text-align:center;" | 1.769 <br>(1)

| style="text-align:center;" | 0.050

| style="text-align:center;" | 0.0041

|-

| style="text-align:center;" | Europa <br>Jupiter II

| style="background:black; text-align:center;" |135x135px

| style="background:black; text-align:center;" |215px

| style="text-align:center;" |

| style="text-align:center;" |

| style="text-align:center;" | 3.014

| style="text-align:center;" |

| style="text-align:center;" | 3.551 <br>(2.0)

| style="text-align:center;" | 0.471

| style="text-align:center;" | 0.0094

|-

| style="text-align:center;" | Ganymede <br>Jupiter III

| style="background:black; text-align:center;" |135px

| style="background:black; text-align:center;" |215px

| style="text-align:center;" |

| style="text-align:center;" |

| style="text-align:center;" | 1.942

| style="text-align:center;" |

| style="text-align:center;" | 7.155 <br>(4.0)

| style="text-align:center;" | 0.204

| style="text-align:center;" | 0.0011

|-

| style="text-align:center;" | Callisto <br>Jupiter IV

| style="background:black; text-align:center;" |135x135px

| style="background:black; text-align:center;" |215px

| style="text-align:center;" |

| style="text-align:center;" |

| style="text-align:center;" | 1.834

| style="text-align:center;" |

| style="text-align:center;" | 16.689 <br>(9.4)

| style="text-align:center;" | 0.205

| style="text-align:center;" | 0.0074

|}

Origin and evolution

thumb|right|300px|The relative masses of the Jovian moons. Those smaller than Europa are not visible at this scale, and combined would only be visible at 100× magnification.

Jupiter's regular satellites are believed to have formed from a circumplanetary disk, a ring of accreting gas and solid debris analogous to a protoplanetary disk. They may be the remnants of a score of Galilean-mass satellites that formed early in Jupiter's history.

Visibility

All four Galilean moons are bright enough to be viewed from Earth without a telescope, if only they could appear farther away from Jupiter. (They are, however, easily distinguished with even low-powered binoculars.) They have apparent magnitudes between 4.6 and 5.6 when Jupiter is in opposition with the Sun, and are about one unit of magnitude dimmer when Jupiter is in conjunction. The main difficulty in observing the moons from Earth is their proximity to Jupiter, since they are obscured by its brightness. The maximum angular separations of the moons are between 2 and 10 arcminutes from Jupiter, which is close to the limit of human visual acuity. Ganymede and Callisto, at their maximum separation, are the likeliest targets for potential naked-eye observation.

<gallery mode=packed heights=160>

File:Jupiter-moons.jpg|Jupiter and all of the Galilean moons as seen through a amateur telescope (Meade LX200).

File:Jupiter.mit.Io.Ganymed.Europa.Calisto.Vollmond.10.4.2017.jpg|Jupiter with the Galilean moons and the full Moon as seen around conjunction on 10 April 2017

File:Galilean satellite triple conjunction 2015-01-24.jpg|Two Hubble Space Telescope views of a rare triple transit of Jupiter by Europa, Callisto and Io (24 January 2015)|alt=Small satellites visible against the vastness of the largest planet in the solar system

</gallery>

Orbit animations

GIF animations depicting the Galilean moon orbits and the resonance of Io, Europa, and Ganymede

Latest flyby

See also

  • Jupiter's moons in fiction
  • Colonization of the Jovian System

Notes

References

  • Sky & Telescope utility for identifying Galilean moons
  • Interactive 3D visualisation of Jupiter and the Galilean moons
  • NASA's Stunning Discoveries on Jupiter's Largest Moons | Our Solar System's Moons
  • A Beginner's Guide to Jupiter's Moons
  • Dominic Ford: The Moons of Jupiter. With a chart of the current position of the Galilean moons.