Galilean moons

The Galilean moons (or Galilean satellites) /ɡælɪˈliːən/[1] are the four largest moons of Jupiter—Io, Europa, Ganymede, and
Callisto. They were first seen by Galileo Galilei in December 1609 or January 1610, and recognized by him as satellites of Jupiter in
March 1610.[2] They were the first objects found to orbit a planet other than the Earth.

They are among the largest objects in the Solar System with the exception of the Sun and the eight planets, with radii larger than
any of the dwarf planets. Ganymede is the largest moon in the Solar System, and is even bigger than the planet Mercury, though
only around half as massive. 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 much smaller remaining moons have irregular forms because of their
weaker self-gravitation.

The Galilean moons were observed in either 1609 or 1610 when Galileo made improvements to his telescope, which enabled him to
observe celestial bodies more distinctly than ever.[3] Galileo's observations 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 Ptolemaic world system, a geocentric theory in
which everything orbits around Earth.

Galileo initially named his discovery the Cosmica Sidera ("Cosimo's 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
Montage of Jupiter's four
their present names, derived from the lovers of Zeus, which were suggested by Johannes Kepler, in his Mundus Jovialis, published
Galilean moons, in a composite
in 1614.[4]
image depicting part of Jupiter
and their relative sizes (positions
The four Galilean moons were the only known moons of Jupiter until the discovery of Amalthea, the "fifth moon of Jupiter", in
are illustrative, not actual). From
1892.[5] top to bottom: Io, Europa,
Ganymede, Callisto.

Contents
History
Discovery
Dedication to the Medicis
Name
Determination of longitude
Two Hubble Space Telescope views of a
Members rare triple transit of Jupiter by Europa,
Io Callisto and Io (24 January 2015)

Europa
Ganymede
Callisto
Comparative structure
Size
Latest flyby
Origin and evolution
Visibility
Orbit animations
See also
References
External links

History

Discovery

As a result of improvements Galileo Galilei made to the telescope, with a magnifying capability of 20×,[6] he was able to see celestial
bodies more distinctly than was previously possible. This allowed Galilei to observe in either December 1609 or January 1610 what came
to be known as the Galilean moons.[3][7]

On January 7, 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. He continued to observe these celestial orbs from January 8 to March 2, 1610. In these
observations, he discovered a fourth body, and also observed that the four were not fixed stars, but rather were orbiting Jupiter.[3]

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.[8] Galileo's March 13, 1610, Sidereus Nuncius (Starry Messenger), which announced celestial Galileo Galilei, the
observations through his telescope, does not explicitly mention Copernican heliocentrism, a theory that placed the Sun at the center of discoverer of the four
the universe. Nevertheless, Galileo accepted the Copernican theory.[3] moons

A Chinese historian of astronomy, Xi Zezong, has claimed that a "small reddish star" observed near Jupiter in 362 BCE by Chinese
astronomer Gan De may have been Ganymede. If true, this might predate Galileo's discovery by around two millennia.[9]

The observations of Simon Marius are another noted example of observation, and he later reported observing the moons in 1609.[10] However, because he did not
publish these findings until after Galileo, there is a degree of uncertainty around his records.[10]

Dedication to the Medicis

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.[3] On February 13, 1610, Galileo wrote to the Grand Duke's secretary:

"God graced me with being able, through such a singular sign, to reveal to my Lord my devotion and the desire I have that his
glorious name live as equal among the stars, and since it is up to me, the first discoverer, to name these new planets, I wish, in
imitation of the great sages who placed the most excellent heroes of that age among the stars, to inscribe these with the name
of the Most Serene Grand Duke."[3]

Galileo asked whether he should name the moons the "Cosmian Stars", after Cosimo alone, or the "Medician Stars", which would honor
all four brothers in the Medici clan. The secretary replied that the latter name would be best.[3]

On March 12, 1610, Galileo wrote his dedicatory letter to the Duke of Tuscany, and the next day sent a copy to the Grand Duke, hoping to
obtain the Grand Duke's support as quickly as possible. On March 19, he sent the telescope he had used to first view Jupiter's moons to The Medician stars in the
the Grand Duke, along with an official copy of Sidereus Nuncius (The Starry Messenger) that, following the secretary's advice, named the Sidereus Nuncius (the
'starry messenger'), 1610.
four moons the Medician Stars.[3] In his dedicatory introduction, Galileo wrote:
The moons are drawn in
changing positions.
Scarcely have the immortal graces of your soul begun to shine forth on earth than bright stars offer themselves in the heavens
which, like tongues, will speak of and celebrate your most excellent virtues for all time. Behold, therefore, four stars reserved
for your illustrious name ... which ... make their journeys and orbits with a marvelous speed around the star of Jupiter ... like
children of the same family ... Indeed, it appears the Maker of the Stars himself, by clear arguments, admonished me to call
these new planets by the illustrious name of Your Highness before all others.[3]

Name

Galileo initially called his discovery the Cosmica Sidera ("Cosimo's stars"), in honour of Cosimo II de' Medici (1590–1621).[12] At
Cosimo'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:

I. Principharus (for the "prince" of Tuscany), II. Victripharus (after Vittoria della Rovere), III. Cosmipharus (after Cosimo
de' Medici) and IV. Fernipharus (after Duke Ferdinando de' Medici)[13] – 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. A Jovilabe:[11] an apparatus from
the mid-18th century for
The names that eventually prevailed were chosen by Simon Marius, who discovered the moons independently at the same time as demonstrating the orbits of
Galileo: he named them at the suggestion of Johannes Kepler after lovers of the god Zeus (the Greek equivalent of Jupiter): Io, Jupiter's satellites
Europa, Ganymede and Callisto, in his Mundus Jovialis, published in 1614.[14]

Galileo steadfastly refused to use Marius' 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.[14] Galileo used this system in his notebooks but never
actually published it. The numbered names (Jupiter x) were used until the mid-20th century when other inner moons were discovered, and Marius' names became
widely used.[14]

Determination of longitude

Galileo was able to develop a method of determining longitude based on the timing of the orbits of the Galilean moons.[15] 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. The main problem with
the 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. The method was used by Giovanni Domenico Cassini and Jean Picard to re-map France.[16]

Members
Some models predict that there may have been several generations of Galilean satellites in Jupiter's early history. Each generation of moons to have formed would
have spiraled into Jupiter and been destroyed, due to tidal interactions with Jupiter's proto-satellite disk, with new moons forming from the remaining debris. By the
time the present generation formed, the gas in the proto-satellite disk had thinned out to the point that it no longer greatly interfered with the moons' orbits.[17][18]

Other models suggest that Galilean satellites formed in a proto-satellite disk, in which formation timescales were comparable to or shorter than orbital migration
timescales.[19] Io is anhydrous and likely has an interior of rock and metal.[17] Europa is thought to contain 8% ice and water by mass with the remainder rock.[17]
These moons are, in increasing order of distance from Jupiter:
 Semi-
Orbital period
Diameter
Mass
Density
major Inclination

Name
Image Model of interior (days)[21] (relative Eccentricity
(km) (kg) (g/cm3) axis
(°)[22]
to Io)
(km)[20]

3 660.0

Io
1.769

× 3 637.4
8.93 × 1022 3.528 421 800 0.050 0.0041
Jupiter I (1)
× 3 630.6

Europa
3.551

3 121.6 4.8 × 1022 3.014 671 100 0.471 0.0094

Jupiter II (2.0)

Ganymede 7.155

5 268.2 1.48 × 1023 1.942 1 070 400 0.204 0.0011

Jupiter III (4.0)

Callisto
16.69

4 820.6 1.08 × 1023 1.834 1 882 700 0.205 0.0074

Jupiter IV (9.4)

Io
Io (Jupiter I) is the innermost of the four Galilean moons of Jupiter; with a diameter of 3642 kilometers, it is the fourth-largest
moon in the Solar System, and is only marginally larger than Earth's moon. It was named after Io, a priestess of Hera who became
one of the lovers of Zeus. Nevertheless, it was simply referred to as "Jupiter I", or "The first satellite of Jupiter", until the mid-20th
century.[14]

With over 400 active volcanos, Io is the most geologically active object in the Solar System.[23] Its surface is dotted with more than
100 mountains, some of which are taller than Earth's Mount Everest.[24] 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, recent data from the Galileo orbiter indicate that Io might have its own magnetic field.[25] Io has an
extremely thin atmosphere made up mostly of sulfur dioxide (SO2).[26] If a surface data or collection vessel were to land on Io in
Tupan Patera on Io.
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.[27]

Europa
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.[14]

It has a smooth and bright surface,[28] with a layer of water surrounding the mantle of the planet, thought to be 100 kilometers
thick.[29] The smooth surface includes a layer of ice, while the bottom of the ice is theorized to be liquid water.[30] 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.[31] Heat energy from tidal flexing ensures that the ocean remains liquid and drives geological
activity.[32] Life may exist in Europa's under-ice ocean. So far, there is no evidence that life exists on Europa, but the likely Europa.
presence of liquid water has spurred calls to send a probe there.[33]

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.[35] 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 a cause. The color of the markings, reddish-brown, is theorized to be
caused by sulfur, but scientists cannot confirm that, because no data collection devices have been sent to Europa.[36] Europa is
primarily made of silicate rock and likely has an iron core. It has a tenuous atmosphere composed primarily of oxygen.[37]
Recurring plume erupting from
Ganymede Europa.[34]

Ganymede (Jupiter III), the third Galilean moon, is named after the mythological Ganymede, cupbearer of the Greek gods and
Zeus's beloved.[38] Ganymede is the largest natural satellite in the Solar System at 5262.4 kilometers in diameter, which makes it larger than the planet Mercury –
although only at about half of its mass[39] 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.[40]

Ganymede is composed primarily of silicate rock and water ice, and a salt-water ocean is believed to exist nearly 200  km below Ganymede's surface, sandwiched
between layers of ice.[41] 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, O2, and possibly O3 (ozone), and
some atomic hydrogen.[42][43]

Callisto
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.[44] 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 km wide called Valhalla.[45]

Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide[46] and probably molecular oxygen.[47]
Investigation revealed that Callisto may possibly have a subsurface ocean of liquid water at depths less than 300 kilometres.[48]
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.[49] 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.[50] Ganymede.

Comparative structure
Fluctuations in the orbits of the moons indicate that their mean density Jovian radiation
decreases with distance from Jupiter. Callisto, the outermost and least
dense of the four, has a density intermediate between ice and rock Moon rem/day

whereas Io, the innermost and densest moon, has a density intermediate Io 3600[51]
between rock and iron. Callisto has an ancient, heavily cratered and
Europa 540[51]
unaltered ice surface and the way it rotates indicates that its density is
Callisto's Valhalla impact crater as
equally distributed, suggesting that it has no rocky or metallic core but Ganymede 8[51]
consists of a homogeneous mix of rock and ice. This may well have been seen by Voyager.
Callisto 0.01 [51]
Comparison of (a part of) Jupiter the original structure of all the moons. The rotation of the three inner
and its four largest natural moons, in contrast, indicates differentiation of their interiors with denser
satellites matter at the core and lighter matter above. They also reveal significant alteration of the surface. Ganymede reveals past tectonic
movement of the ice surface which required partial melting of subsurface layers. Europa reveals more dynamic and recent
movement of this nature, suggesting a thinner ice crust. Finally, Io, the innermost moon, has a sulfur surface, active volcanism and
no sign of ice. All this evidence suggests that the nearer a moon is to Jupiter the hotter its interior. The current model is that the moons experience tidal heating as a
result of the gravitational field of Jupiter in inverse proportion to the square of their distance from the giant planet. In all but Callisto this will have melted the interior
ice, allowing rock and iron to sink to the interior and water to cover the surface. In Ganymede a thick and solid ice crust then formed. In warmer Europa a thinner
more easily broken crust formed. In Io the heating is so extreme that all the rock has melted and water has long ago boiled out into space.

Surface features of the four members at different levels of zoom in each row

Size
 Galilean moons compared with moons of other planets (and with Earth; the scale is changed to 1 pixel = 94 km at this resolution).

Latest flyby

Jupiter and Io Io Europa Ganymede Callisto

Jupiter and Galilean moons circa 2007, imaged by New Horizons during flyby.
(greyscale colour)

Origin and evolution

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.[52][53] They may be the remnants of a score of Galilean-mass satellites that
formed early in Jupiter's history.[18][52]

Simulations suggest that, while the disk had a relatively high mass at any given moment, over time a substantial
fraction (several tenths of a percent) of the mass of Jupiter captured from the Solar nebula was processed through it.
However, the disk mass of only 2% that of Jupiter is required to explain the existing satellites.[52] Thus there may have
been several generations of Galilean-mass satellites in Jupiter's early history. Each generation of moons would have
spiraled into Jupiter, due to drag from the disk, with new moons then forming from the new debris captured from the
Solar nebula.[52] By the time the present (possibly fifth) generation formed, the disk had thinned out to the point that it
no longer greatly interfered with the moons' orbits.[18] The current Galilean moons were still affected, falling into and
being partially protected by an orbital resonance which still exists for Io, Europa, and Ganymede. Ganymede's larger
mass means that it would have migrated inward at a faster rate than Europa or Io.[52]
The relative masses of the Jovian moons.
Those smaller than Europa are not visible at
Visibility this scale, and combined would only be visible
at 100× magnification.
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,[54] 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.[55] The maximum angular separations of the moons are between 2 and 10 arcminutes from
Jupiter,[56] 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.

Orbit animations Jupiter and all of the Galilean

moons as seen through an 25 cm
GIF animations depicting the Galilean moon orbits and the resonance of Io, Europa, and Ganymede (10 in) amateur telescope (Meade
LX200).
 The Laplace resonance of Io, Europa and Ganymede The Galilean moons orbiting Jupiter

(conjunctions are highlighted by color changes)    Jupiter ·    Io ·    Europa ·    Ganymede ·

   Callisto

Jupiter with the Galilean moons –

Io, Ganymede, Europa, and
Callisto (near maximum
elongation), respectively – and
the full Moon as seen around
conjunction on 10 April 2017

See also
Jupiter's moons in fiction
Colonization of the Jovian System

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External links
Sky & Telescope utility for identifying Galilean moons (http://www.skyandtelescope.com/observing/objects/javascript/3307071.html)
Interactive 3D visualisation of Jupiter and the Galilean moons (https://thehappykoala.github.io/Harmony-of-the-Spheres/#/category/Solar%20System/
scenario/The%20Jovian%20System)
A Beginner's Guide to Jupiter's Moons (https://thebigbangoptics.com/jupiters-moons/)
Dominic Ford: The Moons of Jupiter (https://in-the-sky.org/jupiter.php). With a chart of the current position of the Galilean moons.

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