There are 79 known moons of Jupiter. The most massive of the moons are the four Galilean moons, which were independently discovered in 1610 by Galileo Galilei and Simon Marius and were the first objects found to orbit a body that was neither Earth nor the Sun. From the end of the 19th century, dozens of much smaller Jovian moons have been discovered and have received the names of lovers or daughters of the Roman god Jupiter or his Greek equivalent Zeus. The Galilean moons are by far the largest and most massive objects to orbit Jupiter, with the remaining 75 known moons and the rings together comprising just 0.003% of the total orbiting mass.
Of Jupiter's moons, eight are regular satellites with prograde and nearly circular orbits that are not greatly inclined with respect to Jupiter's equatorial plane. The Galilean satellites are nearly spherical in shape due to their planetary mass, and so would be considered at least dwarf planets if they were in direct orbit around the Sun. The other four regular satellites are much smaller and closer to Jupiter; these serve as sources of the dust that makes up Jupiter's rings. The remainder of Jupiter's moons are irregular satellites whose prograde and retrograde orbits are much farther from Jupiter and have high inclinations and eccentricities. These moons were probably captured by Jupiter from solar orbits. Twenty-two of the irregular satellites have not yet been officially named.
The physical and orbital characteristics of the moons vary widely. The four Galileans are all over 3,100 kilometres (1,900 mi) in diameter; the largest Galilean, Ganymede, is the ninth largest object in the Solar System, after the Sun and seven of the planets, Ganymede being larger than Mercury. All other Jovian moons are less than 250 kilometres (160 mi) in diameter, with most barely exceeding 5 kilometres (3.1 mi).[note 1] Their orbital shapes range from nearly perfectly circular to highly eccentric and inclined, and many revolve in the direction opposite to Jupiter's spin (retrograde motion). Orbital periods range from seven hours (taking less time than Jupiter does to spin around its axis), to some three thousand times more (almost three Earth years).
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.
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 passed through it. However, only 2% of the proto-disk mass of Jupiter is required to explain the existing satellites. Thus there may have been several generations of Galilean-mass satellites in Jupiter's early history. Each generation of moons might have spiraled into Jupiter, because of drag from the disk, with new moons then forming from the new debris captured from the solar nebula. By the time the present (possibly fifth) generation formed, the disk had thinned so that it no longer greatly interfered with the moons' orbits. The current Galilean moons were still affected, falling into and being partially protected by an orbital resonance with each other, 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.
The outer, irregular moons are thought to have originated from captured asteroids, whereas the protolunar disk was still massive enough to absorb much of their momentum and thus capture them into orbit. Many are believed to have broken up by mechanical stresses during capture, or afterward by collisions with other small bodies, producing the moons we see today.
Chinese historian Xi Zezong claimed that the earliest record of a Jovian moon (Ganymede or Callisto) was a note by Chinese astronomer Gan De of an observation around 364 BC regarding a "reddish star". However, the first certain observations of Jupiter's satellites were those of Galileo Galilei in 1609. By January 1610, he had sighted the four massive Galilean moons with his 20× magnification telescope, and he published his results in March 1610.
Simon Marius had independently discovered the moons one day after Galileo, although he did not publish his book on the subject until 1614. Even so, the names Marius assigned are used today: Ganymede; Callisto; Io; and Europa. No additional satellites were discovered until E. E. Barnard observed Amalthea in 1892.
With the aid of telescopic photography, further discoveries followed quickly over the course of the 20th century. Himalia was discovered in 1904, Elara in 1905, Pasiphae in 1908, Sinope in 1914, Lysithea and Carme in 1938, Ananke in 1951, and Leda in 1974.
By the time that the Voyager space probes reached Jupiter, around 1979, 13 moons had been discovered, not including Themisto, which had been observed in 1975, but was lost until 2000 due to insufficient initial observation data. The Voyager spacecraft discovered an additional three inner moons in 1979: Metis; Adrastea; and Thebe.
No additional moons were discovered for two decades, but between October 1999 and February 2003, researchers found another 34 moons using sensitive ground-based detectors. These are tiny moons, in long, eccentric, generally retrograde orbits, and averaging 3 km (1.9 mi) in diameter, with the largest being just 9 km (5.6 mi) across. All of these moons are thought to have been captured asteroidal or perhaps comet bodies, possibly fragmented into several pieces.
By 2015, a total of 15 additional moons were discovered. Two more were discovered in 2016 by the team led by Scott S. Sheppard at the Carnegie Institution for Science, bringing the total to 69. On 17 July 2018, the International Astronomical Union confirmed that Sheppard's team had discovered ten more moons around Jupiter, bringing the total number to 79. Among these is Valetudo, which has a prograde orbit, but crosses paths with several moons that have retrograde orbits, making an eventual collision—at some point on a billions of years timescale—likely.
The Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto) were named by Simon Marius soon after their discovery in 1610. However, these names fell out of favor until the 20th century. The astronomical literature instead simply referred to "Jupiter I", "Jupiter II", etc., or "the first satellite of Jupiter", "Jupiter's second satellite", and so on. The names Io, Europa, Ganymede, and Callisto became popular in the mid-20th century, whereas the rest of the moons remained unnamed and were usually numbered in Roman numerals V (5) to XII (12).[better source needed] Jupiter V was discovered in 1892 and given the name Amalthea by a popular though unofficial convention, a name first used by French astronomer Camille Flammarion.
The other moons were simply labeled by their Roman numeral (e.g. Jupiter IX) in the majority of astronomical literature until the 1970s. In 1975, the International Astronomical Union's (IAU) Task Group for Outer Solar System Nomenclature granted names to satellites V–XIII, and provided for a formal naming process for future satellites still to be discovered. The practice was to name newly discovered moons of Jupiter after lovers and favorites of the god Jupiter (Zeus) and, since 2004, also after their descendants. All of Jupiter's satellites from XXXIV (Euporie) onward are named after descendants of Jupiter or Zeus, except LIII (Dia), named after a lover of Jupiter. Names ending with "a" or "o" are used for prograde irregular satellites (the latter for highly inclined satellites), and names ending with "e" are used for retrograde irregulars. Some of the most recently confirmed moons have not received names.
Some asteroids share the same names as moons of Jupiter: 9 Metis, 38 Leda, 52 Europa, 85 Io, 113 Amalthea, 239 Adrastea. Two more asteroids previously shared the names of Jovian moons until spelling differences were made permanent by the IAU: Ganymede and asteroid 1036 Ganymed; and Callisto and asteroid 204 Kallisto.
These have prograde and nearly circular orbits of low inclination and are split into two groups:
The irregular satellites are substantially smaller objects with more distant and eccentric orbits. They form families with shared similarities in orbit (semi-major axis, inclination, eccentricity) and composition; it is believed that these are at least partially collisional families that were created when larger (but still small) parent bodies were shattered by impacts from asteroids captured by Jupiter's gravitational field. These families bear the names of their largest members. The identification of satellite families is tentative, but the following are typically listed:
Prograde irregular moons
The moons of Jupiter are listed below by orbital period. Moons massive enough for their surfaces to have collapsed into a spheroid are highlighted in bold. These are the four Galilean moons, which are comparable in size to the Moon. The other moons are much smaller, with the least massive Galilean moon being more than 7000 times more massive than the most massive of the other moons. The irregular captured moons are shaded light gray when prograde and dark gray when retrograde. All orbits are based on the estimated orbit on the Julian date 2457000, or 3 September 2017. As several moons of Jupiter are currently lost, these orbital elements may be only rough approximations. As of 2018, seven satellites are considered to be lost. These are S/2003 J 2, S/2003 J 4, S/2003 J 9, S/2003 J 10, S/2003 J 12, S/2003 J 16, and S/2003 J 23. A number of other moons have only been observed for a year or two, but have decent enough orbits to be easily measurable even in 2018.
|1||XVI||Metis||//||10.5||60 × 40 × 34||≈ 3.6||128852||+7h 10m 16s||2.226||0.0077||1979||Synnott
|2||XV||Adrastea||//||12.0||20 × 16 × 14||≈ 0.2||129000||+7h 15m 21s||2.217||0.0063||1979||Jewitt
|3||V||Amalthea||//||7.1||250 × 146 × 128
|208||181366||+12h 01m 46s||2.565||0.0075||1892||Barnard||Inner|
|4||XIV||Thebe||//||9.0||116 × 98 × 84||≈ 43||222452||+16h 16m 02s||2.909||0.0180||1979||Synnott
|9||XVIII||Themisto†||//||13.5||8||0.069||7393216||+129.87||45.762||0.2115||1975/2000||Kowal & Roemer/
Sheppard et al.
|12||LXXI||Ersa†||//||15.9||2||0.0015||11453004||+250.40||30.606||0.0944||2018||Sheppard et al.||Himalia|
|13||LXV||Pandia†||//||16.2||2||0.0015||11494801||+251.77||28.155||0.1800||2017||Sheppard et al.||Himalia|
|16||LIII||Dia†||//||16.3||4||0.0090||12570424||+287.93||27.584||0.2058||2001||Sheppard et al.||Himalia|
|17||XLVI||Carpo†||//||16.2||3||0.0045||17144873||+458.62||56.001||0.2735||2003||Sheppard et al.||Carpo|
|18||— (lost)||S/2003 J 12‡||17.0||1||0.00015||17739539
|2003||Sheppard et al.||Ananke (unconfirmed)|
|19||LXII||Valetudo†||//||16.9||1||18928095||+532.00||34.014||0.2219||2016||Sheppard et al.||Valetudo|
|20||XXXIV||Euporie‡||//||16.4||2||0.0015||19088434||−538.78||144.694||0.0960||2002||Sheppard et al.||Ananke|
|21||LX||Eupheme‡||//||16.9||2||0.0015||19621780||−561.52||146.363||0.2507||2003||Sheppard et al.||Ananke|
|22||LV||S/2003 J 18‡||16.5||2||0.0015||20219648||−587.38||146.376||0.1048||2003||Gladman et al.||Ananke|
|23||LII||S/2010 J 2‡||17.5||1||20307150||−588.36||150.4||0.307||2010||Veillet||Ananke|
|24||XLII||Thelxinoe‡||//||16.4||2||0.0015||20453753||−597.61||151.292||0.2684||2003||Sheppard et al.||Ananke|
|25||XXXIII||Euanthe‡||//||16.5||3||0.0045||20464854||−598.09||143.409||0.2000||2002||Sheppard et al.||Ananke|
|26||XLV||Helike‡||//||16.1||4||0.0090||20540266||−601.40||154.586||0.1374||2003||Sheppard et al.||Ananke|
|27||XXXV||Orthosie‡||//||16.7||2||0.0015||20567971||−602.62||142.366||0.2433||2002||Sheppard et al.||Ananke|
|28||LXVIII||S/2017 J 7‡||16.6||2||0.0015||20571458||−602.77||143.438||0.2147||2017||Sheppard et al.||Ananke|
|29||LIV||S/2016 J 1‡||17.0||3||0.0015||20595483||−603.83||139.839||0.1377||2016||Sheppard et al.||Ananke|
|30||LXIV||S/2017 J 3‡||16.5||2||0.0015||20639315||−605.76||147.915||0.1477||2017||Sheppard et al.||Ananke|
|31||XXIV||Iocaste‡||//||15.5||5||0.019||20722566||−609.43||147.248||0.2874||2001||Sheppard et al.||Ananke|
|32||— (lost)||S/2003 J 16‡||16.4||2||0.0015||20743779||−610.36||150.769||0.3184||2003||Gladman et al.||Ananke|
|33||XXVII||Praxidike‡||//||14.9||7||0.043||20823948||−613.90||144.205||0.1840||2001||Sheppard et al.||Ananke|
|34||XXII||Harpalyke‡||//||15.9||4||0.012||21063814||−624.54||147.223||0.2440||2001||Sheppard et al.||Ananke|
|35||XL||Mneme‡||//||16.4||2||0.0015||21129786||−627.48||149.732||0.3169||2003||Gladman et al.||Ananke|
|36||XXX||Hermippe‡||//||15.6||4||0.0090||21182086||−629.81||151.242||0.2290||2002||Sheppard et al.||Ananke|
|37||XXIX||Thyone‡||//||15.9||4||0.0090||21405570||−639.80||147.276||0.2525||2002||Sheppard et al.||Ananke|
|38||LXX||S/2017 J 9‡||16.1||2||0.0015||21429955||−640.90||152.661||0.2288||2017||Sheppard et al.||Ananke|
|40||L||Herse‡||//||16.6||2||0.0015||22134306||−672.75||162.490||0.2379||2003||Gladman et al.||Carme|
|41||XXXI||Aitne‡||//||16.0||3||0.0045||22285161||−679.64||165.562||0.3927||2002||Sheppard et al.||Carme|
|42||LXVII||S/2017 J 6‡||16.4||2||0.0015||22394682||−684.66||155.185||0.5569||2017||Sheppard et al.||Pasiphae (fringe member)|
|43||LXXII||S/2011 J 1‡||16.7||1||22401817||−694.98||163.341||0.2328||2011||Sheppard et al.||Carme|
|44||XXXVII||Kale‡||//||16.4||2||0.0015||22409207||−685.32||165.378||0.2011||2002||Sheppard et al.||Carme|
|45||XX||Taygete‡||//||15.6||5||0.016||22438648||−686.67||164.890||0.3678||2001||Sheppard et al.||Carme|
|46||LXI||S/2003 J 19‡||16.8||2||0.0015||22696750||−698.55||166.657||0.2572||2003||Gladman et al.||Carme|
|47||XXI||Chaldene‡||//||16.0||4||0.0075||22713444||−699.33||167.070||0.2916||2001||Sheppard et al.||Carme|
|48||LVIII||Philophrosyne‡||//||16.7||2||0.0015||22720999||−699.68||141.812||0.0932||2003||Sheppard et al.||Pasiphae|
|49||— (lost)||S/2003 J 10‡||16.8||2||0.0015||22730813
|2003||Sheppard et al.||Carme|
|50||— (lost)||S/2003 J 23‡||16.8||2||0.0015||22739654||−700.54||148.849||0.3930||2004||Sheppard et al.||Pasiphae|
|51||XXV||Erinome‡||//||16.1||3||0.0045||22986266||−711.96||163.737||0.2552||2001||Sheppard et al.||Carme|
|52||XLI||Aoede‡||//||15.6||4||0.0090||23044175||−714.66||160.482||0.4311||2003||Sheppard et al.||Pasiphae|
|53||XLIV||Kallichore‡||//||16.3||2||0.0015||23111823||−717.81||164.605||0.2041||2003||Sheppard et al.||Carme|
|54||LXVI||S/2017 J 5‡||16.5||2||0.0015||23169389||−720.49||164.331||0.2842||2017||Sheppard et al.||Carme|
|55||LXIX||S/2017 J 8‡||17.0||1||0.0015||23174446||−720.73||164.782||0.3118||2017||Sheppard et al.||Carme|
|56||XXIII||Kalyke‡||//||15.5||5||0.019||23180773||−721.02||165.505||0.2139||2001||Sheppard et al.||Carme|
|59||XXXII||Eurydome‡||//||16.3||3||0.0045||23230858||−723.36||149.324||0.3769||2002||Sheppard et al.||Pasiphae|
|60||LXIII||S/2017 J 2‡||16.9||2||0.0015||23240957||−723.83||166.398||0.2360||2017||Sheppard et al.||Carme|
|61||XXXVIII||Pasithee‡||//||16.8||2||0.0015||23307318||−726.93||165.759||0.3288||2002||Sheppard et al.||Carme|
|62||LI||S/2010 J 1‡||16.5||2||23314335||−724.34||163.2||0.320||2010||Jacobson et al.||Carme|
|63||XLIX||Kore‡||//||16.6||2||0.0015||23345093||−723.72||137.371||0.1951||2003||Sheppard et al.||Pasiphae|
|64||XLVIII||Cyllene‡||//||16.3||2||0.0015||23396269||−731.10||140.148||0.4115||2003||Sheppard et al.||Pasiphae|
|65||LVI||S/2011 J 2‡||16.9||1||23400981||−731.32||148.77||0.3321||2011||Sheppard et al.||Pasiphae|
|66||XLVII||Eukelade‡||//||16.0||4||0.0090||23483694||−735.20||163.996||0.2828||2003||Sheppard et al.||Carme|
|67||LIX||S/2017 J 1‡||16.8||2||0.0015||23483978||−734.15||149.197||0.3969||2017||Sheppard et al.||Pasiphae|
|68||— (lost)||S/2003 J 4‡||16.7||2||0.0015||23570790
|2003||Sheppard et al.||Pasiphae|
|70||XXXIX||Hegemone‡||//||16.0||3||0.0045||23702511||−745.50||152.506||0.4077||2003||Sheppard et al.||Pasiphae|
|71||XLIII||Arche‡||//||16.3||3||0.0045||23717051||−746.19||164.587||0.1492||2002||Sheppard et al.||Carme|
|72||XXVI||Isonoe‡||//||16.0||4||0.0075||23800647||−750.13||165.127||0.1775||2001||Sheppard et al.||Carme|
|73||— (lost)||S/2003 J 9‡||17.0||1||0.00015||23857808||−752.84||164.980||0.2761||2003||Sheppard et al.||Carme|
|74||LVII||Eirene‡||//||15.9||4||0.0090||23973926||−758.34||165.549||0.3070||2003||Sheppard et al.||Carme|
|76||XXXVI||Sponde‡||//||16.7||2||0.0015||24252627||−771.60||154.372||0.4431||2002||Sheppard et al.||Pasiphae|
|77||XXVIII||Autonoe‡||//||15.6||4||0.0090||24264445||−772.17||151.058||0.3690||2002||Sheppard et al.||Pasiphae|
|78||XIX||Megaclite‡||//||15.0||5||0.021||24687239||−792.44||150.398||0.3077||2001||Sheppard et al.||Pasiphae|
|79||— (lost)||S/2003 J 2‡||16.6||2||0.0015||28570410
|2003||Sheppard et al.||Pasiphae (unconfirmed)|
The first spacecraft to visit Jupiter were Pioneer 10 in 1973, and Pioneer 11 a year later, taking low-resolution images of the four Galilean moons. The Voyager 1 and Voyager 2 probes visited Jupiter in 1979, discovering the volcanic activity on Io and the presence of water ice on the surface of Europa. The Cassini probe to Saturn flew by Jupiter in 2000 and collected data on interactions of the Galilean moons with Jupiter's extended atmosphere. The New Horizons spacecraft flew by Jupiter in 2007 and made improved measurements of its satellites' orbital parameters.
The Galileo spacecraft was the first to enter orbit around Jupiter, arriving in 1995 and studying it until 2003. During this period, Galileo gathered a large amount of information about the Jovian system, making close approaches to all of the Galilean moons and finding evidence for thin atmospheres on three of them, as well as the possibility of liquid water beneath the surfaces of Europa, Ganymede, and Callisto. It also discovered a magnetic field around Ganymede.
Note: some semi-major axis were computed using the µ value, while the eccentricities were taken using the inclination to the local Laplace plane
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