On 7th January 1610, the famous scientist Galileo Galilei (1564-1642) telescopically discovered the Jupiter moons of Io, Europa and Callisto when he first pointed his newly devised optical telescope towards the heavens. Galileo, however, did not discover the fourth main satellite, Ganymede, until 13th January, as ay first he thought was just a nearby field star. This initial discovery was very profound on Galileo, as over subsequent nights he found they were like some mini-solar system, with the four moons orbiting around Jupiter itself. This immediately showed the Copernican view of the Solar System were essentially correct. Galileo soon named them the Medicean Stars in honour of his patron, the Duke of Tuscany, Cosimo de Medici III (1642-1723). In more modern times, these four satellites have become to be known collectively as the Galilean Satellites.



(Jup. Radii)
I. Io 1.769 421,756 02.295 3,642.6 5.7
II Europa 3.552 671,079 04.693 3,128.2 5.3
III Ganymede 7,155 1,070,428 07.486 5,264.8 4.6
IV Callisto 16.689 1,882,700 13.167 4,820.6 5.7

We find Ganymede is much larger in size than the innermost planet Mercury, and all of them are bigger than our own nearby Moon. Figure. 1. (below) shows the comparison of the four main Jovian Moons, and presents these sizes against our Moon, Mercury and Jupiter.

All four moons show discernible small disks averaging about 1 arcsec across. These disks are readily visible in 10.5cm., and can be confirmed by various stellar occultations or when in transit by the shadows cast onto the upper Jovian atmosphere.

The following section generally summarises the four principal moons. Given is some information in observing them, and current understand their natures. A shorter summary is on the other much smaller and fainter moons, however, these latter ones are mostly invisible in amateur visual telescopes.


IO (I)


IO, pronounced eye-oo or the Americanised ee-oo, is the innermost moon of the four main Galilean Satellites orbiting Jupiter.

Established as the forth largest moon in the Solar System, this 3,642 km moon orbits Jupiter in 01hr 18min 27.5sec. Its fairly rapid motion is distinguishable after only a few minutes, often highlighted by the relative positions of the other three satellites. Telescopically, Io appears to the eye as a small yellow or yellow-orange disk, and even small apertures, is clearly not stellar.

In mythology, Io was the priestess of the highly venerated and worshipped goddess, Hera. Her association with Jupiter is...

Due to the proximity to Jupiter, whose distance is never more than Jupiter 2.95 radii away. Io regularly transits across the disk or disappear behind Jupiter itself. Io also projects its shadow upon the Jovian atmosphere.


Discoverer : Galileo Galilei
Equatorial : 3,642.6±0.4 km.
Polar : 3,631.4±0.4 km.
Period (P) : 1.769138 days / 01d 18h 27m 33.5s
Orbital Velocity : 17.334 km.s-1
Eccentricity (e) : 0.0041
Inclination (i) : 0.036°
Mass : 8.933±0.015×1022 kg.
Escape Velocity : 2.56 km.s-1
Mean Density : 3.530±0.006
Mean Distance : 421,756 km.
Sidereal Rotation : Synchronous
Max. Apparent Diameter : 1.22″ (arcsec)
Min. Apparent Diameter : 0.81″ (arcsec)
Maximum Magnitude : 5.74
Minimum Magnitude : 6.11

We knew little about Io physical surface until the Pioneer 10 and 11 spacecraft in the 1973 first revealed Ios general nature. Much of what we presently discovered in better detail was with Voyager 1 in March 1979 followed later by Voyager 2 in July 1979.

This particular moon remains one of the most bizarre worlds in our entire Solar System, whose quite sickly appearance is mostly morbid yellow-orange in colour. Across the surface of the orb are intermixed many black and white coloured regions randomly scattered, so that it looks more like pizza with olives than any large astronomical body! These rich colours are mainly due to molten or solid Sulphur (or Sulfur) and sulphur compounds, whose various multi-states produces the differing colours. Io remains one of the few planetary bodies, other than Earth and Saturn moon Enceladus, to show significant active volcanism. Some four-hundred volcanic vents have so far been identified. These volcanoes regularly spews molten material that splatters it across the surface. We view large amounts of sticky sulphur-based materials on the surface.

Sulphur by its odd chemical nature can exhibit many so-called phase transitions or allotropes, so that the colours can range from white, black, red, or yellows, and can be mixtures, and so producing many other colour combinations. Io is continuously pulled and kneaded by the enormity of the tidal forces exerted by Jupiter. This keeps the inner core hot and easily explains the continuous volcanic activity. Ios surface is thought to change completely every 20,000 years or so. (Real Estate buyers beware!) This moon was first examined in detail by both the Voyager spacecraft in 1979 and later by the recent Galileo spacecraft.



EUROPA is second out of the four bright Jovian system moons and is the smallest among them. It was discovered by Galileo on the same night as Io and Callisto on 7th January 1610.


Discoverer : Galileo Galilei
Equatorial : 3,128.2±8 km.
Polar : 3,121.8±8 km.
Period (P) : 3.551810 days or 03d 13h 14m 36.4s
Orbital Velocity : 13.740 km.s-1
Eccentricity (e) : 0.0101
Inclination (i) : 0.0464°
Mass : 4.797±0.015×1022 kg.
Escape Velocity : 2.03 km.s-1
Mean Density : 2.99±0.05
Mean Distance : 671,079 km.
Sidereal Rotation : Synchronous
Max. Apparent Diameter : 1.04″ (arcsec)
Min. Apparent Diameter : 0.69″ (arcsec)
Maximum Magnitude : 5.31
Minimum Magnitude : 6.27

This slightly dirty off-white coloured ice world is easily visible in small telescopes, and displays a small featureless disk 0.7 to 1.0 arcsec across. It always remains within 3.8 arcmin of Jupiter or 4.7 Jovian radii. Europas mean diameter is 3128 km. across, being only 350 km. or 10% smaller than our Moon (3474 km.) By its observed visual magnitude, white-coloured Europa is always slightly brighter than either Io and Callisto, though it is often calculated and stated as being 0.7 magnitude fainter than the largest Jovian moon, Ganymede (III).

Europa throughout its orbit can undergo continuous events of mutual eclipses followed by transits, that averages once every 1.75 days, being almost exactly twice in number of events compared to the much shorter orbital period of Io. Europa orbits Jupiter every 3 days 13.2 hours at the mean distance of 670,900 km. – 1.7 times the Earth-Moon distance – or about 600,000 km. above the Jovian cloud-tops. Its mean velocity around Jupiter is 13.7 km.s-1, matching closely with Jupiters mean velocity of 13.1 km.s-1around the Sun.

Like Io, Europa is kneaded by strong tidal forces generated by the huge bulk of Jupiter. Although these the same tidal distortions of Io are less dramatic, they have likely been significant to allow the existence of oceans below its hard surface. They have also formed the great fissures and fractures appear across the surface. It seems that liquid water, has or still, flows from the subsurface or the interior of this moon, which have become frozen solid on the surface. In recent years, there has been speculation that these likely oceans maybe possibly a place for extraterrestrial aquatic life.


Rape of Europa Europa was born as one of the many daughters of the original Titan gods Oceanus and Tethys. Europa is named after a mythical beautiful and desirable Phoenician noblewoman, who was once courted by Zeus, and later became the queen of Crete.

She features importantly in both Greek and Cretan legends, and is especially highlighted around the famous story of her relationship with Zeus (Jupiter). Most prominent, is the story was her being abducted by him, who appeared as a splendid white bull, then taking her to Crete, by swimming across the sea, then raping her. The nature of their union has inspired many works of literature and in art. For example, one of the renown paintings was made by the Italian painter known as Titian (Tiziano Vecelli (1488 (or 1490)-1576) — as stylised The Rape of Europa (1562). (See Side Figure.)


Ganymede Voyage 1

GANYMEDE is the largest in the Jovian satellite system being the third outermost moon of the four Jovian moons. It is also the largest moon in the entire Solar System, whose 5,264 kilometre diameter just exceeds the planet Mercury 4,879 km. size by about 8%. This moon was discovered by Galileo who found Ganymede on the 11th January 1610.

The adoption of the Greek mythological name was first proposed by Simon Marius. Ganymede was a virile and handsome Greek god, who was the important nectar cup-bearer for Zeus and the many other immortal gods in Olympus. He was born mortal, but when he was immediately beheld and desired by Zeus (Jupiter). Zeus then transformed himself into an eagle to carry him to Olympus. On arrival, Ganymede was elevated to immortality, and then quickly replaced Zeuss female consort, Hebe, taking Ganymede as his beautiful young male lover. In some ancient astronomical legends, Ganymede is also represented as the zodiacal constellation of Aquarius the Water Bearer, which Zeus did formally honour for his service. He is also closely associated with the constellation, Aquila the Eagle, who in some peoples views, acted as the messenger. Alternatively the eagle is represented as Zeus himself stealing away Ganymede, where he now resides at Olympus. When Ganymedes services are required, he personally delivers wine or mead at the feet of the only king of the Greek gods, Zeus.


Discoverer : Galileo Galilei
Equatorial : 5,264.8±20.0 km.
Polar : 5,264.6±20 km.
Period (P) : 7.154553 days or 07d 03h 42m 33.4s
Velocity : km.s-1
Eccentricity (e) : 0.0006
Inclination (i) : 0.186°
Mass : 1.482±0.001×1023 kg.
Escape Velocity : 2.74 km.s-1
Mean Density : 1.94±0.02
Mean Distance : 1,070,428 km.
Sidereal Rotation : Synchronous
Max. Apparent Diameter : 1.74″ (arcsec)
Min. Apparent Diameter : 1.16″ (arcsec)
Maximum Magnitude : 4.59
Minimum Magnitude : 5.60

Were it not in orbit around Jupiter, Ganymede would be considered as a planet in its own right. The moon orbits Jupiter once every 07 days 04 hours or so, orbiting at the mean distance of 1.07 million kilometres or about three times the Earth-Moon distance. The orbit is remarkably circular, which is locked in the ratio of 1:2 with closer Europa. This satellite is locked into synchronous rotation by Jupiters immense gravitation, so the axial rotation is the same as the period. Telescopically, Ganymede displays an obvious yellowish coloured disk and is clearly not stellar. At maximum the disk is 1.7 arcsec, similar to Neptune apparent diameter. Size varies by about 0.6 arcsec depending on the Jupiter-Earth distance and Jupiters distance from the Sun.

Ganymede consists mainly of rock and ice. The topography of the surface has several varied regions. Some of the surface is heavily cratered and has dark areas similar to the mare of the moon. The largest is the somewhat circular Galileo Regio that is about 3,200 kilometres across. (See top right-hand side of the inserted image.) Such regions are thought to be from an older period some four billion years ago during the formation of the Solar System. Other places are much lighter and are probably are younger, whose terrain consists of regular parallel ridges and grooves covering significant length of the disk. This region is also cratered but these are fairly weathered. Many of the new freshly impact craters display large ray structures, whose inside structures are slightly lighter than the surrounding plains. How these were formed is not well understood, though it seems they are likely produced by internal tectonic forces like compressing or moving plates. Ganymede also has several and an different regions that include mountainous terrain and small dark areas in the mid- to polar southern hemisphere.

Ganymede is significantly influenced by the gravitational tides exerted by Jupiter, and for this reason, some planetary astronomers have speculated that there might exist a narrow ocean of underground salt water somewhere between the surface and core. For some time it has been known of a magnetic field. This suggest that there is still a small liquid iron metallic inner core and is not composed of ices. Although this moon has no significant atmosphere to speak of, the Hubble Space Telescope found evidence by spectroscopy of a tenuous amount of ozone. This seems to have been created by the interaction of either magnetic field with the surface water ice. It is possible that this temperature could generate unusual conditions inside Ganymede itself. These four main satellites are interesting in this regard, as their individual densities decrease away from the planet. Though Ganymede is darker and seem more rocky, the whole moon is actually slightly less dense than the much whiter icy surface of the nearer Europa. This immediately says that the history and composition of each satellites differs from moon to moon, and may have varied during their evolutions or their original formation.


Callisto Voyager 2

CALLISTO is the last of the four main Galilean satellites to orbit Jupiter.


Discoverer : Galileo Galilei
Equatorial : 4,820.6±1.5 km.
Polar : 4,806±10 km.
Period (P) : 16.689018 days/ 16d 16h 32m 11.2s
Orbital Velocity : 8.204 km.s-1
Eccentricity (e) : 0.007
Inclination (i) : 0.281°
Mass : 1.076±0.001×1023 kg.
Escape Velocity : 2.44 km.s-1
Mean Density : 1.851±0.004
Mean Distance : 1,882,700 km.
Sidereal Rotation : Synchronous
Max. Apparent Diameter : 1.60″ (arcsec)
Min. Apparent Diameter : 1.07″ (arcsec)
Maximum Magnitude : 5.73
Minimum Magnitude : 6.72

This moon is very unlike the other three, being much darker in colour and is more heavily cratered than the Moon or the innermost planet, Mercury. In appearance, the dark surface leaves the impression that it is less interesting than either Io, Europa and Ganymede, but this view is slightly misleading. By composition, Callisto contains mostly mixtures of both rock and ices.

Callisto appears as a small 1½ arcsec disk, which is just glimpsed in 7.5cm. (3-inch) and obvious in 10.5cm. (4-inch). Mutual transits and occultations with the main disk of Jupiter occur twice each month, being spaced between each event by eight days (8.344509 days). This means that from any fixed location only two events in a row can be seen, with the third one being unsighted.

A very weak planetary atmosphere of oxygen and carbon dioxide. From the radiation from the Sun, the surface temperature rises only to 134K (−139°C)



Although sixty-nine (2018) moons are currently or provisionally recognised as Jovian satellites, only the four main Galilean moons are probably of interest to most visual observers. Galileo was to discover the first four in 1610, but it was not until 272 years later, on 9th September 1892, that E.E. Barnard (1857-1923) stubbled upon the fifth moon, V. Amalthea. This was followed eight years later by VI. Himelia on 1904, being found by Charles Dillon Perrine (1867-1951), and he followed this with VII. Elara on 3rd December 1905. Amazingly, these two latter moons found by Perrine were not given proper names until the IAU designated them in 1975!

Satellite numbers then slowly increased, with seven by 1905, eight by 1914, eleven in 1938 and twelve by 1951. In 1974/75 another two moons were discovered by Earth-bound observed, making the total fourteen. Next in 1979 were the discovery of the innermost moons by Voyager 1 & 2 ; being XV. Adrastea, XIV. Thebe and XVI. Metis. This number remained stable until 2000, until XVII. Callirrhoe was found by Canadian born Brett J. Gladman and colleagues on 4th November 1999.

At first the object was thought to be an asteroid, but on 18th July 2000, Tim Spahr realised that Callirrhoe was a Jovian moon. Next during 2001 another group of observers headed by Scott Sheppard (University of Hawaii) began looking for other distant Jovian moons. They imaging the area around the planet using 2.2-metre reflector on 23rd to 26th November and discovered nine satellites that were designated S/2000 J2 to S/2000 J11. A tenth satellite was discovered by them on 5th December. The general orbits of the retrograde orbiting moons were determined by CCD observations by S. Dahm and A. Evans made between 27th December 2000 and 1st January 2001 — so the discovery dates in most references are given as happening in 2001. This increased the number of Jovian satellites to twenty-six. Except S/2000 J11, which has the quite dissimilar prograde orbit, all these moons were assigned Roman numerals (XIX to XXVII) and proper names. S/2000 J11 is still yet to be named or have an accurate orbital calculation.

Sheppard and colleagues then began another new series of searches using the 3.6-metre Canada-France-Hawaii and 2.2-metre University of Hawaii atop of Mona Kea in Hawaii. On the nights of 9th-11th December and 17th-19th December 2001, another eleven satellites. Orbits were calculated between 10th-11th March, and the discoveries were announced by the I.A.U. Central Bureau for Astronomical Telegrams CBAT 7900 on 16th May 2002. They were designated S/2001 J1 to S/2001 J11. Names for these Jovian moon were announced in the CBAT 8177 on 8th August 2003.

Another series of discoveries by Sheppard found another ten (Nos. 39 to 48). These were formally named on 30th March 2005 in CBAT 8502, the last being satellite Cyllene. The last to be named (till 2009) was the 49th, Kobe, which appeared in CBAT 8826 on 5th April 2007.

Nature of the Other Non-Galilean Moons

By proximity to Jupiter, XVI. Metis is the closest of all the Jovian moons, which was discovered on images produced by Voyager 1 interplanetary spacecraft by Stephen P. Synnott in March 1979. Distance is about 127,690 kilometres and merely 15,296 km. above the Jovian cloud tops. This just pips XV. Adrastra only by 1,000 km. V. Amelthea is the next in distance, some 52,000 km higher than Metis.

By distance, furthest away is the tiny one kilometre sized moon, S/2003 J2 (2009). Distance averages some 29.5 million km. from Jupiter and whose period is 981 days or 2.68 years — longer than Mars takes to orbit the Sun! This is certainly an asteroid that was captured, based primarily on the orbital inclination and retrograde orbit. By size, only three of the sixty-three moons are larger than 100 km. across. The largest of these lesser moons is the orangish coloured and potato-shaped, V. Amelthea (250×146×128 km.), followed by VI. Himelia (170 km.) and XIV. Thebe (116×98×84 km.)

All these other satellites are of little interest because of their mostly meagre sizes. To the visual observer all of them are either often impossibly hard to find or are invisible — and even with the largest of telescopes. Such difficult are not exactly helped by the sheer brightness of Jupiter. Most appear as tiny pinpoints around 18th magnitude. Table 2 below shows the sixty-two orbiting moons. It is speculated that many of these outer moons placed so far from Jupiter are captured asteroids, which have strayed too close to the planet.



I Io 1,769 421,756 3,642.6 5.7
II Europa 3.552 671,079 3,128.2 5.3
III Ganymede 7.155 1,070,428 5,264.8 4.6
IV Callisto 16.69 1,882,700 4,820.6 5.7
V Amalthea 0.498 181,366 250×156×128 14.1
VI Himalia 250.37 11,460,000 170 -.-
VII Elara 261.14 11,778,030 86 -.-
VIII Pasiphae 741.09r 23,609,040 60 16.6
IX Sinope 762.33r 24,057,860 38 11.6
X Lysithea 259.89 11,740,560 36 11.7
XI Carme 721.82r 23,197,990 46 11.3
XII Ananke 642.02r 21,454,950 28 12.2
XIII Leda 241.75 11,187,780 16 13.5
XIV Thebe 0.675 221,889 116x98x84 -.-
XV Adrastra 0.298 128,690 20x16x14 -.-
XVI Metis 0.295 127,690 60x40x64 -.-
XVII Callirrhoe 722.62r 23,214,990 8.6 20.7
XVIII Themisto 129.87 7,393,200 8 14.4
XIX Megaclite 792.44r 24,687,200 5 15.0
XX Taygete 686.67r 22,438,650 5 15.4
XXI Chaldene 699.33r 22,713,400 4 15.7
XXII Harpalyke 624.54r 21,063,900 4 15.2
XXIII Kalyke 721.03r 23,180,800 5 15.3
XXIV Iocaste 609.43r 20,722,600 5 14.5
XXV Erinome 711.96r 22,966,200 3 16.0


XXVI Isonoe 750.13r 23,800,600 4 15.9
XXVII Praxidike 613.90r 20,823,900 7 15.0
XXVIII Autonoe 772.17r 24,264,400 4 15.4
XIX Thyone 639.80r 21,405,600 4 15.7
XXX Hermippe 629.81r 21,182,100 4 15.5
XXXI Aitne 679.64r 22,285,200 3 16.1
XXXII Eurydome 723.36r 23,230,900 3 16.1
XXXIII Euanthe 598.09r 20,464,850 3 16.2
XXXIV Euporie 538.78r 19,088,400 2 16.5
XXXV Orthosie 602.62r 20,568,000 2 16.5
XXXVI Sponde 771.60r 24,252,600 2 16.4
XXXVII Kale 685.32r 22,409,200 2 16.4
XXXVIII Pasithee 726.93r 23,307,318 2 16.6
XXXIX Hegemone 745.50r 23,702,500 3 15.9
XL Mneme 627.48r 21,129,800 2 16.3
XLI Aoede 714.66r 23,044,200 4 15.8
XLII Thelxinoe 597.61r 20,453,800 2 16.4
XLIII Arche 746.19r 23,717,100 3 16.4
XLIV Kallichore 717.81r 23,111,800 2 16.8
XLV Helike 601.40r 20,540,300 4 16.0
XLVI Carpo 456.62 17,144,900 3 15.6
XLVII Eukelade 735.20r 23,483,700 4 15.0
XLVIII Cyllene 731.20r 23,396,300 2 16.2
XLIX Kore 776.02r 23,345,100 2 16.7





50 S/2003 J2 981.55r 29,540,000 2 16.6
51 S/2003 J3 561.52r 19,622,000 2 16.9
52 S/2003 J4 739.29r 23,571,000 2 16.4
53 S/2003 J5 758.34r 23,970,000 4 15.6
54 S/2003 J9 752.84r 23,857,800 1 17.2
55 S/2003 J10 700.13r 22,731,000 2 16.7
56 S/2003 J11 287.93 12,570,424 2
57 S/2003 J12 489.72r 17,883,000 1 17.2
58 S/2003 J15 699.68r 22,721,000 2 16.8
59 S/2003 J16 610.36r 20,744,000 2 16.3
60 S/2003 J17 672.75r 22,134,000 2 -.-
61 S/2003 J18 569.73r 19,813,000 2 16.5
62 S/2003 J19 699.13r 22,709,000 2 16.7
63 S/2003 J23 700.54r 22,740,000 2 16.7
64 S/2010 J1 723.2 23,314,335 2 16.7
65 S/2010 J2 588.1 20 307 150 1 17.2
66 S/2011 J1 580.7 20,155,290 1 16.8
67 S/2011 J2 726.8 23,329,710 1 17.0
68 S/2016 J1 726.8 20,595,480 3 24.0
69 S/2017 J1 735.21 23,483,980 2 23.9

Disclaimer : The user applying this data for any purpose forgoes any liability against the author. None of the information should be used for either legal or medical purposes. Although the data is accurate as possible some errors might be present. Onus of its use is placed solely with the user.


Last Update : 29th March 2018

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