Orbital parameters

Satellite	Period (Days)	Inclination (Degrees)	Eccentricity	Density (kg/m3)	Albedo
Metis	0.294780	~0	<0.041	N/A	0.05
Adrastea	0.29826	~0	~0	N/A	0.05
Amalthea	0.498179	0.40	0.003	N/A	0.05
Thebe	0.6745	0.8	0.0015	N/A	0.05
Io	1.769138	0.040	0.041	3,530	0.61
Europa	3.551810	0.470	0.0101	2,990	0.64
Ganymede	7.154553	0.195	0.0015	1,940	0.42
Callisto	16.689018	0.281	0.007	1,851	0.20
Leda	238.72	27	0.163
Himalia	250.5662	28	0.163
Lysithea	259.22	29	0.107
Elara	259.6528	28	0.207
Ananke	631 R	147	0.169
Carme	692 R	163	0.207
Pasiphae	735 R	148	0.378
Sinope	758 R	153	0.275

Values for the smaller moons are approximate. *R indicates retrograde motion

Metis

Metis ( "MEE tis" ) is the innermost of Jupiter's known satellites:

orbit: 128,000 km from Jupiter
diameter: 40 km
mass: 9.56e16 kg

Metis was a Titaness who was the first wife of Zeus (Jupiter).

Discovered by Synnott in 1979 (Voyager 1).

Metis and Adrastea lie within Jupiter's main ring. They may be the source of the material comprising the ring.

Small satellites within a planet's rings are sometimes called "mooms".

Adrastea

Adrastea ("a DRAS tee uh") is the second of Jupiter's known satellites:

orbit: 129,000 km from Jupiter
diameter: 20 km (23 x 20 x 15)
mass: 1.91e16 kg

Adrastea, the distributor of rewards and punishments, was the daughter of Jupiter and Ananke.

Discovered by graduate student David Jewitt (working under Danielson) in 1979 (Voyager 1).

Metis and Adrastea orbit inside the synchronous orbit radius and inside the Roche limit. They may be small enough to avoid tidal disruption but their orbits will eventually decay.

Adrastea is one of the smallest moons in the solar system.

Amalthea

Amalthea ("am al THEE uh") is the third of Jupiter's known satellites:

orbit: 181,300 km from Jupiter
diameter: 189 km (270 x 166 x 150)
mass: 7.17e18 kg

Amalthea was the nymph who nursed the infant Jupiter with goat's milk.

Discovered by Barnard 1892 September 9 using the 36 inch (91 cm) refractor at Lick Observatory. Amalthea was the last moon to be discovered by direct visual observation (as opposed to photography).

Amalthea and Himalia are Jupiter's fifth and sixth largest moons; they are about the same size but only 1/15 the size of next larger one, Europa.

Like most of Jupiter's moons, Amalthea rotates synchronously; its long axis is pointed toward Jupiter.

Amalthea is the reddest object in the solar system. The reddish color is apparently due to sulfur originating from Io.

Its size and irregular shape imply that Amalthea is a fairly strong, rigid body. Its composition is probably more like an asteroid's than like the Galilean moons.

Like Io, Amalthea radiates more heat than it receives from the Sun (probably due to the electrical currents induced by Jupiter's magnetic field).

Thebe

Thebe ("THEE bee") is the fourth of Jupiter's known satellites:

orbit: 222,000 km from Jupiter
diameter: 100 km (100 x 90)
mass: 7.77e17 kg

Thebe was a nymph, daughter of the river god Asopus.

Discovered by Synnott in 1979 (Voyager 1).

Io

Io ( "EYE oh" ) is the fifth of Jupiter's known satellites and the third largest; it is the innermost of the Galilean moons. Io is slightly larger than Earth's Moon.

orbit: 422,000 km from Jupiter
diameter: 3630 km
mass: 8.93e22 kg

The pronounciation "EE oh" is also acceptable.

Io was a maiden who was loved by Zeus (Jupiter) and transformed into a heifer in a vain attempt to hide her from the jealous Hera.

Discovered by Galileo and Marius in 1610.

In contrast to most of the moons in the outer solar system, Io and Europa may be somewhat similar in bulk composition to the terrestrial planets, primarily composed of molten silicate rock. Recent data from Galileo indicates that Io has a core of iron (perhaps mixed with iron sulfide) with a radius of at least 900 km.

Io's surface is radically different from any other body in the solar system. It came as a very big surprise to the Voyager scientists on the first encounter. They had expected to see impact craters like those on the other terrestrial bodies and from their number per unit area to estimate the age of Io's surface. But there are very few, if any, impact craters on Io.

Instead of craters, Voyager 1 found hundreds of volcanic calderas. Some of the volcanoes are active! Striking photos of actual eruptions with plumes 300 km high were sent back by both Voyagers and Galileo. This may have been the most important single discovery of the Voyager missions; it was the first real proof that the interiors of other "terrestrial" bodies are actually hot and active. The material erupting from Io's vents appears to be some form of sulfur or sulfur dioxide. The volcanic eruptions change rapidly. In just four months between the arrivals of Voyager 1 and Voyager 2 some of them stopped and others started up. The deposits surrounding the vents also changed visibly.

Recent images taken with NASA's Infrared Telescope Facility on Mauna Kea, Hawaii show a new and very large eruption. A large new feature near Ra Patera has also been seen by HST. Images from Galileo also show many changes from the time of Voyager's encounter. These observations confirm that Io's surface is very active indeed.

Io has an amazing variety of terrains: calderas up to several kilometers deep, lakes of molten sulfur, mountains which are apparently NOT volcanoes, extensive flows hundreds of kilometers long of some low viscosity fluid (some form of sulfur?), and volcanic vents. Sulfur and its compounds take on a wide range of colors which are responsible for Io's varigated appearance.

Analysis of the Voyager images led scienties to believe that the lava flows on Io's surface were composed mostly of various compounds of molten sulfur. However, subsequent ground-based infra-red studies indicate that they are too hot for liquid sulfur. One current theory is that Io's lavas are molten silicate rock. Recent HST observations indicate that the material may be rich in sodium. Or there may be a variety of different materials in different locations.

Some of the hottest spots on Io may reach temperatures of 700 K (even 900 K has been reported), though the average is much lower, about 130 K. These hot spots are the principal mechanism by which Io loses its heat.

The energy for all this activity probably derives from tidal interactions between Io, Europa, Ganymede and Jupiter. These three moons are locked into resonant orbits such that Io orbits twice for each orbit of Europa which in turn orbits twice for each orbit of Ganymede. Though Io, like Earth's Moon always faces the same side toward its planet, the effects of Europa and Ganymede cause it to wobble a bit. This wobbling stretches and bends Io by as much as 100 meters (a 100 meter tide!) and generates heat the same way a coat hanger heats up when bent back and forth. (Lacking another body to perturb it, the Moon is not heated by Earth in this way.)

Io also cuts across Jupiter's magnetic field lines, generating an electric current. Though small compared to the tidal heating, this current may carry more than 1 trillion watts. It also strips some material away from Io which forms a torus of intense radiation around Jupiter. Particles escaping from this torus are partially responsible for Jupiter's unusually large magnetosphere.

Recent data from Galileo indicate that Io may have its own magnetic field as does Ganymede. Io has a thin atmosphere composed of sulfur dioxide and perhaps some other gases.

Unlike the other Galilean satellites, Io has little or no water. This is probably because Jupiter was hot enough early in the planets' life span to drive off the volatile elements in the vicinity of Io but not so hot to do so on moons that are farther out.

Europa

Europa ("yoo ROH puh") is the sixth of Jupiter's known satellites and the fourth largest; it is the second of the Galilean moons. Europa is slightly smaller than the Earth's Moon.

orbit: 670,900 km from Jupiter
diameter: 3138 km
mass: 4.80e22 kg

Europa was a Phoenician princess abducted to Crete by Zeus, who had assumed the form of a white bull, and by him the mother of Minos.

Discovered by Galileo and Marius in 1610.

Europa and Io are somewhat similar in bulk composition to the terrestrial planets: primarily composed of silicate rock. Recent data from Galileo indicate that Europa has a layered internal structure perhaps with a small metallic core.

But Europa's surface is not at all like anything in the inner solar system. It is exceedingly smooth: few features more than a few hundred meters high have been seen. The prominent markings seem to be only albedo features or very low relief.

There are very few craters on Europa; only three craters larger than 5 km in diameter have been found. This would seem to indicate a young and active surface. However, the Voyagers mapped only a fraction of the surface at high resolution.

The images of Europa's surface strongly resemble images of sea ice on Earth. It is possible that beneath Europa's surface ice there is a layer of liquid water, perhaps as much as 50 km deep, kept liquid by tidally generated heat. If so, it would be the only place in the solar system besides Earth where liquid water exists in significant quantities. Europa's most striking aspect is a series of dark streaks crisscrossing the entire globe. The larger ones are roughly 20 km across with diffuse outer edges and a central band of lighter material. The latest theory of their origin is that they are produced by a series of volcanic eruptions or geysers.

Recent observations with HST reveal that Europa has a very tenuous atmosphere (1e^-11 bar) composed of oxygen. Of the 61 moons in the solar system only four others (Io, Ganymede, Titan and Triton) are known to have atmospheres. Unlike the oxygen in Earth's atmosphere, Europa's is almost certainly not of biologic origin. It is most likely generated by sunlight and charged particles hitting Europa's icy surface producing water vapor which is subsequently split into hydrogen and oxygen. The hydrogen escapes leaving the oxygen. The Voyagers didn't get a very good look at Europa. But it is a principal focus of the Galileo mission. Images from Galileo's first two close encounters with Europa seem to comfirm earlier theories that Europa's surface is very young: very few craters are seen, some sort of activity is obviously occuring. There are regions that look very much like pack-ice on polar seas during spring thaws on Earth. The exact nature of Europa's surface and interior is not yet clear but the evidence is now strong for a subsurface 'ocean'.

Galileo has found some evidence of a weak magnetic field (perhaps 1/4 of the strength of Ganymede's).

Ganymede

Ganymede ("GAN uh meed") is the seventh and largest of Jupiter's known satellites. Ganymede is the third of the Galilean moons.

orbit: 1,070,000 km from Jupiter
diameter: 5262 km
mass: 1.48e23 kg

Ganymede was a Trojan boy of great beauty whom Zeus carried away to be cupbearer to the gods.

Discovered by Galileo and Marius in 1610.

Ganymede is the largest satellite in the solar system. It is larger in diameter than Mercury but only about half its mass. Ganymede is much larger than Pluto.

Before the Galileo encounters with Ganymede it was thought that Ganymede and Callisto were composed of a rocky core surrounded by a large mantle of water or water ice with an ice surface (and that Titan and Triton were similar). Preliminary indications from the Galileo data now suggest that Callisto has a uniform composition while Ganymede is differentiated into a three layer structure: a small molten iron or iron/sulfer core surrounded by a rocky silicate mantle with a icy shell on top. In fact, Ganymede may be similar to Io with an additional outer layer of ice.

Ganymede's surface is a roughly equal mix of two types of terrain: very old, highly cratered dark regions , and somewhat younger lighter regions marked with an extensive array of grooves and ridges. Their origin is clearly of a tectonic nature, but the details are unknown. In this respect, Ganymede may more similar to the Earth than either Venus or Mars (though there is no evidence of recent tectonic activity).

HST has recently found evidence of a very tenuous oxygen atmosphere on Ganymede very similar to the one found on Europa (note that this is definitely NOT evidence of life).

Similar ridge and groove terrain is seen on Enceladus, Miranda and Ariel. The dark regions are similar to the surface of Callisto.

Extensive cratering is seen on both types of terrain. Craters both overlay and are cross cut by the groove systems indicating the the grooves are quite old. Relatively young craters with rays of ejecta are also visible.

The craters are quite flat, lacking the ring mountains and central depressions common to craters on the Moon and Mercury. This is probably due to the relatively weak nature of Ganymede's icy crust which can flow over geologic time and thereby soften the relief. Ancient craters whose relief has disappeared leaving only a "ghost" of a crater are known as palimpsests.

Galileo's first flyby of Ganymede discovered that Ganymede has its own magnetosphere field embedded inside Jupiter's huge one. This is probably generated in a similar fashion to the Earth's: as a result of motion of conducting material in the interior.

Callisto

Callisto ("ka LIS toh") is the eighth of Jupiter's known satellites and the second largest. It is the outermost of the Galilean moons.

orbit: 1,883,000 km from Jupiter
diameter: 4800 km
mass: 1.08e23 kg

Callisto was a nymph, beloved of Zeus and hated by Hera. Hera changed her into a bear and Zeus then laced her in the sky as the constellation Ursa Major.

Discovered by Galileo and Marius in 1610.

Callisto is only slightly smaller than Mercury but only a third of its mass.

Unlike Ganymede, Callisto seems to have little or no internal structure; it is a more or less uniform mixture of ice (40%) and rock/iron (60%). Titan and Triton are probably similar.

Callisto's surface is covered entirely with craters. The surface is old, like the highlands of the Moon and Mars.

The largest craters are surrounded by a series of concentric rings which look like huge cracks but which have been smoothed out by eons of slow movement of the ice. The largest of these has been named Valhalla. 4000 km in diameter, Valhalla is a dramatic example of a multi-ring basin, the result the result of a massive impact. Other examples are Callisto's Asgard, Mare Orientale on the Moon and Caloris Basin on Mercury.

Like Ganymede, Callisto's ancient craters have collapsed. They lack the high ring mountains, radial rays and central depressions common to craters on the Moon and Mercury. Detailed images from Galileo show that, in some areas at least, small craters have mostly been obliterated. This suggests that some processes have been at work more recently, even if its just slumping.

Another interesting feature is Gipul Catena, a long series of impact craters lined up in a straight line. This was probably caused by an object that was tidally disrupted as it passed close to Jupiter (much like Comet SL 9) and then impacted on Callisto.

Galileo has detected no evidence of a magnetic field.

Unlike Ganymede, with its complex terrains, there is little evidence of tectonic activity on Callisto. While Callisto is very similar in bulk properties to Ganymede, it apparently has a much simpler geologic history. The different geologic histories of the two has been an important problem for planetary scientists.

Leda

Leda ("LEE duh") is the ninth of Jupiter's known satellites and the smallest:

orbit: 11,094,000 km from Jupiter
diameter: 16 km
mass: 5.68e15 kg

Leda was queen of Sparta and the mother, by Zeus in the form of a swan, of Helen and Pollux.

Discovered by Kowal in 1974.

Leda, Ananke, and Sinope are among the smallest moons in the solar system.

Himalia

Himalia ("hih MAL yuh") is the tenth of Jupiter's known satellites:

orbit: 11,480,000 km from Jupiter
diameter: 186 km
mass: 9.56e18 kg

Himalia was a nymph who bore three sons of Zeus (Jupiter).

Discovered by Perrine in 1904.

Unlike the inner satellites, the orbits of Leda, Himalia, Lysithea and Elara are significantly inclined to Jupiter's equator (about 28 degrees).

Lysithea

Lysithea ("ly SITH ee uh") is the eleventh of Jupiter's known satellites:

orbit: 11,720,000 km from Jupiter
diameter: 36 km
mass: 7.77e16 kg

Lysithea was a daughter of Oceanus and one of Zeus' lovers.

Discovered by Nicholson in 1938.

Elara

Elara ("EE lar uh") is the twelfth of Jupiter's known satellites:

orbit: 11,737,000 km from Jupiter
diameter: 76 km
mass: 7.77e17 kg

Elara was the mother by Zeus of the giant Tityus.

Discovered by Perrine in 1905.

Leda, Himalia, Lysithea and Elara may be remnants of a single asteroid that was captured by Jupiter and broken up.

Ananke

Ananke ("a NANG kee") is the thirteenth of Jupiter's known satellites:

orbit: 21,200,000 km from Jupiter
diameter: 30 km
mass: 3.82e16 kg

Ananke was the mother of Adrastea, by Jupiter.

Discovered by Nicholson in 1951.

Ananke, Carme, Pasiphae and Sinope have unusual but similar orbits.

Carme

Carme ("KAR mee") is the fourteenth of Jupiter's known satellites:

orbit: 22,600,000 km from Jupiter
diameter: 40 km
mass: 9.56e16 kg

Carme was the mother, by Zeus of Britomartis, a Cretan goddess.

Discovered by Nicholson in 1938.

Ananke, Carme, Pasiphae and Sinope are especially unusual in that their orbits are retrograde.

Pasiphae

Pasiphae ("pah SIF ah ee") is the fifteenth of Jupiter's known satellites:

orbit: 23,500,000 km from Jupiter
diameter: 50 km
mass: 1.91e17 kg

Pasiphae was the wife of Minos and mother, by a white bull, of the Minotaur.

Discovered by P. Melotte in 1908.

Ananke, Carme, Pasiphae and Sinope have orbits highly inclined to Jupiter's equator (about 150 degrees).

Sinope

Sinope ("sah NOH ") is the outermost of Jupiter's known satellites:

orbit: 23,700,000 km from Jupiter
diameter: 36 km
mass: 7.77e16 kg

Sinope was a woman said to have been unsuccessfully (!) courted by Zeus.

Discovered by Nicholson in 1914.

Ananke, Carme, Pasiphae and Sinope may be remnants of a single asteroid that was captured by Jupiter and broken up.