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 |
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 ("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 ( "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
(right) 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 (right).
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 ("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 ("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 (but still ancient) 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 ("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 ("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 ("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 ("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 ("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 ("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 ("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 ("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 ("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.
Sun
Venus
Mars
Saturn
Neptune