Jupiter is the fifth planet from the Sun and by far the largest. Jupiter is more than twice as massive as all the other planets combined (the mass of Jupiter is 318 times that of Earth).
orbit: 778,330,000 km (5.20 AU) from Sun
diameter: 142,984 km (equatorial)
mass: 1.900e27 kg
Jupiter (a.k.a. Jove; Greek Zeus) was the King of the Gods, the ruler of Olympus and the patron of the Roman state. Zeus was the son of Cronus (Saturn).
Jupiter is the fourth brightest object in the sky (after the Sun, the Moon and Venus). It has been known since prehistoric times as a bright "wandering star". But in 1610 when Galileo first pointed a telescope at the sky he discovered Jupiter's four large moons Io, Europa, Ganymede and Callisto (now known as the Galilean moons) and recorded their motions back and forth around Jupiter. This was the first discovery of a center of motion not apparently centered on the Earth. It was a major point in favor of Copernicus's heliocentric theory of the motions of the planets (along with other new evidence from his telescope: the phases of Venus and the mountains on the Moon). Galileo's outspoken support of the Copernican theory got him in trouble with the Inquisition. Today anyone can repeat Galileo's observations (without fear of retribution :-) using binoculars or an inexpensive telescope.
Jupiter was first visited by Pioneer 10 in 1973 and later by Pioneer 11, Voyager 1, Voyager 2 and Ulysses. The spacecraft Galileo orbited Jupiter for eight years. It is still regularly observed by the Hubble Space Telescope.
The gas planets do not have solid surfaces, their gaseous material simply gets denser with depth (the radii and diameters quoted for the planets are for levels corresponding to a pressure of 1 atmosphere). What we see when looking at these planets is the tops of clouds high in their atmospheres (slightly above the 1 atmosphere level).
Jupiter is about 90% hydrogen and 10% helium (by numbers of atoms, 75/25% by mass) with traces of methane, water, ammonia and "rock". This is very close to the composition of the primordial Solar Nebula from which the entire solar system was formed. Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium.
Our knowledge of the interior of Jupiter (and the other gas planets) is highly indirect and likely to remain so for some time. (The data from Galileo's atmospheric probe goes down only about 150 km below the cloud tops.)
Jupiter probably has a core of rocky material amounting to something like 10 to 15 Earth-masses.
Above the core lies the main bulk of the planet in the form of liquid metallic hydrogen. This exotic form of the most common of elements is possible only at pressures exceeding 4 million bars, as is the case in the interior of Jupiter (and Saturn). Liquid metallic hydrogen consists of ionized protons and electrons (like the interior of the Sun but at a far lower temperature). At the temperature and pressure of Jupiter's interior hydrogen is a liquid, not a gas. It is an electrical conductor and the source of Jupiter's magnetic field. This layer probably also contains some helium and traces of various "ices".
The outermost layer is composed primarily of ordinary molecular hydrogen and helium which is liquid in the interior and gaseous further out. The atmosphere we see is just the very top of this deep layer. Water, carbon dioxide, methane and other simple molecules are also present in tiny amounts.
Recent experiments have shown that hydrogen does not change phase suddenly. Therefore the interiors of the jovian planets probably have indistinct boundaries between their various interior layers.
Three distinct layers of clouds are believed to exist consisting of ammonia ice, ammonium hydrosulfide and a mixture of ice and water. However, the preliminary results from the Galileo probe show only faint indications of clouds (one instrument seems to have detected the topmost layer while another may have seen the second). But the probe's entry point (left) was unusual -- Earth-based telescopic observations and more recent observations by the Galileo orbiter suggest that the probe entry site may well have been one of the warmest and least cloudy areas on Jupiter at that time.
Data from the Galileo atmospheric probe also indicate that there is much less water than expected. The expectation was that Jupiter's atmosphere would contain about twice the amount of oxygen (combined with the abundant hydrogen to make water) as the Sun. But it now appears that the actual concentration much less than the Sun's. Also surprising was the high temperature and density of the uppermost parts of the atmosphere.
Jupiter and the other gas planets have high velocity winds which are confined in wide bands of latitude. The winds blow in opposite directions in adjacent bands. Slight chemical and temperature differences between these bands are responsible for the colored bands that dominate the planet's appearance. The light colored bands are called zones; the dark ones belts. The bands have been known for some time on Jupiter, but the complex vortices in the boundary regions between the bands were first seen by Voyager. The data from the Galileo probe indicate that the winds are even faster than expected (more than 400 mph) and extend down into as far as the probe was able to observe; they may extend down thousands of kilometers into the interior. Jupiter's atmosphere was also found to be quite turbulent. This indicates that Jupiter's winds are driven in large part by its internal heat rather than from solar input as on Earth.
The vivid colors seen in Jupiter's clouds are probably the result of subtle chemical reactions of the trace elements in Jupiter's atmosphere, perhaps involving sulfur whose compounds take on a wide variety of colors, but the details are unknown.
The colors correlate with the cloud's altitude: blue lowest, followed by browns and whites, with reds highest. Sometimes we see the lower layers through holes in the upper ones.
The Great Red Spot (GRS) has been seen by Earthly observers for more than 300 years (its discovery is usually attributed to Cassini, or Robert Hooke in the 17th century). The GRS is an oval about 12,000 by 25,000 km, big enough to hold two Earths. Other smaller but similar spots have been known for decades. Infrared observations and the direction of its rotation indicate that the GRS is a high-pressure region whose cloud tops are significantly higher and colder than the surrounding regions. Similar structures have been seen on Saturn and Neptune. It is not known how such structures can persist for so long.
Jupiter radiates more energy into space than it receives from the Sun. The interior of Jupiter is hot: the core is probably about 20,000 K. The heat is generated by the Kelvin-Helmholtz mechanism, the slow gravitational compression of the planet. (Jupiter does NOT produce energy by nuclear fusion as in the Sun; it is much too small and hence its interior is too cool to ignite nuclear reactions.) This interior heat probably causes convection deep within Jupiter's liquid layers and is probably responsible for the complex motions we see in the cloud tops. Saturn and Neptune are similar to Jupiter in this respect, but oddly, Uranus is not.
Jupiter is just about as large in diameter as a gas planet can be. If more material were to be added, it would be compressed by gravity such that the overall radius would increase only slightly. A star can be larger only because of its internal (nuclear) heat source. (But Jupiter would have to be at least 80 times more massive to become a star.)
Jupiter has a huge magnetic field, much stronger than Earth's. Its magnetosphere extends more than 650 million km (past the orbit of Saturn!). (Note that Jupiter's magnetosphere is far from spherical -- it extends "only" a few million kilometers in the direction toward the Sun.) Jupiter's moons therefore lie within its magnetosphere, a fact which may partially explain some of the activity on Io. Unfortunately for future space travelers and of real concern to the designers of the Voyager and Galileo spacecraft, the environment near Jupiter contains high levels of energetic particles trapped by Jupiter's magnetic field. This "radiation" is similar to, but much more intense than, that found within Earth's Van Allen belts. It would be immediately fatal to an unprotected human being.
The Galileo atmospheric probe discovered a new intense radiation belt between Jupiter's ring and the uppermost atmospheric layers. This new belt is approximately 10 times as strong as Earth's Van Allen radiation belts. Surprisingly, this new belt was also found to contain high energy helium ions of unknown origin.
Jupiter has rings like Saturn's, but much fainter and smaller (right). They were totally unexpected and were only discovered when two of the Voyager 1 scientists insisted that after traveling 1 billion km it was at least worth a quick look to see if any rings might be present. Everyone else thought that the chance of finding anything was nil, but there they were. It was a major coup. They have since been imaged in the infra-red from ground-based observatories and by Galileo.
Unlike Saturn's, Jupiter's rings are dark (albedo about .05). They're probably composed of very small grains of rocky material. Unlike Saturn's rings, they seem to contain no ice.
Particles in Jupiter's rings probably don't stay there for long (due to atmospheric and magnetic drag). The Galileo spacecraft found clear evidence that the rings are continuously resupplied by dust formed by micrometeor impacts on the four inner moons, which are very energetic because of Jupiter's large gravitational field. The inner halo ring is broadened by interactions with Jupiter's magnetic field.
In July 1994, Comet Shoemaker-Levy 9 collided with Jupiter with spectacular results (left). The effects were clearly visible even with amateur telescopes. The debris from the collision was visible for nearly a year afterward with HST.
When it is in the nighttime sky, Jupiter is often the brightest "star" in the sky (it is second only to Venus, which is seldom visible in a dark sky). The four Galilean moons are easily visible with binoculars; a few bands and the Great Red Spot can be seen with a small astronomical telescope.
Jupiter's Satellites
Jupiter has 63 known satellites (as of Feb 2004): the four large Galilean moons plus many more small ones some of which have not yet been named:
Jupiter is very gradually slowing down due to the tidal drag produced by the Galilean satellites. Also, the same tidal forces are changing the orbits of the moons, very slowly forcing them farther from Jupiter.
Io, Europa and Ganymede are locked together in a 1:2:4 orbital resonance and their orbits evolve together. Callisto is almost part of this as well. In a few hundred million years, Callisto will be locked in too, orbiting at exactly twice the period of Ganymede (eight times the period of Io).
Jupiter's satellites are named for other figures in the life of Zeus (mostly his numerous lovers).
Many more small moons have been discovered recently but have not as yet been officially confirmed or named. The most up to date info on them can be found at Scott Sheppard's site.
Distance Radius Mass
Satellite (000 km) (km) (kg) Discoverer Date
--------- -------- ------ ------- ---------- -----
Metis 128 20 9.56e16 Synnott 1979
Adrastea 129 10 1.91e16 Jewitt 1979
Amalthea 181 98 7.17e18 Barnard 1892
Thebe 222 50 7.77e17 Synnott 1979
Io 422 1815 8.94e22 Galileo 1610
Europa 671 1569 4.80e22 Galileo 1610
Ganymede 1070 2631 1.48e23 Galileo 1610
Callisto 1883 2400 1.08e23 Galileo 1610
Leda 11094 8 5.68e15 Kowal 1974
Himalia 11480 93 9.56e18 Perrine 1904
Lysithea 11720 18 7.77e16 Nicholson 1938
Elara 11737 38 7.77e17 Perrine 1905
Ananke 21200 15 3.82e16 Nicholson 1951
Carme 22600 20 9.56e16 Nicholson 1938
Pasiphae 23500 25 1.91e17 Melotte 1908
Sinope 23700 18 7.77e16 Nicholson 1914
1. 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".
2. 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.
3. 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: 3.5e18 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.
Earlier, it was thought that its size and irregular shape should imply that Amalthea is a fairly strong, rigid body. But measurements of it's mass made during Galileo's last orbit indicate otherwise. It now appears that Amalthea's density is only about the same as water and since it is unlikely to be composed of ice it is most likely a loose "rubble pile" with a lot of empty spaces.
4. 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)
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 cup bearer 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/sulfur 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 (left), and somewhat younger (but still ancient) lighter regions marked with an extensive array of grooves and ridges (right). Their origin is clearly of a tectonic nature, but the details are unknown. In this respect, Ganymede may be more similar to the Earth than either Venus or Mars (though there is no evidence of recent tectonic activity).
Evidence for a tenuous oxygen atmosphere on Ganymede, very similar to the one found on Europa, has been found recently by HST (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. The density of cratering indicates an age of 3 to 3.5 billion years, similar to the Moon. Craters both overlay and are cross cut by the groove systems indicating that the grooves are quite ancient, too. Relatively young craters with rays of ejecta are also visible (left).
Unlike the Moon, however, 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 (right).
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 placed 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 internal structure; however there are signs from recent Galileo data that the interior materials have settled partially, with the percentage of rock increasing toward the center. Callisto is about 40% ice and 60% rock/iron. Titan and Triton are probably similar.
Callisto's surface is covered entirely with craters. The surface is very old, like the highlands of the Moon and Mars. Callisto has the oldest, most cratered surface of any body yet observed in the solar system; having undergone little change other than the occasional impact for 4 billion years.
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 (right). Nearly 3000 km in diameter, Valhalla is a dramatic example of a multi-ring basin, the result of a massive impact. Other examples are Callisto's Asgard (left), 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 (left) 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 (right). 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.
Callisto has a very tenuous atmosphere composed of carbon dioxide.
Galileo has detected evidence of a weak magnetic field which may indicate some sort of salty fluid below the surface.
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; (it may be related to the orbital and tidal evolution of Ganymede).
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
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 (left). Therefore, the surface is very young.
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 by Galileo (bottom left image on this page) 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 (right). 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 (below right), mountains which are apparently NOT volcanoes (left), 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 variegated appearance.
Analysis of the Voyager images led scientists 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 idea 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 as high as 2000 K 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 evolution of the solar system to drive off the volatile elements in the vicinity of Io but not so hot to do so 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. Unlike Io, however, Europa has a thin outer layer of ice. 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 with 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 precise age of Europa's surface is an open question.
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 many moons in the solar system only five others (Io, Ganymede, Callisto, 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 confirm earlier theories that Europa's surface is very young: very few craters are seen, some sort of activity is obviously occurring. 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 that Europa has a weak magnetic field (perhaps 1/4 of the strength of Ganymede's). And most interestingly, it varies periodically as it passes thru Jupiter's massive magnetic field. This is very strong evidence that there is a conducting material beneath Europa's surface, most likely a salty ocean
Jupiter has 63 known satellites (as of Feb 2004): the four large Galilean moons plus many more small ones some of which have not yet been named:
Jupiter is very gradually slowing down due to the tidal drag produced by the Galilean satellites. Also, the same tidal forces are changing the orbits of the moons, very slowly forcing them farther from Jupiter.
Io, Europa and Ganymede are locked together in a 1:2:4 orbital resonance and their orbits evolve together. Callisto is almost part of this as well. In a few hundred million years, Callisto will be locked in too, orbiting at exactly twice the period of Ganymede (eight times the period of Io).
Jupiter's satellites are named for other figures in the life of Zeus (mostly his numerous lovers).
Many more small moons have been discovered recently but have not as yet been officially confirmed or named. The most up to date info on them can be found at Scott Sheppard's site.
Distance Radius Mass
Satellite (000 km) (km) (kg) Discoverer Date
--------- -------- ------ ------- ---------- -----
Metis 128 20 9.56e16 Synnott 1979
Adrastea 129 10 1.91e16 Jewitt 1979
Amalthea 181 98 7.17e18 Barnard 1892
Thebe 222 50 7.77e17 Synnott 1979
Io 422 1815 8.94e22 Galileo 1610
Europa 671 1569 4.80e22 Galileo 1610
Ganymede 1070 2631 1.48e23 Galileo 1610
Callisto 1883 2400 1.08e23 Galileo 1610
Leda 11094 8 5.68e15 Kowal 1974
Himalia 11480 93 9.56e18 Perrine 1904
Lysithea 11720 18 7.77e16 Nicholson 1938
Elara 11737 38 7.77e17 Perrine 1905
Ananke 21200 15 3.82e16 Nicholson 1951
Carme 22600 20 9.56e16 Nicholson 1938
Pasiphae 23500 25 1.91e17 Melotte 1908
Sinope 23700 18 7.77e16 Nicholson 1914
Jupiter's Inner Moons
1. 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".
2. 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.
3. 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: 3.5e18 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.
Earlier, it was thought that its size and irregular shape should imply that Amalthea is a fairly strong, rigid body. But measurements of it's mass made during Galileo's last orbit indicate otherwise. It now appears that Amalthea's density is only about the same as water and since it is unlikely to be composed of ice it is most likely a loose "rubble pile" with a lot of empty spaces.
4. 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)
Jupiter's Satellite : 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 cup bearer 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/sulfur 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 (left), and somewhat younger (but still ancient) lighter regions marked with an extensive array of grooves and ridges (right). Their origin is clearly of a tectonic nature, but the details are unknown. In this respect, Ganymede may be more similar to the Earth than either Venus or Mars (though there is no evidence of recent tectonic activity).
Evidence for a tenuous oxygen atmosphere on Ganymede, very similar to the one found on Europa, has been found recently by HST (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. The density of cratering indicates an age of 3 to 3.5 billion years, similar to the Moon. Craters both overlay and are cross cut by the groove systems indicating that the grooves are quite ancient, too. Relatively young craters with rays of ejecta are also visible (left).
Unlike the Moon, however, 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 (right).
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
Jupiter's Satellite : 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 placed 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 internal structure; however there are signs from recent Galileo data that the interior materials have settled partially, with the percentage of rock increasing toward the center. Callisto is about 40% ice and 60% rock/iron. Titan and Triton are probably similar.
Callisto's surface is covered entirely with craters. The surface is very old, like the highlands of the Moon and Mars. Callisto has the oldest, most cratered surface of any body yet observed in the solar system; having undergone little change other than the occasional impact for 4 billion years.
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 (right). Nearly 3000 km in diameter, Valhalla is a dramatic example of a multi-ring basin, the result of a massive impact. Other examples are Callisto's Asgard (left), 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 (left) 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 (right). 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.
Callisto has a very tenuous atmosphere composed of carbon dioxide.
Galileo has detected evidence of a weak magnetic field which may indicate some sort of salty fluid below the surface.
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; (it may be related to the orbital and tidal evolution of Ganymede).
Jupiter's Satellite : 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
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 (left). Therefore, the surface is very young.
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 by Galileo (bottom left image on this page) 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 (right). 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 (below right), mountains which are apparently NOT volcanoes (left), 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 variegated appearance.
Analysis of the Voyager images led scientists 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 idea 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 as high as 2000 K 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 evolution of the solar system to drive off the volatile elements in the vicinity of Io but not so hot to do so farther out.
Jupiter's Satellite : 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. Unlike Io, however, Europa has a thin outer layer of ice. 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 with 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 precise age of Europa's surface is an open question.
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 many moons in the solar system only five others (Io, Ganymede, Callisto, 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 confirm earlier theories that Europa's surface is very young: very few craters are seen, some sort of activity is obviously occurring. 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 that Europa has a weak magnetic field (perhaps 1/4 of the strength of Ganymede's). And most interestingly, it varies periodically as it passes thru Jupiter's massive magnetic field. This is very strong evidence that there is a conducting material beneath Europa's surface, most likely a salty ocean
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