Inclination of Axis - 3.08E,

Density - 1.3 g/cm³,

Known Satellites - min 27? (11 new discoveries Nov. 2000)

Jupiter, a giant gas-rich planet of the outer solar system, is the center of a system of a least 16 ice and rock satellites and a narrow ring of much smaller particles. Part of the reason Jupiter became so much larger than the rocky inner planets is that it formed in a cool region of the ancient solar nebula where water ice was stable. Jupiter consists principally of hydrogen and helium; a rocky core is embedded deep within the planet.

Above the core of Jupiter (and Saturn) 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.

The brightly colored atmosphere is banded and has large semipermanent storm systems. The Great Red Spot 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 Great Red Spot is an oval about 12,000 by 25,000 km, big enough to hold two Earths.

3 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). Data from the Galileo atmospheric probe also indicate that there is much less water than expected. However, recent data suggests that the probe entry site may well have been one of the warmest and least cloudy areas on Jupiter at that time, so stay tuned.

Jupiter's four large moons Io, Europa, Ganymede and Callisto, discovered by Galileo in 1610.

Io, the innermost Galilean satellite, has a very young surface and is presently volcanically active. Tidal flexing of this moon provides a continued input of energy to melt parts of the interior of this rocky planet. There are no impact craters on Io. The surface of Io must be younger than a Million years old, and is continually being resurfaced by volcanic activity. Also, the surface is very colorful, mottled with red, yellow, white and orange black markings. The surface composition on Io consists largely of sulfurwith deposits of frozen sulfur dioxide. The surface on Io is mostly flat plains rising no more than 1 km. Moutain ranges up to 9 km high have also been observed.

Europa has a relatively smooth, but fractured, icy surface. No vestiges of the intense bombardment remain on Europa; only a few small craters have been identified. Resurfacing by the outpouring of watery lavas and the flow of solid ice have shaped the surface features. Tidal flexing of Europa has kept the interior warm. 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.

Ganymede, the largest Galilean satellite, has a varied surface dominated by heavily cratered terrain and large swathes of younger, intensely grooved terrain, apparently formed by flooding old crust with volcanic flows of liquid water. Data from Galileo indicates that 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. The latest data suggest that Ganymede, which is bigger than two of the solar system's nine planets, should be added to the growing list of worlds with evidence of liquid water under the surface.

Callisto, the outermost of the large satellites, is dominated by heavily cratered terrain. The satellites of Jupiter appear to have condensed in a thermal gradient centered on Jupiter. Therefore, the inner moons are silicate-rich , and ice-poor whereas the outer moons are ice-rich.