jupiter

Jupiter is more massive than all of the other planets in the solar system combined, plus their satellites, the asteroids, and all the comets! A mini solar system unto itself. It consists almost entirely of Hydrogen and Helium like our sun. Jupiter is attended by four of the biggest moons in the solar system, the Galilean satellites Io, Europa, Ganymede, and Callisto, all of which are larger than Pluto, plus a veritable swarm of other tiny moonlets.
Jupiter's deep gaseous atmosphere merges into a layer of liquid hydrogen. Within the planet pressure and temperature are so high that there is no clear boundary between the gas and liquid. The atmosphere is composed primarily of hydrogen and helium in nearly the same abundances found in the Sun and other stars. This enormous planet rotates faster than any other in the solar system, so fast that its shape is noticeably flattened out of a sphere. The banded and colorful atmosphere contains many gigantic oval-shaped storms. One of these, the Great Red Spot, is larger than two Earths and has persisted in Jupiter's atmosphere at least since the 1700s. The atmosphere is probably a few hundred miles in depth, pulled toward the surface by the intense gravity. Closer to the surface, the gases become more dense, and likely turn into a compound of slurry. Jupiter's rocky core lies well below the "surface" and is very hot (around 36,000 degrees F.) due to gravitational compression (compression is a heating process). But Jupiter is much too small and cool to ignite nuclear fusion reactions which are required to become a star.
Jupiter's extremely fast rotation flattens the globe at the poles and drives extremely changeable weather patterns in the clouds which envelope the planet. The clouds are likely made of ammonia ice crystals, changing to ammonia droplets further down. It is estimated that the temperature of the cloud tops are about -280 degrees F. Overall, Jupiter's average temperature is -238 degrees F. Since Jupiter is only tilted slightly more then 3 degrees on its axis, seasonal fluctuations are minimal.
Jupiter is basically a turbulent, stormy, whirlpool of wind, banded with variable belts and a giant "Red Spot." This giant Red Spot is an oval shaped, counter-clockwise moving storm and is four times larger than our Earth. The storm is by far the largest of similar ovals found on other parts of Jupiter and the other gas giants. Jupiter's wind appears to be driven by internal heat rather than from solar insolation.
Jupiter was the first target for Pioneers 10 and 11 in 1972 and 1973 and Voyagers 1 and 2 in 1979. From 1995 to 2003, the Galileo orbiter toured the Jupiter system. Jupiter's mass makes it an attractive waypoint for gravity assist trajectories, which allow spacecraft to reach otherwise inaccessible points in the solar system. The solar orbiter Ulysses in 1992 and Saturn orbiter Cassini in 2000 took advantage of Jupiter gravity assists. The Pluto mission New Horizons will be the next spacecraft to fly by Jupiter in February 2007.
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.