The Gas Giants
A gas giant (also known as a Jovian planet after the planet Jupiter,) is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus and Neptune. Astronomers sometimes categorize Uranus and Neptune as "ice giants", in order to emphasize the differences in composition between them and larger gas giants like Jupiter and Saturn. They are the furthest planets from the Sun. Compared to the rocky planets, gas giants are extremely large and massive.
Despite their size, gas giants are low-density planets because they are composed almost entirely of gas. The gas giants are large planets that have a thick atmosphere of hydrogen and helium. The hydrogen and helium in "traditional" gas giants like Jupiter and Saturn constitutes most of the planet. They possibly have a dense molten core made of rocky elements. The gases that make up these planets become thinner and thinner with increasing distance from the planets' centers, eventually becoming indistinguishable from the surrounding space.
JUPITER, named after the Roman god Jupiter, is the fifth planet from the Sun and the largest planet in the Solar System. Its mass is only one-thousandth of that of the Sun but is two and a half times the mass of all the other planets in the Solar System combined. Jupiter is primarily composed of hydrogen and helium, although helium only comprises about a tenth of the mass. It may also have a rocky core of heavier elements, but lacks a well-defined solid surface.
Because of its rapid rotation, it possesses a slight but noticeable bulge around the equator. The outer atmosphere is divided into several bands at different latitudes, resulting in turbulence and storms along their boundaries. Jupiter's most visible and well known feature is the Great Red Spot, a giant storm that is known to have existed since at least the 17th century. Ganymede, the largest of these moons, has a diameter greater than that of the planet Mercury. Jupiter has been explored on several occasions by robotic spacecraft, most notably during the early Pioneer and Voyager flyby missions and later by the Galileo orbiter. The most recent probe to visit Jupiter was the Pluto-bound New Horizons spacecraft in late February 2007.
Jupiter has at least 67 moons, including the four large moons called the Galilean moons that were first discovered by Galileo Galilei in 1610 and a faint ring system that was discovered in 1979 by the Voyager 2 mission. The ring system is composed of three main segments: an inner torus of particles known as the halo, a relatively bright main ring, and an outer gossamer ring. These rings appear to be made of dust, rather than ice as with Saturn's rings. The main ring is probably made of material ejected from the satellites Adrastea and Metis.
SATURN, named for the Roman god of agriculture, is the sixth planet from the Sun, at a distance of about 1.4 billion km (886 million miles) or 9.5 AU, and the second largest planet in the Solar System, after Jupiter. The planet's average radius is about nine times that of Earth. While only one-eighth the average density of Earth, with its larger volume Saturn is just over 95 times more massive.
Saturn's interior is probably composed of a core of iron, nickel and rock, surrounded by a deep layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium and an outer layer composed of ammonia and methane. Saturn looks pale yellow due to ammonia crystals in its upper atmosphere. The outer atmosphere is generally uninteresting and lacking in contrast, although long-lived features can appear. Wind speeds on Saturn can reach 1,800 km/h (1,100 mph), faster than on Jupiter, but not as fast as those on Neptune.
There are sixty-two known moons orbiting Saturn, of which fifty-three are officially named. Saturn's most famous feature is its prominent ring system that consists of nine continuous main rings and three arcs, composed mostly of ice particles with a smaller amount of rocky debris and dust. Titan, Saturn's largest and the Solar System's second largest moon, is larger than the planet Mercury and is the only moon in the Solar System to retain a substantial atmosphere.
