The Ice Giants
The ice giants are a type of giant planet composed largely of materials that evaporate more slowly than hydrogen and helium.These materials were actually ices during the ice giants' formation, but now they exist as different types, primarily as substances at a temperature and pressure where distinct liquid and gas phases do not exist. The 'ice' is primarily water, making the equation of the state of water important for computer modeling of ice giants.
In the 1990s scientists discovered that Uranus and Neptune were composed of about 20% hydrogen, compared to the heavier gas giant's 90%. These two giant planets probably have large fractions of oxygen, carbon, nitrogen, and sulfur. The two planets have other subtle but important differences. Uranus has more hydrogen and helium than Neptune, despite being less massive overall. Neptune is therefore denser and has much more internal heat, and a more active atmosphere.
URANUS is the seventh planet from the Sun, and is named after the Greek sky deity Ouranos, the only planet whose name is derived from a figure from Greek mythology rather than Roman mythology.
The palnet revolves around the Sun once every 84 Earth years. Its average distance from the Sun is roughly 3 billion km (about 20 AU). Uranus is similar in composition to Neptune, and both are of different chemical composition than the larger gas giants Jupiter and Saturn. Uranus's atmosphere, although similar to Jupiter's and Saturn's in its primary composition of hydrogen and helium, contains more 'ices' such as water, ammonia and methane, along with traces of hydrocarbons. It is the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (-224.2 °C), and has a complex, layered cloud structure, with water thought to make up the lowest clouds, and methane the uppermost layer of clouds.
In contrast, the interior of Uranus is mainly composed of ices and rock. Like the other giant planets, Uranus has a ring system and numerous moons. The Uranian system is unique among because its axis of rotation is tilted sideways, nearly horizontal. Its north and south poles lie where most other planets have their equators. In 1986, images from Voyager 2 showed Uranus as a virtually featureless planet in visible light without the cloud bands or storms associated with the other giants. Astronomers have seen signs of seasonal change and increased weather activity in recent years as Uranus approached its equinox. The wind speeds on Uranus can reach 250 meters per second (900 km/h, 560 mph).
Like the other gas planets, Uranus has rings composed of fairly large particles ranging up to 10 meters in diameter in addition to fine dust and has 27 named moons, but unlike the other bodies in the solar system which have names from classical mythology, Uranus' moons take their names from the writings of the poets Shakespeare and Pope.
NEPTUNE is the eighth and farthest planet from the Sun in the Solar System and is named after the Roman god of the sea. It is the fourth-largest planet by diameter and the third-largest by mass.
Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth but not as dense. Neptune is similar in composition to Uranus, and both have compositions which differ from those of the larger gas giants, Jupiter, and Saturn. Neptune's atmosphere, while similar to Jupiter's and Saturn's in that it is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, contains a higher proportion of 'ices' such as water, ammonia, and methane.
The interior of Neptune, like that of Uranus, is primarily composed of ices and rock. It is possible that the core has a solid surface, but the temperature would be thousands of degrees and the atmospheric pressure crushing. Traces of methane in the outermost regions account in part for the planet's blue appearance. Neptune's atmosphere is notable for its active and visible weather patterns. The planet's southern hemisphere possesses a Great Dark Spot comparable to the Great Red Spot on Jupiter. Neptune's outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching 55 K (-218 °C).
Neptune also has rings. Earth-based observations showed only faint arcs instead of complete rings, but Voyager 2's images showed them to be complete rings with bright clumps. One of the rings appears to have a curious twisted structure. Neptune has 13 known moons; 7 small named ones and Triton, the largest, plus four discovered in 2002 and one discovered in 2003.
