NEPTUNE

eighth planet in average distance from the Sun. It was named for the Roman god of the sea. The sea-god's three-pronged trident () serves as the planet's astronomical symbol. Neptune's distance from the Sun varies between 29.8 and 30.4 astronomical units (AUs). Its diameter is nearly four times that of the Earth (Table), but because of its great distance Neptune cannot be seen from the Earth without the aid of a telescope. Neptune's deep blue colour is due to the absorption of red light by methane gas in its atmosphere. It receives less than half as much sunlight as Uranus, but heat escaping from its interior makes Neptune slightly warmer than the latter. The heat liberated may also be responsible for Neptune's stormier atmosphere, which exhibits the fastest winds seen on any planet in the solar system. Neptune holding his trident, classical sculpture; in the Lateran Museum, Rome Latin Neptunus, in Roman religion, originally the god of freshwater; by 399 Bc he was identified with the Greek Poseidon and thus became a deity of the sea. His female counterpart, Salacia, was perhaps originally a goddess of leaping springwater, subsequently equated with the Greek Amphitrite. Neptune's festival (Neptunalia) took place in the heat of the summer (July 23), when water was scarcest; thus, its purpose was probably the propitiation of the freshwater deity. Neptune had a temple in the Circus Flaminius at Rome; one of its features was a sculptured group of marine deities headed by Poseidon and Thetis. In art Neptune appears as the Greek Poseidon, whose attributes are the trident and the dolphin. the eighth major planet from the Sun. Discovered in 1846, it was named after the Roman god of the sea. Neptune travels around the Sun in an elliptical orbit at an average distance of 4.504 billion km (2.799 billion miles). At this distance, it takes the planet 165 years to complete one orbit. Neptune is more than 17 times as massive as the Earth, and its volume exceeds that of the latter by 44 times. It has an equatorial diameter of roughly 49,528 km (30,775 miles). Neptune consists largely of hydrogen and helium. It has no apparent solid surface, but it may have a core of ice and rocky material. The planet's atmosphere, particularly the outer layers, contains substantial amounts of methane gas. Absorption of red light by the atmospheric methane is responsible for Neptune's deep blue colour. Considerable atmospheric turbulence was detected by the U.S. Voyager 2 space probe during its 1989 flyby of Neptune. Winds moving at 300 metres per second (700 miles per hour) and higher were observed by the spacecraft, as was an enormous storm systemthe so-called Great Dark Spotin the planet's southern hemisphere. Above this system drift delicate clouds resembling those of the cirrus variety, and closer to the south pole are broad cloud bands of varying shades of blue. Because of its great distance from the Sun, Neptune receives only a limited amount of solar radiation. Temperature measurements, however, suggest that the planet has an internal heat source just like Jupiter and Saturn. Even though Neptune receives about 2.5 times less sunlight than neighbouring Uranus, its effective radiating temperature is roughly the same as that of the latter (about 59 K). As in the case of the other giant outer planets and the Earth, Neptune is surrounded by a magnetic field. Though not nearly as strong as those of the other planets, Neptune's field traps solar wind and galactic cosmic-ray particles (e.g., energetic protons and electrons) in a belt around the planet that is somewhat akin to the Earth's Van Allen radiation belts. By analyzing radio emissions from the Neptunian magnetic field, astronomers have determined that the rotational period of the planet is 16.11 hours. Neptune's axis of rotation is tilted approximately 29 to the plane of the planet's orbit around the Sun, which is similar to those of the Earth and Mars (23.5 and 25, respectively). Voyager 2 observations revealed that Neptune is encircled by at least four rings, which are populated largely by dust-sized particles. The particles of the outermost ring are not distributed uniformly; there are five bright arclike regions of greater density distributed along a 45 segment of the ring. Neptune also has eight known satellites. Only two of these, Triton and Nereid, had been observed prior to the Voyager 2 flyby. Triton is the largest of the eight satellites and is almost as big as the Earth's Moon. The other Neptunian satellites range in diameter from 58 to 416 km (36 to 258 miles). Apart from Triton, the moons of Neptune are irregularly shaped and have very dark surfaces. Additional reading Patrick Moore, The Planet Neptune (1988), provides a good summary of pre-Voyager knowledge of the planet. A set of articles in Science, 246(4936):14171501 (Dec. 15, 1989), comprises the initial report of the Voyager findings at Neptune; more detailed reports of the findings are in a set of articles in Journal of Geophysical Research, Supplement, 96:18, 90319,268 (Oct. 30, 1991); and Eric Burgess, Far Encounter: The Neptune System (1991), a popular work which also contains some discussion of Pluto. Mark Littmann, Planets Beyond: Discovering the Outer Solar System, updated and rev. ed. (1990), chronicles the history of the discovery of Uranus, Neptune, and Pluto. Ellis D. Miner