TRITON

in Greek mythology, a merman, demigod of the sea; he was the son of the sea god, Poseidon, and his wife, Amphitrite. According to the Greek poet Hesiod, Triton dwelt with his parents in a golden palace in the depths of the sea. Sometimes he was not particularized but was one of many Tritons. He was represented as human down to his waist, with the tail of a fish. Triton's special attribute was a twisted seashell, on which he blew to calm or raise the waves. the largest of Neptune's known satellites. It measures about 2,735 km (1,700 miles) across, having a diameter several hundred kilometres smaller than that of the Earth's Moon. Triton moves in a retrograde orbit (i.e., one that is opposite the direction of Neptune's rotation) at a distance of about 354,450 km (220,300 miles) from the planet. Triton revolves so slowly that each of its seasons lasts more than 40 years. Though discovered by the English astronomer William Lassell in 1846, little was known about Triton until 1989, when the U.S. Voyager 2 probe flew to within 38,600 km (24,000 miles) of it. Voyager's observations revealed that Triton has an extremely thin atmosphere of nitrogen and methane. Data from the probe also indicated that the satellite has a surface temperature of -240 C (-400 F), making it perhaps the coldest object in the solar system. Triton is covered with enormous expanses of ice pitted by what appear to be a few meteorite craters as well as by some rugged areas. Other topographic features include long, ridgelike structures that occasionally cut across one another. Scientists attribute Triton's unique landscape to a phenomenon termed icy volcanism. They theorize that a significant amount of the thick layer of nitrogen and methane frost covering the surface evaporates during those seasons when the surface is exposed to sunlight. The decrease in the volume of icy matter results in a reduction in the amount of pressure exerted on subsurface reservoirs of liquid nitrogen and methane. When the confining pressure drops sufficiently, some of this nitrogen and methane vaporizes and is ejected from fissures in the overlying layer of ice. Exposure to the subfreezing temperature of the atmosphere causes the nitrogen and methane to pass directly from the gaseous to the solid phase, resulting in the further accumulation of frost at the surface. U.S. nuclear-powered submarine that was the first vessel to circumnavigate the globe underwater. At the time of its launch in August 1958, the Triton was the largest and most powerfully engined submarine. Its two nuclear reactors gave the ship a speed of 30 knots (nautical miles per hour) and a range of about 110,000 miles (180,000 km) without refueling. It was 447.5 feet (136 m) long and displaced 5,900 tons. The Triton completed its circumnavigation on May 10, 1960. The submarine was decommissioned in 1969 and laid up at Norfolk, Va.