The largest satellite of Neptune, 2707 km in diameter; also known as Neptune I. It lies 354 760 km from Neptune’s centre, and orbits in a retrograde direction every 5.877 days at an inclination of 156°.9 to Neptune’s equator. Its period of axial rotation is the same as its orbital period. Triton was discovered by W. Lassell in 1846, a few weeks after the discovery of Neptune itself. Triton has a tenuous nitrogen atmosphere with some methane. The surface pressure, however, is only about 16 μbar, little more than 10−5 of the Earth’s atmospheric pressure, and its surface temperature is −235°C, the coldest surface so far measured in the Solar System. Triton has a young icy surface with an average albedo of 0.72, one of the brightest surfaces of any planetary satellite. Few impact craters are seen on Triton, but it has many different types of terrain including smooth plains, hummocky plains, cantaloupe terrain, long linear features, and also a large, probably seasonal, polar cap of nitrogen ice. The Voyager 2 space probe photographed dark plumes rising from dark spots on the surface to an altitude of 8 km before drifting downwind. These plumes may be geyser-like eruptions from subsurface pockets of liquid nitrogen or methane.