Neptune is the outermost of the gas giants. Like Uranus, it has an atmosphere rich in hydrogen, helium and methane, and a faint ring system. Neptune was discovered in 1846, after its position had been predicated based on disturbance to the motion of Uranus caused by its gravitational pull. The planet remained poorly know until the space probe Voyger2 flew past in 1989.
Rings and Moons
Neptune has at least four rings and eight moons. The rings, which are too see faint to see with telescopes from the Earrth, were photographed by the Voyager 2 space probe. The outermost ring, known as Adams, contains several denser clumps of material termed rings arcs.
Atmosphere and Climate
The atmospheres of Neptune and Uranus are similar, although Neptune’s stormier and appears blue (rather than blue-green) because there is more methane in its upper levels. In 1989 Voyager 2 photographed a large dark spot, reminiscent of Jupiter’s of Jupiter’s Great Red Spot, in the southern Hemisphere. Five years later, when the Hubble space Telescope observed Neptune, the spot had vanished but another dark spot, this time in the Northern Hemisphere, appeared late in 1994. Bright streaks of methane cirrus clouds were recorded by both Voyager and Hubble. The compositions of the main cloud deck is not known with the certainly, but hydrogen sulfide and ammonia are probably present. Neptune’s interior is thought to resemble that of Uranus, consisting of water, ammonia, and methane above a rocky core.
Of Neptune’s eight known moons, Triton is the only one of any significance. It’s also the largest having a diameter of 1, 680 miles (2,700). The surface is covered with nitrogen and methane frost and at- 3918oF (-235oC), is the coldest known place in the solar System. Liquid nitrogen bursts through the surface to create geysers that leave dark streaks. Triton has a retrograde orbit (it moves in the opposite direction to Neptune) and is thought to have been a separate body, like Pluto, that was captured by Neptune’s gravity, Triton is actually larger than Pluto, and the two may be physically similar.
Neptune never becomes brighter than 8th magnitude and so is invisible to the naked eye, but it can located with binoculars, through which it resembles a faint stars. Sightings of Neptune can be confirmed by observing its gradual movement against the background of stars over several nights. Through a telescope, the planet appears as a featureless bluish disk about two-thirds the apparent size of Uranus. Neptune comes to opposition (see p.15) anly two and a half days later each year.