The planet Neptune

Neptune is like Uranus in many ways, but has its own unique features. Because of Pluto's highly elliptical orbit, it is currently the most distant planet from the Sun, at a separation of about 30 Astronomical Units. The full disk of Neptune in shown in the adjacent image (Ref).

Neptune has been particularly challenging to study from the ground because its disk is small and badly blurred by the Earth's atmosphere at that distance. In spite of this, ground-based astronomers had learned a great deal about this planet since its position was first predicted by Adams and Leverrier in 1845. However, our most detailed information about Neptune has come from the Voyager 2 flyby in 1989.

General Features

Neptune is currently the most distant planet from the Sun, with an orbital radius of 30 Astronomical Units and an orbital period of 165 years. Its diameter is about four times that of the Earth, which makes it the 4th largest planet. It is slightly smaller than Uranus, but its density is 1.6 g/cc (compared with 1.2 g/cc for Uranus), which makes it the 3rd most massive planet. The relatively low density indicates large concentrations of hydrogen and helium, but Uranus and Neptune both have much larger concentrations of heavier elements than Jupiter and Saturn. As for all the gas giant planets, models suggest rocky cores of maybe 15 Solar masses, but there is no direct confirmation of this.

The bluish color of the adjacent image is, as for Uranus, because of methane in the atmosphere, which absorbs red light, leaving the light scattered from Neptune preferentially enhanced at blue wavelengths. The period of rotation is about 16 hours, comparable to that of Uranus and much slower than for Jupiter and Saturn. The temperatures at the cloud tops are about -216 degrees Celsius, slightly warmer than for Uranus. Neptune, like Jupiter and Saturn but unlike Uranus, has an internal heat source and produces 2.7 times more heat than it absorbs.

The Surface and the Interior

The Voyager 2 flyby in 1989 revealed strong winds, bright, high-altitude clouds, and two large dark spots attributed to long-lived giant storm systems. Tracking of these features indicated wind speeds as large as 730 miles per hour. The largest dark storm systems, called the "Great Dark Spot", is shown in the image adjacent left. It resembles Jupiter's Great Red Spot (more info).

The interior is presumed to contain a rocky core with an icy mantle topped by a deep layer of liquid hydrogen. Voyager 2's instruments detected a complex magnetic field. Like Uranus, the field is tipped with respect to the axis of rotation and offset from the center (the tilt is 50 degrees for Neptune, compared with 60 degrees for Uranus). However, the field is somewhat weaker than for Uranus.

As for Uranus, it is speculated that this magnetic field my originate in a conducting shell not far below the clouds, rather than deep in the interior as for Jupiter or the Earth. In that case, the conducting material would not be metallic hydrogen, as for Jupiter, or iron and nickel, as for the Earth. As noted earlier for Uranus, a mixture of water, methane, and ammonia under the right pressure could be responsible.

The Rings

Neptune's rings were first detected in star occultation experiments from Earth in 1983, but they were very difficult to study before the data from Voyager 2. The following image (Ref) shows the faint rings of Neptune (the light from the body of the planet is blocked off in the black region to allow the rings to be seen clearly).

Only the two brightest rings show clearly in this image. Voyager 2 found additional fainter rings. The rings rotate in the same direction as the planet and are nearly in the equatorial plane.

The material in one of the rings is clumpy (more dense in some regions than others). This gave the impression in ground-based observations that the rings were arcs instead of complete rings, but the Voyager 2 data showed that they were complete rings. The rings of Neptune contain much more dust-size grains than the corresponding rings of Saturn and Uranus, and are quite narrow (about as narrow as the Saturn F-ring).

The Moons

Neptune has two large moons that are easily seen from Earth, Triton and Nereid. Voyager 2 discovered six additional moons. One of these is actually larger than Nereid, but could not be seen easily from Earth because it orbits close to Neptune.

Triton

The image on the right (Ref) shows a portion of the southern region of Triton, Neptune's largest moon, taken in 1989 by Voyager. Triton is comparable in size with our own moon, and has a thin atmosphere, mostly of nitrogen. The polar ice cap in this image is probably mostly nitrogen ice. Triton orbits Neptune with retrograde motion, which probably means that it is a captured object.

Voyager 2 found some of the most varied terrain in the Solar System, a thin atmosphere, and even evidence for ice volcanoes on Triton. The dark streaks seen in this image are material spread downwind from recent volcanic eruptions. Giant faults cross the surface of Triton. The favored mechanism for the volcanoes is that the Sun heats darkened methane ice on the surface, which heats underlying nitrogen ice that vents through the surface volcanoes (more info).

The New Moons of Neptune

The six newly-discovered moons orbit with direct motion nearly in the equatorial plane. Most are closer to Neptune than its rings. Because this lies inside the Roche limit, these moons could not have formed by accretion in their present location. They must have formed elsewhere before coming to their present orbits, though we are not certain where.