Neptune, like the rest of the gas giant planets in the Solar System, can be broken up into various layers. The composition of Neptune changes depending on which of these layers you’re looking at. The outermost layer of Neptune is the atmosphere, forming about 5-10% of the planet’s mass, and extending up to 20% of the way down to its core.
Beneath the atmosphere is the planet’s large mantle. This is a superheated liquid region where temperatures can reach as high as 2,000 to 5,000 K (1727 – 4727 °C; 3140 – 8540 °F). The mantle is equivalent to 10 – 15 Earth masses and is rich in water, ammonia and methane. This mixture is referred to as icy even though it is a hot, dense fluid, and is sometimes called a “water-ammonia ocean”.
Increasing concentrations of methane, ammonia and water are found in the lower regions of the atmosphere. Unlike Uranus, Neptune’s composition has a higher volume of ocean, whereas Uranus has a smaller mantle. Like the other gas/ice giants, Neptune is believed to have a solid core, the composition of which is still subject to guesswork. However, the theory that it is rocky and metal-rich is consistent with current theories of planet formation.
In accordance with these theories, the core of Neptune is composed of iron, nickel and silicates, with an interior model giving it a mass about 1.2 times that of Earth. The pressure at the center is estimated to be 7 Mbar (700 GPa), about twice as high as that at the center of Earth, and with temperatures as high as 5,400 K. At a depth of 7000 km, the conditions may be such that methane decomposes into diamond crystals that rain downwards like hailstones.
Neptune’s atmosphere forms about 5% to 10% of its mass and extends perhaps 10% to 20% of the way towards the core, where it reaches pressures of about 10 GPa – or about 100,000 times that of Earth’s atmosphere. At high altitudes, Neptune’s atmosphere is 80% hydrogen and 19% helium, with a trace amount of methane.
As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue, although Neptune’s is darker and more vivid. Because Neptune’s atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune’s more intense coloring.
In most pictures, Neptune seems to be a deeper and more brilliant blue than Uranus, which looks like a pale turquoise dot. But Neptune is likely similarly pale as its neighbor and just appears darker in images because of its greater distance from the sun.
Neptune's atmosphere is made up mostly of hydrogen and helium with just a little bit of methane. Neptune's neighbor Uranus is a blue-green color due to such atmospheric methane, but Neptune is a more vivid, brighter blue, so there must be an unknown component that causes the more intense color.
At high altitudes, the composition of Uranus and Neptune is very similar. The blue color of both the ice giants can be contributed to the presence of methane in their upper atmospheres. Methane absorbs most of the red light incident on the planets which makes them appear blue in color. But there is a difference in the color of these planets: Uranus has a cyan blue color (blue-green) whereas Neptune has a vivid azure blue color (bright blue).
Neptune has the wildest and strangest weather in the entire Solar System. It has huge storms with extremely high winds. Its atmosphere has dark spots which come and go, and bright cirrus-like clouds which change rapidly. Neptune has an average temperature of -353 Fahrenheit (-214 Celsius). On Earth sunlight drives our weather, but Neptune is so far away that it receives a thousand times less sunlight than Earth does. How Neptune gets the energy for such intense weather is still a mystery.
Just like Jupiter and Saturn, Neptune has bands of storms that circle the planet. While the wind speeds on Jupiter can reach 550 km/hour – twice the speed of powerful hurricanes on Earth, that’s nothing compared to Neptune. Astronomers have clocked winds on Neptune traveling at 2,100 km/hour.
So why can the winds on Neptune reach such huge speeds? Astronomers think that the cold temperatures on Neptune might have something to do with that after all. The cold temperatures might decrease the friction in the system, so that winds can get going fast on Neptune.
During its 1989 flyby, NASA’s Voyager 2 spacecraft discovered the Great Dark Spot on Neptune. Similar to Jupiter’s Great Red Spot, this is an anti-cyclonic storm measuring 13,000 km x 6,600 km across. A few years later, however, the Hubble Space Telescope failed to see the Great Dark Spot, but it did see different storms. This might mean that storms on Neptune don’t last as long as they do on Jupiter or even Saturn.
The more active weather on Neptune might be due, in part, to its higher internal heat. Although Neptune is much more distant than Uranus from the Sun, receiving 40% less sunlight, temperatures on the surface of the two planets are roughly similar. In fact, Neptune radiates 2.61 times as much energy as it receives from the Sun. This is enough heat to help drive the fastest winds in the Solar System.
Neptune has at least 14 moons and six known narrow rings. Each of the myriad particles that constitute the rings can be considered a tiny moon in its own orbit. The four moons nearest the planet orbit within the ring system, where at least some of them may interact gravitationally with the ring particles, keeping them from spreading out.
Prior to Voyager 2’s encounter, Neptune’s only known moons were Triton, discovered visually through a telescope in 1846, and Nereid, discovered in telescopic photographs more than a century later, in 1949. (Neptune’s moons are named after figures in Greek mythology usually connected with Poseidon [the Roman god Neptune] or with water.) With a diameter nearly that of Earth’s Moon, Triton is, by far, Neptune’s largest satellite—more than six times the size of its largest known sibling, Proteus, discovered by Voyager 2 in 1989. Triton is the only large moon of the solar system that travels around its planet in retrograde fashion. Moreover, whereas the orbits of the largest moons in the solar system are inclined less than about 5° to their planet’s equator, Triton’s orbit is tilted more than 157° to Neptune’s equator. Nereid, which revolves more than 15 times farther from Neptune on average than does Triton, has the most eccentric orbit of any known moon. At its greatest distance, Nereid is nearly seven times as far from Neptune as at its smallest distance. Even at its closest approach, Nereid is nearly four times the distance of Triton.
Evidence that Neptune has one or more rings arose in the mid-1980s when stellar occultation studies from Earth occasionally showed a brief dip in the star’s brightness just before or after the planet passed in front of it. Because dips were seen only in some studies and never symmetrically on both sides of the planet, scientists concluded that any rings present do not completely encircle Neptune but instead have the form of partial rings, or ring arcs.
Neptune revolves around the sun once every 165 years. As it revolves, Neptune traces an ellipse. (An ellipse is roughly oval-shaped.) The planet travels a huge distance to complete just one orbit.
Neptune’s average orbital distance from the sun is enormous. In fact, this distance is about 30 times greater than the corresponding value for Earth.
Neptune is the eighth and farthest planet from the sun. Beyond Neptune’s orbit is a region that is traversed by many trans-Neptunian objects. Of these trans-Neptunian objects, the most familiar is Pluto. It sometimes ventures closer to the sun than Neptune.
Neptune’s magnetic field rotates once in about 16 hours. This is the length of the planet’s “day.” The rate of rotation of the visible features in Neptune’s atmosphere varies greatly depending on the features’ latitudes.