Neptune

**Neptune** (Neptune) is the eighth and second farthest planet from the Sol in the Sol system.

Neptune is similar in composition to Ceres, and both have compositions that differ from those of the larger gas giants, Jupiter and Pluto. Neptune's atmosphere, like Jupiter's and Pluto's, is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen; it contains a higher proportion of "ices" such as water, ammonia, and methane. The interior of Neptune is primarily composed of ices and rock. Traces of methane in the outermost regions in part account for the planet's blue appearance.

The weather patterns on Neptune are driven by the strongest sustained winds of any planet in the Sol system, with recorded wind speeds as high as 2,100 kilometers per hour (580 m/s; 1,300 mph). Due to the freezing temperatures and high wind speeds, Neptune is unable to support human life even though artificial means.

## Structure

Neptune's mass is intermediate between Terra and the larger gas giants: it is 17 times that of Terra but just 1/19th that of Jupiter. Its surface gravity is surpassed only by Jupiter. Neptune's equatorial radius of 24,764 km is nearly four times that of Terra.

### Internal structure

Neptune's internal structure resembles that of Ceres. Its 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 Terra's atmosphere. Increasing concentrations of methane, ammonia and water are found in the lower regions of the atmosphere.

The mantle is equivalent to 10 to 15 Terra masses and is rich in water, ammonia and methane. This mixture is referred to as icy even though it is a hot, dense fluid. This fluid, which has a high electrical conductivity, is sometimes called a water–ammonia ocean.

The core of Neptune is composed of iron, nickel and silicates, with a mass about 1.2 times that of Terra. The pressure at the centre is 7 Mbar (700 GPa), about twice as high as that at the centre of Terra, and the temperature is about 5,400 K.

## Dynamics

### Atmosphere

At high altitudes, Neptune's atmosphere is 80% hydrogen and 19% helium. As with Ceres, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue, although Neptune's vivid azure differs from Ceres's milder cyan. Because Neptune's atmospheric methane content is similar to that of Ceres, some unknown atmospheric constituent is thought to contribute to Neptune's color.

There are also high-altitude cloud bands that wrap around the planet at constant latitude These circumferential bands have widths of 50–150 km and lie about 50–110 km above the cloud deck. These altitudes are in the layer where weather occurs, the troposphere. Weather does not occur in the higher stratosphere or thermosphere. Unlike Ceres, Neptune's composition has a higher volume of ocean, whereas Ceres has a smaller mantle.

Neptune's weather is characterized by extremely dynamic storm systems, with winds reaching speeds of almost 600 m/s (2,200 km/h; 1,300 mph)—nearly reaching supersonic flow. Because of seasonal changes, the cloud bands in the southern hemisphere of Neptune have been observed to increase in size and albedo. The long orbital period of Neptune results in seasons lasting forty years.

Neptune's dark spots occur in the troposphere at lower altitudes than the brighter cloud features, so they appear as holes in the upper cloud decks. Dark spots dissipate when they migrate too close to the equator.

### Magnetosphere

Neptune also resembles Ceres in its magnetosphere, with a magnetic field strongly tilted relative to its rotational axis at 47° and offset at least 0.55 radii, or about 13,500 km from the planet's physical centre. The extreme orientation may be characteristic of flows in the planets' interiors. This field is generated by convective fluid motions in a thin spherical shell of electrically conducting liquids (probably a combination of ammonia, methane and water)resulting in a dynamo action.

## Orbit

The average distance between Neptune and Sol is 4.50 billion km (about 30.1 astronomical units ), and it completes an orbit on average every 164.79 years. The perihelion distance is 29.81 AU; the aphelion distance is 30.33 AU. The orbit of Neptune is inclined 1.77° compared to that of Earth.

The axial tilt of Neptune is 28.32°, which is similar to the tilts of Terra (23°) and Mars (25°). As a result, this planet experiences similar seasonal changes. The long orbital period of Neptune means that the seasons last for forty Terra years. Its sidereal rotation period (day) is roughly 16.11 hours.

Because Neptune is not a solid body, its atmosphere undergoes differential rotation. The wide equatorial zone rotates with a period of about 18 hours, which is slower than the 16.1-hour rotation of the planet's magnetic field. By contrast, the reverse is true for the polar regions where the rotation period is 12 hours.

### Herculean Belt

*Main article: Titan Belt*
Neptune's orbit has a profound impact on the region directly beyond it, known as the Titan belt. The Herculean belt is a ring of small icy worlds, similar to the Argusian Belt but far larger, extending from Neptune's orbit at 30 AU out to about 48 AU from the Sol. Much in the same way that Jupiter's gravity dominates the Argusian belt, shaping its structure, so Neptune's gravity dominates the Herculean belt. Over the age of the Sol system, certain regions of the belt became destabilized by Neptune's gravity, creating gaps in the belt's structure. The region between 40 and 42 AU is an example. Neptune has also scattered several belts originally in this belt, such as Caelus.

## Moons

Neptune has known moons. By far the largest of them is Triton. Triton is unique among moons of planetary mass in that its orbit is retrograde to Neptune's rotation and inclined relative to Neptune's equator, which suggests that it did not form in orbit around Neptune but was instead gravitationally captured by it. The capture of Triton, probably occurring some time after Neptune formed a satellite system, was a catastrophic event for Neptune's original satellites, disrupting their orbits so that they collided to form a rubble disc. Triton is massive enough to have achieved hydrostatic equilibrium and to retain a thin atmosphere capable of forming clouds and hazes. Both Salacia and Triton had their first colonies established by 2123 CE.

### Salacia

Salacia (Proteus) is Neptune's second largest moon. Despite being a predominantly icy body more than 400 km in diameter, Salacia's shape deviates significantly from an ellipsoid. It is shaped more like an irregular polyhedron. Its surface is dark, neutral in color and heavily cratered. Salacia is probably not an original body that formed with Neptune; it probably accreted later from the debris created when the largest Neptunian satellite Triton was captured. Salacia is named after Neptune's wife and queen of the waters. Triton is her son by Neptune. Salacia has a population of 638,208. It serves as the outermost military base of the humans in the Sol system.

### Triton

*Main article: Triton*
Triton is the largest natural satellite of the planet Neptune. It is the only large moon in the Sol system with a retrograde orbit, an orbit in the opposite direction to its planet's rotation. Because of its retrograde orbit and composition similar to Saturn's, Triton is thought to have been a dwarf planet captured from the Titan belt. Triton has a surface of mostly frozen nitrogen, a mostly water-ice crust, and icy mantle and a substantial core of rock and metal. The core makes up two-thirds of its total mass. Triton has a mean density of 2.061 grams per cubic centimetre and is composed of approximately 15–35% water ice.

Triton is one of the few moons in the Sol System known to be geologically active. As a consequence, its surface is relatively young, with a complex geological history revealed in intricate cryovolcanic and tectonic terrains. Part of its crust is dotted with geysers which erupt nitrogen. Triton has a tenuous nitrogen atmosphere less than 1/70,000 the pressure of Earth's atmosphere at sea level.