Neptune

* 0.990
* 0.980

12 other dwarf moons

## Interior structure

Neptune is the outer of Sol's two ice giants - gaseous worlds composed primarily of volatile materials heavier than hydrogen and helium, such as methane, water, and ammonia, which are known in planetary science as 'ices'. It has no solid surface, instead having a steadily thickening atmosphere composed of hydrogen, helium and methane which gives way to a thick, liquid, ice-rich mantle as depth increases, ending eventually in a solid rocky core. The planet has some of the fastest winds in the Solar system, with top speeds of over 2100 km/h occurring in its dynamic and impressive storm systems.

## Atmosphere

Neptune's atmosphere consists mainly of hydrogen and helium. A small percentage (about 1.5%) of methane is also present and gives the planet its peculiar blue colour, by absorbing the red end of the spectrum of visible light. Compared with Uranus, which has a similar amount of methane in its atmosphere, the blue colour of Neptune's atmosphere is much more vivid. This might be caused by an additional component which is currently unknown. The distribution of gases in Neptune's atmosphere is not homogeneous, as in the equatorial region the amount of methane, ethane, and ethyne is ten to a hundred times higher.

Drawing the line between the "surface" and "atmosphere" of Neptune is not an easy task because there are no clearly definable boundaries. The definition of the limit between surface and atmosphere is therefore purely arbitrary: the atmosphere begins above the altitude where the pressure is equal to the pressure found at sea level on Earth.

The atmosphere is divided into four different layers:

* The lowest layer is called troposphere. In this region temperature decreases as altitude increases. This temperature gradient is caused by the heat emanating from the planet's core, which has a greater influence than the light coming from the distant Sun.
* Above the troposphere lies the stratosphere, where temperature increases with altitude because sunlight is now more influential than the planet's inner heat.
* The next layer is the thermosphere, which is characterized by lower pressures and temperatures of 750 K. These temperatures are so high that they can't be explained by sun's radiation alone. One possible explanation for this phenomenon could be the atmosphere’s interaction with ion’s in the planet’s magnetic field.
* The outer layer is the exosphere, which is only slightly cooler than the thermosphere (about 600K) and is heated by the same anomalous process as the thermosphere.

Neptune's atmosphere is subject to differential rotation, which means that the equatorial zone rotates with slower speed - its period being 18 hours - while the polar zones rotate much faster and have a period of only 12 hours. This difference in velocity, paired with the contrast between Neptune's unusually high inner temperature and the coldness of space, is responsible for storms and wind shears of spectacular intensity. These winds can rage across the atmosphere with speeds of 2100 km/h. Even the storms of Jupiter pale in comparison with these phenomena. The huge anticyclonic storm named "Great Dark Spot" discovered in 1989 proved to be short-lived, as it had already waned when the Hubble Space Telescope was used to observe Neptune. On this occasion another very similar storm, called "Scooter", was discovered on the northern hemisphere. This suggests that storms on Neptune have shorter lifespans compared to those found on Jupiter.

## Axial tilt

Neptune has a moderate axial tilt of 28.32°. Because of its long orbital period, the planet's seasons last for 40 years. When one pole of the planet is exposed to sunlight it warms up enough to allow methane to sublimate and leak into space. The Neptunian magnetic field is dramatically offset from the planet's center of mass and not aligned with its rotational axis.

## Moons and rings

Neptune has a system of 13 known moons, the largest of which by far is Triton. Triton has a retrograde orbit around Neptune and is thought to be a former Kuiper Belt Object like Pluto that was captured by Neptune when it passed too close to the planet. Neptune also has a system of rings, though they are the smallest of any major planet, and rich in dust like the rings of Jupiter. The rings are extremely dark, reflecting less than 2% of the radiation that strikes them. The large outer ring is not smooth and continuous, but rather has several short clumps of material known as 'arcs' along one section. In about 3.6 billion years, Triton's orbit will have decayed inside the Roche limit and the moon will be torn apart into a colossal ring system perhaps more spectacular than Saturn's.

## Discovery of Neptune

Neptune is too dim to be observed with the naked eye. The invention of the telescope made the observation of this celestial body possible, but it was not identified as a planet until 1846. In fact Galileo Galilei himself had already seen Neptune, but he assumed that it was a blue fixed star. After the discovery of Uranus by William Herschel in 1781 scientists monitored the orbit of the newly discovered planet for several decades and they noticed irregularities that seemed to contradict the Newtonian Law of Gravitation. The astronomer Urbain Le Verrier realized that these perturbations could have been caused by another planet orbiting even further beyond than Uranus and eventually published a theoretical prediction of the planet's position. The British astronomer and mathematician John Adams reached the same conclusion independently, which sparked a search for the new planet. After a British team of astronomers had made several attempts to observe this mysterious planet it was the German astronomer Johann Galle who - from the Berlin Observatory - observed Neptune for the first time knowing that it was a planet. Le Verrier had sent him his papers when he realized that no astronomer in the French scientific community was interested in the project. This marked a new development that deeply influenced the way astronomy was conceived and practiced. A planet had been discovered "on paper", which was a huge achievement for the theoretical branch of astronomy, and Newton's gravitational theory had been confirmed beyond a reasonable doubt.

* Johann Gottfried Galle
* Urbain Le Verrier

## Sources

1. ↑ The sections "Interior structure", "Axial tilt" and "Moons and rings" have been adapted from the Space Engine ingame wiki.
2. ↑ http://www.space.com/18922-neptune-atmosphere.html
3. ↑ http://solarsystem.nasa.gov/planets/neptune/indepth
4. ↑ https://www.researchgate.net/publication/234243535\_Heating\_of\_Neptune's\_Thermosphere\_by\_the\_Magnetospheric\_Convection\_Electric\_Field
5. ↑ http://www.universetoday.com/22070/surface-of-neptune/
6. ↑ https://en.wikipedia.org/wiki/Discovery\_of\_Neptune

## Images

Close-up of NeptuneView of Neptune with its second-innermost dwarf moon Thalassa in the distanceThe dwarf moon DespinaNeptune's crescent and its innermost dwarf moon NaiadThe polar region of Neptune with visible auroraAurora seen from the "surface" of Neptune's outermost cloud layerNeptune's translucent rings and its dwarf moon DespinaNeptune seen from the orbit of its largest moon TritonNeptune seen as a tiny dot in the distance from its outermost dwarf moon NesoNeptune seen from the surface of its dwarf moon Thalassa