NEPTUNE

Image of Neptune taken by Voyager 2 in 1989. The structures of the clouds and the Great Dark Spot can be seen. (Calvin J. Hamilton)

Neptune is the eight planet of the Solar System; it is named after the Roman god of the sea, the Greek Poseidon. slightly smaller than Uranus, it is part of the jovian planets.

It has been observed, for the first time, on September 23, 1846, by J.G. Galle and L. d'Arrest. Its position had been predicted both by the English astronomer Adams and by the French Le Verrier, who independently calculated it, by studying the orbit of Uranus. These astronomers had in fact noticed that Uranus did not rigorously follow the laws of celestial mechanics; the notable perturbations of its orbit caused by Jupiter and Saturn could not completely explain these anomalies. It had therefore been hypothesized the presence of another planet beyond the orbit of Uranus.
 

Image of the two hemispheres of Neptune in true colors, taken by the Hubble Space Telescope. One can see light cloud strips.  (HST)

Until now Neptune has been visited by just a single probe, Voyager 2. It got close to the planet in August 1989. Almost everything we know about Neptune is due to the Voyager. Recently the Hubble Space Telescope acquired interesting images of the planet.

Neptune has a diameter of 49,492 km at the Equator, and its mass is as large as 102,400 billion of billion tons (1,024 10²⁹ g), more than 17 times the mass of the Earth. The mean density is 1.64, the largest among the jovian planets.
Neptune orbits at a distance of 4,504 million km from the Sun, and it completes one revolution in 164.79 years. Its orbit is inclined by 1.77 degrees on the Ecliptic, and it has an eccentricity of 0.0097.
The planet rotates in 16 hours and 7 minutes about its axis, which is inclined by 28.31 degrees with respect to the orbital plane.
 
 

The atmosphere
 

Neptune is surrounded by a dense atmosphere, rich in clouds just like the ones of the other giant planets.
On Neptune too several Jupiter like spots can be seen, which are atmospheric whirlpools. The most important of these, revealed by Voyager in 1989, was called Great Dark Spot: it had the dimensions of the Earth and it was similar to the Great Red Spot of Jupiter. However, when observed by HST in 1994, it had disappeared.
 

The Great Dark Spot of Neptune, surrounded by light clouds.  The cloud system rotates counter clockwise. (NASA/JPL)

Evolution of the Great Dark Spot during two Neptune rotations (about 36 hours). The light clouds rapidly move. They are composed by frozen methane.  (NASA/JPL)

 

Above the Great Dark Spot a similar system is located, but of smaller dimensions: the Small Dark Spot. It is believed that it is, too, an atmospheric storm. (Calvin J. Hamilton)

 

True color image taken by Voyager 2, showing the feature of the clouds near the pole.  (NASA/JPL)

The clouds are driven by very strong winds, the strongest in the Solar System: near the Great Dark Spot they can reach 2,000 km/h. The atmosphere of the planet has a composition much similar to that of Uranus: hydrogen (85%), helium (13%) and methane (2%); this last is the responsible of the blue color of the planet, since it absorbs the red radiation and reflects  the blue one. Neptune's albedo is 0.41.

 
 

The structure
 

Neptune and Triton in a Voyager image.  (Calvin J. Hamilton)

The structure of Neptune is different from that of Jupiter and Saturn. It is probably similar to that of Uranus, that is the planet must be composed by several ices and rock, while hydrogen constitutes just 15% and helium is present in small amount. Just like Uranus, even Neptune does not seem to have an internal stratification, but, like the first one, almost certainly contains a small rocky nucleus.

The magnetic field of Neptune, like the one of Uranus, is very inclined with respect to the rotation axis, 47 degrees. Moreover, its center is displaced by at leas 13,500 km from the geometric center of the planet. This can be due to the motion of some conductive fluid inside the planet, at average depths.
 
 

The rings
 

Neptune too has a ring system: they are thin and faint like the ones of Jupiter and Uranus. If they are observed from the Earth, they manifest as faint arcs beside the planet and only the Voyager probe could reveal their true nature.
The rings are composed by small dust grains, maybe produced as the result of the disruption of small meteorites in the impact with the moons of Neptune.
 

The rings of Neptune imaged by Voyager in 1989, at the distance of 280 thousand km. The two main rings are visible, and, inside, a third fainter ring. (NASA/JPL)

A Neptune ring. (NASA/JPL)

 
 

ANIMATIONS

The Great Dark Spot, AVI, 1.1 Mb (NASA)

Rotation of Neptune, AVI, 2.8 Mb (NASA/JPL)

Neptune clouds. MPEG, 480 Kb (STScI)