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Neptune is the outermost gas giant, and it is believed to have a similar structure and composition
to Uranus.
RIGHT: Neptune as seen in 1989 by Voyager 2, the only spacecraft to have visited it so far. |
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However it looks more like a blue version of Jupiter, with a variety of subtle light and dark bands in the clouds, and winds blowing at up to 2000km per hour. There was even a feature similar to Jupiter's Great Red Spot, which was immediately named the Great Dark Spot. About 50km above the main cloud deck there are bright clouds of methane ice, which appear as white streaks next to the Great Dark Spot in the picture above. The Spot could not be seen in photographs taken by the Hubble Space Telescope in 1994, although a new dark spot was seen a few months later, indicating that the atmosphere can change rapidly.
Neptune has faint rings, which like those of Uranus consist of dark-coloured material. Their composition is unknown, but they also appear to be capable of rapid change as more recent observations show that the outermost ring has faded significantly.
Neptune is the first planet to have been discovered by a deliberate search. After Uranus had been discovered, its orbit was soon calculated. However, it was found to be deviating slightly from its predicted position in space, and it was suggested by a number of astronomers that this could be due to the gravitational influence of an unknown planet further out.
The probable position of this new planet was first calculated by John Couch Adams, a mathematician at St. John's College, Cambridge, in 1845. He passed his conclusions to the Astronomer Royal, George Airy, who eventually asked a Cambridge professor, James Challis, to search for the planet. Challis did little, and found nothing. In the meantime, however, the French mathematician Urbain Le Verrier had made similar calculations, and in 1846 astronomers at the Berlin observatory found Neptune very close to where he had predicted it ought to be. Subsequently, it became clear that Challis had actually seen the new planet during his search, but had failed to realise what he was looking at.
 Triton, as seen by Voyager 2 |
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 The Great Dark Spot |
Triton, the largest moon of Neptune, is covered with frozen nitrogen and methane. The surface is very flat, with a pinkish polar snowcap, and blue-tinged ice around the equator. Dark streaks on the surface are believed to be caused by geysers of nitrogen ice and gas erupting through the ground and then being scattered by winds in the tenuous atmosphere. One Voyager photo shows a plume rising 8km high from one of these eruptions.
Neptune is too faint to be seen with the naked eye, and a fairly powerful telescope is needed to show it as anything other than a star-like point. Even then, you'd need something like the Hubble to show any detail.
URL: http://www.randomnotes.co.uk/Astronomy/Neptuneprint.htm