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Neptune is the eighth planet from the Sun and orbits the Sun every 165 years at a mean distance of 30.1 times that of the Earth. It has a diameter of 48,000 km and a mass 17 times that of the Earth. It is the furthest of the giant gaseous planets from the Sun and has a rotation period of about 19 hours. The structure of the planet is a rocky core surrounded by a jacket of ice which is, in turn, surrounded by an 8000 km-deep atmosphere. This atmosphere is composed mainly of molecular hydrogen with clouds of methane. The temperature of what is seen as the disk is -220°C.
The discovery of Neptune
The story of the discovery of Neptune is an intriguing one which is as much a story about people and their characters as it is about science.
During the 19th century observations of the positions of the planet Uranus were seen to be in disagreement with the predicted ephemeris. Two mathematicians, a Frenchman Urbain Leverrier and an Englishman John Couch Adams, analysed these small departures from the predicted positions assuming that they were due to the gravitational pull of another, unknown planet. Adams and Leverrier worked independently and both predicted the presence of a new planet in essentially the same place in the sky.
Leverrier had the good fortune to communicate his predictions to Galle at Berlin who searched and discovered Neptune in 1846. Adams had attempted to interest the Astronomer Royal, Airy, in his calculations but, due to a clash of personalities, Airy did not consider Adams's work important. He suggested that Adams should ask Challis, at Cambridge, to undertake a search. Challis used the Northumberland telescope, which is still in Cambridge, to search for the new planet. In fact Challis did observe Neptune but, as he was engaged in a systematic search of a large area of sky and looking for changes in the position of one of the objects he had charted, he missed the fact that one of the brightest objects in the search field showed a small disc and was indeed Neptune.
Initially Leverrier was credited with the prediction and it was only some years later that Adams was given joint credit for the first predicted discovery of a new planet in the solar system.
Voyager 2 at Neptune
From the Earth Neptune can be seen only as a small greenish disc. Almost all of our detailed knowledge of Neptune comes from the close encounter by the Voyager 2 spacecraft in 1989. Voyager 2 had visited Jupiter, Saturn and Uranus; Neptune was its last calling post. It had taken 12 years to get there and yet passed within 3000 miles of the planet's surface. The signals received on the Earth had a strength of less than 0.0000000000000001 watts and yet the pictures showed fantastic details.
Neptune's atmosphere was seen to have a banded structure similar to that found on the other gas giant planets. A giant hurricane (the Great Dark Spot, see right), with a diameter equal to the Earth's, was seen together with other cloud formations including some extensive, and very beautiful cirrus clouds high (50 km) above the main clouds. Neptune was shown to have a very active cloud system, possibly even more active than that of Jupiter with wind speeds of up to 2000 km/h.
In 1994, observations made by Hubble Space Telescope showed that the Great Dark Spot had disappeared and that a new spot had appeared in the northern hemisphere.
Observations from the Earth had shown that Neptune possessed some ring arcs; the Voyager pictures showed that there are at least 4 rings and many dust particles were detected in the plane of the rings. The supposed arcs were found to be brighter clumps in each complete ring.
Neptune has a magnetic field which is not aligned with the rotation axis. It is believed that this field is generated in a spherical shell near the surface of the planet. Aurorae were seen associated with this magnetic field.
Before the Voyager encounter Neptune was known to have two satellites, Triton and Nereid with diameters of 3800 and 300 km. Voyager found six more with diameters ranging from 50 to 200 km. All of these small bodies orbit Neptune close to its equatorial plane and in the same direction as the planet's rotation. Nereid and Triton, however, both have orbits that are inclined to the equatorial plane by 30 and 20 degrees. Triton also has the unique property that its direction is retrograde (it is the only large satellite in the solar system to travel around its planet in the reverse direction from the planet's rotation). This suggests that these two satellites did not condense at the same time as Neptune but were captured at some later time.
Triton is only a little smaller than our Moon and has the coldest surface in the solar system at just 38K or -235°C. The pictures of Triton returned by Voyager were probably the most exciting of the whole 12 year journey. They show vast canyons, craters and peaks with frozen pools of ice and ammonia and long fissures that look like trans-continental highways. Most of the nitrogen in the atmosphere is condensed into ice which covers the surface. Pinkish areas at the poles are believed to be methane ice whilst the origin of the greenish 'cantaloupe terrain' has yet to be explained.
The most surprising discovery was of volcanoes. These are quite different from the volcanoes on the Earth. Instead of red-hot magma being ejected it is nitrogen gas, evaporated from its liquid state that is being vented and is carrying with it darker carbon compounds from below the satellite's surface.
The picture above shows a variety of these features, although in the reproduction they may be hard to see. The road-like fissures can easily be seen. Two volcanoes show up as dark plumes of carbonaceous material.