THE SURFACE OF NEPTUNE
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eptune is about five times too faint to be seen by the naked eye. The best pictures of Neptune from Earth show discrete bright clouds and a bright haze over the south pole of the planet. The U.S. Voyager 2 spacecraft confirmed these sightings when it reached Neptune in 1989. It was flying less than 5,000 km (3,100 miles) above the planet's cloud tops. Its cameras revealed many features, including a large, dark storm system in the southern hemisphere. This feature, named the Great Dark Spot, had disappeared by 1994. But a new similar Great Dark Spot now exists in the northern hemisphere.
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he orbit of Neptune around the Sun is even more nearly circular than the Earth's orbit. The planet's average distance from the Sun is 4,497,000,000 km (2,794,000,000 miles). It takes 164.793 Earth years to make one trip around the Sun.
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he rotation period of Neptune's magnetic field, which is presumed to trace the rotation of the planet's core, was found by Voyager 2 to be 16.11 hours. The jet-stream wind speeds on Neptune reach 2,400 km (1,500 miles) per hour, moving in the opposite direction that the planet rotates. These are the strongest retrograde winds seen on any planet in the solar system.
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eptune has a diameter of 49,500 km (30,750 miles) and a mass 17.22 times that of the Earth. This means that the planet is slightly smaller and heavier than Uranus. It has an average density of 1.67/cm3.
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he atmosphere of Neptune consists mainly of hydrogen and helium, but about 2.5% -3% of the atmosphere is methane CH4. The cirrus clouds seen in the atmosphere probably consist of crystals of methane rather than of water ice. The presence of methane is what gives Neptune its deep blue color. Data collected by Voyager 2 suggests that temperatures on Neptune rise with increasing depth. Scientists had expected the temperature of Neptune to be about -228 degrees C (-378 degrees F), but infrared measurements made by Voyager 2 indicated a higher temperature. Neptune, like Jupiter and Saturn appears to have an internal energy source.
