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Nearly 4.5 billion kilometers (3 billion miles) from the Sun, Neptune orbits the Sun once in 165 years, and therefore has made not quite a full circle around the Sun since it was discovered. With an equatorial diameter of 49,528 kilometers (30,775 miles), Neptune is the smallest of our solar system's gas giants. Even so, its volume could hold nearly 60 Earths. Neptune is the densest of the four giant planets, about 64 percent heavier than if it were composed entirely of water.

The most obvious feature of the planet in Voyager pictures is its blue color, the result of methane in the atmosphere. Methane preferentially absorbs the longer wavelengths of sunlight (those near the red end of the spectrum). What are left to be reflected are colors at the blue end of the spectrum.

While methane is not the only constituent in Neptune's atmosphere, it is one of the most important. Methane cycles through the atmosphere like this:

  • Solar ultraviolet radiation destroys methane high in Neptune's atmosphere by converting it to hydrocarbons such as ethane, acetylene and haze particles of more complex polymers.
  • The haze particles sink to the cold lower stratosphere, where they freeze and become ice particles.
  • The hydrocarbon ice particles gently fall into the warmer troposphere, where they evaporate back into gases.
  • The hydrocarbon gases mix deeper into the atmosphere where the temperature and pressure are higher, mix with hydrogen gas and regenerate methane.
  • Buoyant, convective methane clouds then rise great distances to the base of the stratosphere or higher, returning methane vapor to the stratosphere.

    Throughout the process there is no net loss of methane in Neptune's atmosphere.

Neptune is a dynamic planet, even though it receives only 3 percent as much sunlight as Jupiter does. Several large, dark spots are reminiscent of Jupiter's hurricane-like storms. The largest spot is big enough for Earth to fit neatly inside it. Designated the "Great Dark Spot" by its discoverers, the feature appears to be an anticyclone similar to Jupiter's Great Red Spot. Neptune's Great Dark Spot is comparable in size, relative to the planet, and at the same latitude (the Great Dark Spot is at 22 degrees south latitude) as Jupiter's Great Red Spot. However, Neptune's Great Dark Spot is far more variable in size and shape than its Jupiter counterpart. Bright, wispy "cirrus-type" clouds overlaying the Great Dark Spot at its southern and northeastern boundaries may be analogous to lenticular clouds that form over mountains on Earth.

At about 42 degrees south, a bright, irregularly shaped, eastward-moving cloud circles much faster than does the Great Dark Spot, "scooting" around Neptune in about 16 hours. This "scooter" may be a cloud plume rising between cloud decks.

Another spot, designated "D2" by Voyager's scientists, is located far to the south of the Great Dark Spot, at 55 degrees south. It is almond-shaped, with a bright central core, and moves eastward around the planet in about 16 hours.

Voyager also measured heat radiated by Neptune's atmosphere. The atmosphere above the clouds is hotter near the equator, cooler in the mid-latitudes and warm again at the south pole. Temperatures in the stratosphere were measured to be 750 kelvins (900 degrees F), while at the 100 millibar pressure level, they were measured to be 55 K (-360 degrees F). Heat appears to be caused, at least in part, by convection in theatmosphere that results in compressional heating: Gases rise in the mid-latitudes where they cool, then drift toward the equator and the pole, where they sink and are warmed.

Long, bright clouds, reminiscent of cirrus clouds on Earth, can be seen high in Neptune's atmosphere. They appear to form above most of the methane, and consequently are not blue.

At northern low latitudes (27 degrees north), Voyager captured images of cloud streaks casting their shadows on cloud decks estimated to be about 50 to 100 kilometers (30 to 60 miles) below. The widths of these cloud streaks range from 50 to 200 kilometers (30 to 125 miles), and the widths of the shadows range from 30 to 50 kilometers (20 to 30 miles). Cloud streaks were also seen in the southern polar regions (71 degrees south), where the cloud heights were about 50 kilometers (30 miles).

Most of the winds on Neptune blow in a westward direction, which is retrograde, or opposite to the rotation of the planet. Near the Great Dark Spot, there are retrograde winds blowing up to 1500 miles an hour -- the strongest winds measured on any planet, including windy Saturn.

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