The Planet Neptune:

A Mysterious Stormy Gas Ball


NASA Voyager 2 Image of the planet Neptune
Neptune's blue-green atmosphere was shown in greater detail than ever before as the Voyager 2 spacecraft approached its encounter with the giant gas planet. The color image above, snapped from a distance of about 10 million miles, revealed complex and puzzling atmospheric features.

The Great Dark Spot at the center is about 8,000 miles by 4,000 miles in size. That means the spot is as long as the entire Earth is wide.

Bright, wispy, cirrus-type clouds hovering near the Great Dark Spot are higher in altitude than the dark material, which defines its boundaries. A thin veil fills part of the spot's interior.

The bright cloud at the lower (southern) edge of the spot measures about 600 miles north to south. The small, bright cloud below the spot was referred to as the "scooter." It was rotating faster than the Great Dark Spot, gaining about 30 degrees eastward (toward the right) in longitude every rotation.

Bright streaks of cloud at the latitude of the Great Dark Spot, the small clouds overlying it, and a dimly visible dark protrusion at its western end are examples ofdynamic weather patterns on Neptune, which can change significantly during one rotation of the planet — about l8 hours.

[click image to enlarge]
The Sun, our star at the center of our Solar System, is surrounded by nine major bodies known as planets. The eighth planet out from the Sun is a giant ball of gas we call Neptune. It circles our star at a distance of about three billion miles. That makes it the most remote of the gas giant planets in the outer regions of the Solar System.

Human beings have explored Neptune by remote control. The planet was visited once by Voyager 2, an unmanned robot probe from Earth, which flew within 3,100 miles of Neptune in 1989.

That visit to Neptune and its moons came 143 years after the planet was first seen in Earth's night sky. John Couch Adams in England in 1845 and Urbain Jean Joseph Leverrier in France in 1846, unknown to each other, independently calculated where an eighth planet would have to be in order to explain slight variations in the orbit of the planet Uranus, seventh out from the Sun.

Adams' prediction was good, but was received skeptically in England and therefore was not published until after the planet had been discovered elsewhere.Leverrier told Johann Gottfried Galle and Heinrich Louis d'Arrest where to look and, on the night of Sept. 23, 1846, in Berlin, they found the new planet. It was within one degree of the place where Leverrier had predicted.

As you might imagine, English and French asronomers fussed for months over who should receive credit for the find and who should get to name the new planet. Leverrier wanted to name it after himself. Calmer heads prevailed eventually and the new planet was named Neptune, for the Roman sea god. Adams and Leverrier both received credit for their calculations.

It is possible that Galileo may have spotted Neptune more than two centuries earlier, but he did not recognize it as a planet.

What we see from Earth. Neptune reaches a maximum brightness in Earth's night sky of magnitude 7.8. Unfortunately, that's about five times too faint to be seen by the naked eye.

Through a large telescope, the planet looks like a small blue disk. It is 2.3 seconds of arc in diameter. Our very best pictures from Earth show discrete bright clouds and a bright haze over the south pole of the planet.

During its close-up look, the U.S. interplanetary spacecraft Voyager 2 confirmed the cloud and haze sightings on August 25, 1989, as it flew less than 3,100 miles above the planet's cloud tops.

Voyager's cameras revealed a plethora of interesting features. One was a large, dark storm in the southern hemisphere. Dubbed the Great Dark Spot, that southern hemisphere feature had disappeared by 1994 when the Hubble Space Telescope spotted a new Great Dark Spot in Neptune's northern hemisphere.

That suggests that Neptune's spots change from year to year. That's different from the planet Jupiter's so-called Great Red Spot, which remains fairly constant.

A NASA Image of the Scooter Storm on the Planet Neptune Voyager 2 also saw a smaller dark storm with a bright core of feathery clouds, a small bright cloud feature named Scooter, well-defined banding on the planet, and numerous wispy cirrus-like clouds. Some of the cirrus clouds cast shadows on deeper cloud decks below.

That observation was the first detection of vertical relief in the atmosphere of an outer planet. The cirrus clouds changed rapidly, often forming and dissipating within several hours. Such surprisingly quick changes suggest Neptune's the weather is as dynamic and variable as that of our own planet Earth.

What we know about Neptune. Neptune's orbit around the Sun is even more nearly circular than Earth's. Neptune's average distance from the Sun is 2,794,000,000 miles, with a small eccentricity. The orbit is inclined slightly from the plane of the solar system (the ecliptic). Neptune's axis of rotation is tipped only about 29 degrees, which is not much different from Earth's 23 degree tilt.

The planet takes 164.793 years to make one trip around the Sun.

A NASA Image of the Planet Neptune The rotation period of Neptune's magnetic field, which astronomers had guessed would follow the rotation of the planet's core, was found by Voyager 2 to be 16.11 hours. However, most of the clouds in Neptune's atmosphere have longer periods of rotation, ranging from 12 hours in the southern hemisphere to 18 hours near the equator.

That means the jet-stream wind speeds on Neptune reach 1,500 miles per hour and they move in a retrograde direction, opposite the direction of rotation. They are the strongest retrograde winds seen so far on any planet in the solar system.

The planet's dimensions. Neptune has a diameter of 30,750 miles. Its mass is 17.22 times the mass of Earth. So, the planet is slightly smaller and heavier than the planet Uranus. It is a little more dense that Uranus.

Neptune's atmosphere is mostly hydrogen and helium. However, about 2.5 to 3 percent of the atmosphere is methane. The cirrus clouds seen in the atmosphere may be crystals of methane, rather than water ice as in Earth's cirrus clouds.

Why does Neptune appear blue? Methane's strong absorption features dominate the spectrum of the planet, giving Neptune its deep blue color. Neptune's spectrum also shows molecular hydrogen and stratospheric ethane. Microwave observations of the spectrum suggest the presence of ammonia.

Those microwave observations indicate that temperatures on Neptune rise with increasing depth. That's like Uranus. Scientists had thought the temperature of Neptune would be about -378 degrees F. However, infrared measurements by Voyager 2 revealed a temperature of -360 degrees F. That suggestes that Neptune, like Jupiter and Saturn, but unlike Uranus, probably has an internal energy source.

A NASA Image of a tiny dark moon of the Planet Neptune Voyager 2 discovered Neptune's magnetic field is tilted 46 degrees from the planet's rotation axis. It is offset from the center of the planet. That tells us the magnetic field strength varies across the surface of Neptune.

The unexpected orientation of the field resembles the magnetic field of Uranus. Until the observation of Neptune's field, the orientation of Uranus' magnetic field had been thought to be linked to that planet's unusual orientation, with a rotation axis parallel to the plane of the ecliptic.

Now scientists think the magnetic fields are generated in a slushy ammonia-rich mantle layer, rather than in the planets' cores.

The planet was visited once by Voyager 2 in 1989. Should we go back for another look?


Should we go back for another look? »

Learn more:
NASA Solar System Exploration - Neptune »
Jupiter Icy Moons Orbiter (JIMO) proposed to fly in 2015 »
Neptune interplanetary probe proposed to fly in 2015 »

Read more about the Solar System . . .
Star: The Sun  
Inner Planets: Mercury Venus Earth Mars  
Outer Planets: Jupiter Saturn Uranus Neptune Pluto
Other Bodies: Moons Asteroids Comets  
Beyond: Pioneers Voyagers