Statistics: 

• Planet Diameter:  49,528 km, 3.9 Earth Diameters
• Planetary Mass:  1.03x10²⁶ kg, 17.2 Earth masses
• Average Distance from Sun:  4.497 billion km, 30.06 AU
• Length of Day:  ~16 hours
• Length of Year:  164.8 Earth Years
• Atmosphere:  Gas Giant; mainly hydrogen & helium
• Day/Night Surface Temperature:  59.3 degrees Kelvin

 History of Exploration
 Neptune's Atmosphere
 Rings & Moons of Neptune

The discovery of Neptune was a great tribute to the work of Isaac Newton and the laws of gravity.  Many astronomers had noted deviations and irregularities in the orbit of Uranus, and it was concluded that the only thing that could account for these perturbations in the orbit of Uranus was an eighth massive world resting beyond Uranus.  In 1845, an English astronomer named John C. Adams predicted the positions for this new planet; however, for reasons explored in the History section, Adams' findings were not made public for quite some time.  Interestingly enough, there is also evidence that Neptune was observed as far back as 1612 by Galileo, but he never made any conclusions as to the planet's true nature as he thought it was another star.

Observations of Neptune from the Earth are very difficult.  This is because Neptune is so far away from us that its angular size (that is, the amount of area it appears to occupy in the sky) is so small.  Therefore, astronomers have performed most Earth-based studies of Neptune by examining Neptune as it occults another object.  Important data such as Neptune's diameter, chemical composition, and temperature can be obtained by this method.

In 1989, our knowledge of Neptune expanded when the Voyager 2 space probe rendezvoused with the gas giant world.  Among some of the new discoveries included more accurate measurements of Neptune's rotational period, the fact that Neptune contains an active internal heat source, and confirmation of a ring system around the planet.

Atmosphere of Neptune

In many ways the atmosphere of Neptune is very much like that of its near-sister planet Uranus.  The overall density of Neptune has been determined to be about 1.6 grams per cubic centimeter, which means that Neptune should have a higher proportion of heavy elements than Uranus.  Like Uranus, Neptune has a large amount of hydrogen in its atmosphere, followed by helium and methane.  It is the methane (CH₄) that gives Neptune its bluish coloring; this is because methane absorbs the redder wavelengths of light particularly well, which results in the reflection of a great deal of the bluer light.

Observations from the Voyager 2 probe revealed an atmosphere that was much more active than that of Uranus.  A good piece of evidence for this is the discovery of a huge storm on the surface of Neptune.  Neptune's Great Dark Spot is about the same size as Jupiter's Great Red Spot.  Another interesting feature of this gargantuan cyclone is that it is "chased" by smaller cloud formations called "Scooters".  It should also be noted that Neptune, at times, has more than one Dark Spot.  Lastly, subsequent observations by the Hubble Space Telescope have revealed that the Great Dark Spot is not entirely stable, as it could not be found in these studies.
 

Observations from Voyager 2 yielded that the higher level clouds of Neptune are made primarily of methane ice.  One last interesting feature of Neptune is that its poles seem to be at the same temperature as the equatorial region of the gas giant; this seems strange when it is the equatorial region of the planet that receives the highest intensity of solar radiation.  Stranger still, the mid-latitude regions of Neptune (those between the poles and equator) are much cooler than the poles and equator.

Rings & Moons of Neptune

Rings -- Though ground-based observations of Neptune on Earth revealed the possible existence of a ring system, it was not until Voyager 2's visit in 1989 that they were directly observed.  Voyager 2 found three distinct rings orbiting Neptune, each of which was very circular in shape; however, some of the ring structure seems odd.  That is, two of the rings show an off-center type of clumping that would appear to make the rings unstable.  Further studies revealed that there exists a great deal of dust in these rings.
 

Moons -- There are many moons in the Neptunian system, but we will concentrate on Neptune's largest moon, Triton, for the purposes of this home page.  Interestingly enough, it was discovered that Triton has a very thin, but detectable, atmosphere comprised mainly of nitrogen gas.  The average density of this moon was calculated to be about 2.07 grams per cubic centimeter, which means that it is roughly 70% rock and 30% water ice.  Triton's southern polar cap is covered with a seasonal ice of frozen nitrogen; in addition, there has been some reddish material observed within this polar ice.  The reddish color indicates to planetary scientists that there exists organic material in these ices.

Other features of Triton include many craters and faults.  There is also evidence for volcanic activity on the surface of Triton.  In fact, in the image of Triton below, one can observe the plumes of some ice volcanos that are venting out a dark material that is being carried away by the thin atmosphere.
 

The orbit of Triton around Neptune is retrograde in nature.  This fact has led many scientists to think that Triton originally formed somewhere else in the solar system and was later captured by Neptune's gravity.  Below is a final parting view of Neptune and Triton from Voyager 2 as it leaves our solar system.
 

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