- Mass: 1.02 x 10^26 kg
- Diameter: 49528 km
- Mean density: 1640 kg/m^3
- Escape velocity: 23300 m/sec
- Average distance from Sun: 30.06 AU
- Rotation period: 19.1 hours
- Revolution period: 164.8 years
- Obliquity: 29.6°
- Orbit inclination: 1.77°
- Orbit eccentricity: 0.009°
- Mean temperature: 48 K°
- Visual geometric albedo: 0.51
- Atmospheric components: .74% hydrogen, 25% helium, 1% methane (at depth)
- Rings: Rings are narrow, and contain concentrations of particles called ring arcs.
Neptune's composition is probably similar to Uranus'. Neptune's atmosphere may not have a distinct internal layering but rather to be more or less uniform in composition. Neptune most likely has a small core of rocky material about the mass of the Earth. Its atmosphere is mostly hydrogen and helium with a small amount of methane. Neptune's blue color is largely the result of absorption of red light by methane in the atmosphere but there is some additional as-yet-unidentified chromophore which gives the clouds their rich blue tint.
Like a typical gas planet, Neptune has rapid winds confined to bands of latitude and large storms or vortices. Neptune's winds are the fastest in the solar system, reaching 2000 km/hour. Neptune radiates more than twice as much energy as it receives from the Sun. Neptune's magnetic field is, like Uranus', oddly oriented and probably generated by motions of conductive material (probably water) in its middle layers.
Neptune has 8 known moons. They are named: Naiad, Thalassa, Despina, Galatea, Larissa, Proteus, Triton, and Nereid
.
The irregular shape of Proteus suggests that it has been cold and rigid throughout its history and subject to significant impact cratering. The satellite has an average radius of about 200 km (120 mi) and is uniformly dark with an albedo of about 6%. This image was taken from a range of 870,000 km (540,000 mi).
With an average radius of only 200 km or120 miles, this tiny half-illuminated satellite appears spectrally gray with an albedo of 6%. Hints of crater-like forms and groove-like lineations can be seen. The graininess of the image is caused by the short exposure necessary to avoid significant smear. This image was obtained August 25, 1989 from a range of 146,000 km or 91,000 miles.
The pink hue of Neptune's largest moon, Triton, is thought to result from a slowly evaporating layer of nitrogen ice. Triton is an oddity among moons in that its orbit is highly tilted to the plane of Neptune's equator, and it is in a retrograde orbit. These facts have led scientists to believe that Triton formed independently of Neptune and was later captured by Neptune's gravity.
This cantaloupe-textured terrain encompasses a region roughly 1000 km or 620 miles across. Complex tectonic and volcanic forces involving icy viscous fluids combined to develop the deformed pattern of this landscape. This view was captured in late August 1989.
Triton's surface is covered with nitrogen and methane ice. Its surface temperature is a mere 38 degrees Celsius above absolute zero. Dark streaks across the south polar cap may be the result of recent geyser-like eruptions of gas, dust, and ice venting from beneath the cap into the satellite's near-vacuum atmosphere. The diameter of Triton is 2700 km or 1674 miles.
Lake-like features along the terminator record a time when these regions of Triton's surface were fluid. This 200x200-km or 124x124-mile view was acquired during Voyager 2's closest approach to Triton.
The rings around Neptune are so faint that when Voyager's cameras captured this image, the brightness of Neptune nearly made them impossible to detect. Earth-based observations showed only faint arcs instead of complete rings, but Voyager 2's images showed them to be complete rings with bright clumps. Like Uranus and Jupiter, Neptune's rings are very dark but their composition is unknown. Neptune's rings have been given names: the outermost is Adams (which contains three prominent arcs now named Liberty, Equality and Fraternity), next is an unnamed ring co-orbital with Galatea, then Leverrier whose outer extensions are called Lassell and Arago, and finally the faint but broad Galle.
A portion of the Adams ring appears to be twisted. Scientists believe it looks this way because the original material in the rings was in clumps that formed streaks as the material orbited Neptune.
Neptune is the eighth planet from the Sun and the fourth largest by diameter. Neptune is smaller in diameter but larger in mass than Uranus. In Roman mythology Neptune was the god of the Sea. After the discovery of Uranus, it was noticed that its orbit was not as it should be in accordance with Newton's laws. It was therefore predicted that another more distant planet must be perturbing Uranus' orbit. Neptune was first observed by Galle and d'Arrest on 1846 Sept 23 very near to the locations independently predicted by Adams and Le Verrier from calculations based on the observed positions of Jupiter, Saturn and Uranus.
An international dispute arose between the English and French (though not, apparently between Adams and Le Verrier personally) over priority and the right to name the new planet; they are now jointly credited with Neptune's discovery. Subsequent observations have shown that the orbits calculated by Adams and Le Verrier diverge from Neptune's actual orbit fairly quickly. Had the search for the planet taken place a few years earlier or later it would not have been found anywhere near the predicted location.
More than two centuries earlier, in 1613, Galileo observed Neptune when it happened to be very near Jupiter, but he thought it was just a star. On two successive nights he actually noticed that it moved slightly with respect to another nearby star. But on the subsequent nights it was out of his field of view. Had he seen it on the previous few nights Neptune's motion would have been obvious to him. But cloudy conditions prevented obsevations on those few critical days.
On its approach to Neptune in August 1989, Voyager 2 captured this image of the fourth and outermost of the giant gas planets. This image shows two of the four oval cloud features tracked by the cameras. The large dark oval near the left edge revolves around Neptune every 18 hours. The bright clouds immediately to the south and east of this oval substantially change their appearance in periods as short as 4 hours. The second dark spot, at lower right edge, revolves around Neptune every 16 hours.
Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25 1989. Much of we know about Neptune comes from this single encounter. But ground-based and HST observations have added a great deal, too. Because Pluto's orbit is so eccentric, it sometimes crosses the orbit of Neptune making Neptune the most distant planet from the Sun for a few years.
The location of clouds in Neptune's atmosphere was used for testing the accuracy of Neptunian weather forecasts to aid in the selection of targets for the narrow-angle camera. Three of the four targeted features are visible here, and all three are close to their predicted locations. The Great Dark Spot with its bright white companion is slightly to the left of center. The small vivid Scooter is below and to the left, and Dark Spot 2 with its shiny core is below Scooter. Strong eastward winds, up to 644 kph (400 mph), caused the smaller dark spot to overtake and pass the larger one every five days.
The three features visible here are among the most interesting on Neptune. At the top is the Great Dark Spot, accompanied by bright, white clouds that change rapidly with time. Below the dark spot is a bright feature that scientists nicknamed "Scooter." Below Scooter is the Small Dark Spot. All three features move eastward at different velocities, so it is rare that they can all be photographed together.
The bright cirrus-like clouds of Neptune change rapidly, often forming and
dissipating over several hours. In this sequence spanning two rotations of
Neptune (~36 hours), Voyager 2 observed cloud evolution in the region around
the Great Dark Spot. The surprisingly rapid changes, which occurred over the 18
hours separating each panel, show that in this region Neptune's weather is
perhaps as dynamic and variable as that of the Earth's. However, the scale is
immense by our standards because the Earth and the Great Dark Spot are similar
in size.
This Voyager 2 high-resolution color image, taken 2 hours before closest
approach, provides obvious evidence of vertical relief in Neptune's bright
cloud streaks. These clouds were observed at 29 degrees North latitude near
Neptune's east terminator. The linear clouds are stretched approximately along
lines of constant latitude and the sun is toward the upper left. The sides of
the clouds facing the sun are brighter than the surrounding cloud deck because
they are more directly exposed to the sun. Shadows are visible on the side
opposite the sun.