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.
Neptune is like Uranus in many ways, but has its own unique features.
Orbit:4,504,000,000 km (30.06 AU) from Sun
Diameter:49,532 km (30,760 miles)(equatorial)
Mass:1.0247e26 kg ((17.2 x Earth's)
Minimum Distance from Sun: 4.46 billion km
(2.77 billion miles)
Maximum Distance from Sun:4.54 billion km
(2.82 billion miles)
Minimum Distance from Earth:4.3 billion km
(2.68 billion miles)
Rotation Period about Axis:17.24 hrs
Revolution Period about the Sun:165 years
Tilt of Axis:29 Degrees 36"
Surface Gravity:11 m/s^2 (1.12 x Earth's)
Temperature at Cloud Tops:-210o C ( -346o F)
Average Cloud Top Temperature (K): 63K
Name in Roman/Greek Mythology:Neptune/Poseidon
Satellites/Rings:8 known moons, faint rings
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 fortunately, recent ground-based and HST observations have added a great deal, too.
Neptune is currently the most distant planet from the Sun ( Because of Pluto's highly elliptical orbit), with an orbital radius of 30 Astronomical Units and an orbital period of 165 years. Its diameter is about four times that of the Earth. It is slightly smaller than Uranus, but its density is 1.6 g/cc (compared with 1.2 g/cc for Uranus), which makes it the 3rd most massive planet. The relatively low density indicates large concentrations of hydrogen and helium, but Uranus and Neptune both have much larger concentrations of heavier elements than Jupiter and Saturn. As for all the gas giant planets, models suggest rocky cores of maybe 15 Solar masses, but there is no direct confirmation of this.
The image above, as for Uranus, because of methane in the atmosphere, which absorbs red light, leaving the light scattered from Neptune preferentially enhanced at blue wavelengths,but there is some additional as-yet-unidentified chromophore which gives the clouds their rich blue tint.The period of rotation is about 16 hours, comparable to that of Uranus and much slower than for Jupiter and Saturn. The temperatures at the cloud tops are about -216 degrees Celsius, slightly warmer than for Uranus. Neptune, like Jupiter and Saturn but unlike Uranus, has an internal heat source and produces 2.7 times more heat than it absorbs.
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, alas, cloudy skies prevented obsevations on those few critical days.
The Surface And Interior:-
The Voyager 2 flyby in 1989 revealed strong winds (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.), bright, high-altitude clouds, and two large dark spots attributed to long-lived giant storm systems. Tracking of these features indicated wind speeds as large as 730 miles per hour. The largest dark storm systems, called the "Great Dark Spot".
Neptune's composition is probably similar to Uranus': various "ices" and rock with about 15% hydrogen and a little helium. Like Uranus, but unlike Jupiter and Saturn, it may not have a distinct internal layering but rather to be more or less uniform in composition.
But there is most likely a small core (about the mass of the Earth) of rocky material. Its atmosphere is mostly hydrogen and helium with a small amount of methane.
Like Jupiter and Saturn, Neptune has an internal heat source,it radiates more than twice as much energy as it receives from the Sun.
The Magnetic Field
Voyager 2's instruments detected a complex magnetic field. Like Uranus, the field is tipped with respect to the axis of rotation and offset from the center (the tilt is 50 degrees for Neptune, compared with 60 degrees for Uranus). However, the field is somewhat weaker than for Uranus.
Neptune's magnetic field is, like Uranus', oddly oriented and probably generated by motions of conductive material (probably water) in its middle layers.
As for Uranus, it is speculated that this magnetic field my originate in a conducting shell not far below the clouds, rather than deep in the interior as for Jupiter or the Earth. In that case, the conducting material would not be metallic hydrogen, as for Jupiter, or iron and nickel, as for the Earth. As noted earlier for Uranus, a mixture of water, methane, and ammonia under the right pressure could be responsible.
Neptune's rings were first detected in star occultation experiments from Earth in 1983, but they were very difficult to study before the data from Voyager 2.
Voyager 2 found additional fainter rings. The rings rotate in the same direction as the planet and are nearly in the equatorial plane.
The material in one of the rings is clumpy (more dense in some regions than others). This gave the impression in ground-based observations that the rings were arcs instead of complete rings, but the Voyager 2 data showed that they were complete rings. The rings of Neptune contain much more dust-size grains than the corresponding rings of Saturn and Uranus, and are quite narrow (about as narrow as the Saturn F-ring).
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.
Neptune has 8 known moons; 7 small ones and Triton.
(distance is from Neptune's center to the ring's inner edge)
1-Neptune's magnetic field is off center and at a large angle to its rotation axis. What processes in the interior generate this oddly shaped field?
2-What accounts for the relative lack of hydrogen and helium in Neptune (and Uranus)?
3-Why are Neptune's winds so strong in spite of the fact that it is so far from the Sun and has a relatively weak internal heat source?
4-What happened to the Great Dark Spot?
5-Can we design a useful Neptune orbiter mission cheap enough to be funded?
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Last Modified : 5 Sep. 2001
Created By#C0115361 Team