Neptune

The Mystic

Neptune Facts

• Neptune is the eighth planet from the Sun and the fourth largest (by diameter):
  • distance from Sun: 4,504,000,000 km (30.06 AU)
  • equatorial diameter: 49,528 km; polar diameter: 48600 km
  • mass: 1.0247e26 kg
  Neptune is smaller in diameter but larger in mass than Uranus.

• In Roman mythology Neptune (Greek: Poseidon) 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's 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's and Le Verrier's 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.

• Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25 1989. Almost everything we know about Neptune comes from this encounter.

• Because Pluto's orbit is so eccentric, it sometimes crosses the orbit of Neptune. Since 1979 Neptune has actually been the most distant planet from the Sun; Pluto will again be the most distant in 1999.

• Neptune's composition is probably similar to Uranus's: 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.

• Neptune's blue color is the result of absorption of red light by methane in the atmosphere.

• 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.

• Like Jupiter and Saturn, Neptune has an internal heat source -- it radiates more than twice as much energy as it receives from the Sun.

• At the time of the Voyager encounter, Neptune's most prominent feature was the Great Dark Spot (picture 18) in the southern hemisphere. It was about half the size as Jupiter's Great Red Spot (about the same diameter as Earth). Neptune's winds blew the Great Dark Spot westward at 300 meters/second (700 mph). Voyager 2 also saw a small irregular white cloud that zips around Neptune every 16 hours or so now known as "The Scooter". It may be a plume rising from lower in the atmosphere but its true nature remains a mystery.

• However, HST observations of Neptune (picture 25) in 1994 show that the Great Dark Spot has disappeared! It has either simply dissipated or is currently being masked by other aspects of the atmosphere. A few months later HST discovered a new dark spot in Neptune's northern hemisphere. This indicates that Neptune's atmosphere changes rapidly (picture 26), perhaps due to slight changes in the temperature differences between the tops and bottoms of the clouds.

• Neptune also has rings. 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 (pictures 22, 23).

• 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 coorbital with Galatea, then Leverrier (whose outer extensions are called Lassell and Arago), and finally the faint but broad Galle.

• Neptune's magnetic field is, like Uranus's, oddly oriented and probably generated by motions of conductive material (probably water) in its middle layers.

• Neptune can be seen with binoculars (if you know exactly where to look) but a large telescope is needed to see anything other than a tiny disk. Mike Harvey's planet finder charts show the current position of Neptune (and the other planets) in the sky, but much more detailed charts will be required to actually find it.

Pictures

1. (above) Full Disk Neptune 93k gif; 29k jpg; 186k gif
2. Neptune 291k gif
3. Thin Crescent of planet Neptune 162k gif
4. Parting view of Neptune and moon Triton 59k gif; 17k gif; 23k jpg
5. Neptune in false color 158k gif; 210k gif
6. Neptune in thin crescent, south view (B&W) 160k gif
7. Far view of Neptune showing clouds (B&W) 30k gif
8. Rise of Triton on Neptune (B&W) 23k gif

9. Scooter with Spots 88k gif; 68k jpg

10. Cloud Evolution 58k gif; 25k jpg
11. Bright Cloud Streaks 140k gif; 29k jpg
12. Clouds in Neptune's atmosphere (B&W) 199k gif

13. Cirrus-like clouds 195k gif

14. wave structures? 63k jpg

15. Zoom on clouds (B&W) 202k gif
16. Small Dark Spot 190k gif; 133k jpg
17. Small Dark Spot 116k gif

18. The Great Dark Spot 423k gif; 153k gif; 97k jpg

19. Neptune's Great Dark Spot 126k gif
20. Dark Spot and Scooter 114k gif
21. Big spot of Neptune with the 'scooter' (B&W) 38k gif 63k gif; 10k jpg
22. Rings 89k gif

23. Twisted Rings 77k gif; 43k jpg; 246k gif

24. Neptune's Rings (B&W) 172k gif
25. HST images of both hemispheres 33k jpg; 146k gif; caption

26. HST images 25k jpg; 108k gif

27. ... more Neptune images

Movies

1. Ring Arc Closeup 1334k quicktime
2. Neptune in Rotation 2800k AVI
3. Neptune's Dark Spot 1100k AVI

Neptune's Satellites

           Distance  Radius    Mass
Satellite  (000 km)   (km)     (kg)   Discoverer   Date
---------  --------  ------  -------  ----------  -----
Naiad            48      29      ?    Voyager 2    1989
Thalassa         50      40      ?    Voyager 2    1989
Despina          53      74      ?    Voyager 2    1989
Galatea          62      79      ?    Voyager 2    1989
Larissa          74      96      ?    Voyager 2    1989
Proteus         118     209      ?    Voyager 2    1989
Triton          355    1350  2.14e22  Lassell      1846
Nereid         5509     170      ?    Kuiper       1949

Neptune's Rings

         Distance   Width
Ring       (km)      (km)     aka
-------  --------   -----   -------
Diffuse    41900       15   1989N3R, Galle
Inner      53200       15   1989N2R, LeVerrier
Plateau    53200     5800   1989N4R, Lassell, Arago
Main       62930     < 50   1989N1R, Adams

More about Neptune and its satellites

• from ASU
• from LANL
• from JPL
• from RPIF
• from RGO
• from NSSDC
• from NASA Spacelink
• Neptune's Ring System
• Voyager Neptune Science Summary from JPL
• Neptunian System Nomenclature Tables
• historical notes about the discovery of Neptune

Open Issues

• 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?
• What accounts for the relative lack of hydrogen and helium in Neptune (and Uranus)?
• What happened to the Great Dark Spot?
• Can we design a useful Neptune orbiter mission cheap enough to be funded?

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