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Neptune

Neptune's Moons

DescriptionNeptune

Neptune, like its twin, Uranus, has an atmosphere composed of hydrogen, helium, methane, and ammonia. The methane creates the blue color. Neptune's internal structure is also the same as that of Uranus.

The core of Neptune is probably composed of liquid rock. Then, farther up, the liquid rock slowly gives way to an ocean, primarily containing hydrogen, helium, and water, but also ammonia and methane. This "ocean" accounts for most of Neptune's bulk. The ocean slowly thins out into the atmosphere.

This atmosphere contains much more detail than Uranus'. It has swirls of clouds and high-altitude cirrus clouds, as well as gigantic storms.

Neptune's High Cirrus CloudsNeptune also has a faint system of rings. The rings are made of pieces of material the size of a car or truck. There is about one thousandth of the matter in Neptune's rings as in Uranus'. If the rings were all rolled into a ball, the ball would be only a couple of miles across. That matter is spread across 125,000 km (77,500 miles).

In the outermost ring of Neptune, there are three anomalies called the "Three Arcs of Neptune." They are areas where the rings are relatively dense. Over the millions of years that these have existed, the particles in the arcs should have spread out until the ring was a uniform density. However, this has not happened. Scientists now believe that these arcs are shepherded by small moons.

Neptune has known moons.

Unique Characteristics

Neptune has a few features that no other planet has. First, it is the last non-controversial planet in the solar system, yet discovered (Pluto is still under debate in some circles). Once Uranus had been discovered, perturbations in its orbit were seen. Calculations were made, and it was predicted that there should be a planet within a certain patch of sky. Within hours, Neptune was discovered in 1846.

Neptune's Great Dark SpotThe next feature that is unique to Neptune is the Great Dark Spot (left). It is a depression in the atmosphere surrounded by high cirrus clouds. Neptune's Great Dark Spot does not appear to be stable, though. A few years after the Voyagers found it, it had disappeared (Hubble Space Telescope pictures at the right). Then, in 1995, it reappeared in Neptune's northern hemisphere.Neptune - No Great Dark Spot (HST)

When the Great Dark Spot was first discovered, it was thought to be moving very slowly. However, when Neptune's rotation rate was finally determined, it showed that the Great Dark Spot wasn't moving slowly with the planet, but was actually moving in a direction opposite the planet's spin at almost supersonic speeds. These are ten times hurricane speeds on Earth.

Mythology

Neptune was the ancient Roman god of the sea. When Neptune claimed domination over the seas, the old sea-god, Nereus, gave Neptune his palace and one of his daughter in marriage.

Data for the Planets

Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune
Mean Orbital Distance (106 km)
57.91
108.2
149.60
227.92
778.57
1433.53
2872.46
4495.06
Average Orbital Velocity (km/s)
47.87
35
29.78
24.13
13.07
9.69
6.81
5.43
Orbital Inclination (from Earth's Orbit)
7.00°
3.4°
0.0°
1.850°
1.304°
2.485°
0.772°
1.769°
Orbital Eccentricity
0.2056
0.007
0.0167
0.0935
0.0489
0.0565
0.0457
0.0113
Equatorial Radius (km)
2439.7
6051.8
6378.1
3397
71,492
60,268
25,559
24,764
Polar Radius (km)
2439.7
6051.8
6,356.8
3375
66,854
54,364
24,973
24,341
Axial Tilt (from Earth's geographic North)
0.01°
177.4°
23.45°
25.19°
3.13°
26.73°
97.77°
28.32°
Mass (1024 kg)
0.3302
4.87
5.9736
0.64185
1898.6
568.46
86.832
102.43
Density (water=1)
5.427
5.243
5.515
3.933
1.326
0.687
1.27
1.638
Escape Velocity (km/s)
4.3
10.36
11.19
5.03
59.5
35.5
21.3
23.5
Gravity (m/s2)
3.70
8.802
9.78
3.716
23.1
9
8.7
11
Sidereal Rotation Period (hours)
1407.6
-5832.5
23.9345
24.6229
9.9250
10.656
-17.24
16.11
Length of Day (hours)
4222.6
2802
24
24.6597
9.9259
10.656
17.24
16.11
Tropical Orbital Period (days)
87.968
224.7
365.256
686.980
4330.595
10,746.94
30,588.740
59,799.9
Average Surface Temperature (Celsius) 167° 464° 15° -65° -110° -140° -195° -200°
Number of Moons
Rings? No No No No Yes Yes Yes Yes
Discoverer Unknown Unknown Unknown Unknown Unknown Unknown William Herschel Johann Gottfried Galle
Discovery Date Prehistory Prehistory Prehistory Prehistory Prehistory Prehistory March 13, 1781 September 23, 1846

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