Neptune

Neptune's Moons

Overview

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 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.

Atmosphere

Neptune is basically one big atmosphere - as far as we know, there is no real surface to the planet. Its composition is nearly all H₂; the rest is approximately:

• H₂: 80.0±3.2%
• He: 19.0±3.2%
• CH₄: 1.5±0.5%
• HD: 0.0192%
• C₂H₆: 0.00015%

Besides this, aerosols of ammonia ice, water ice, and ammonia hydrosulfide exist in the atmosphere. Methane ice may also be present.

The temperature at 1 bar is approximately 72 K, and at 0.1 bar it is 55 K. The density at 1 bar is 450 g/m³. Wind speeds range up to 200 m/s. The scale height of the Neptunian atmosphere is about 19.1-20.3 km.

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.
• The next feature that is unique to Neptune is the Great Dark Spot (right). 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.
  
  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.

Satellites that Have Visited

1. Voyager 2 - 1 flyby in 1989

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 Rings

*Lassell and Arago were originally identified as one ring, designated 1989N4R.

Data for the Planets