What are the distinguishing characteristics of Neptune?

It's blue.

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 .

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

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 in Neptune's rings as in Uranus.

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, 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.

Unique Characteristics:

Neptune is the last non-controversial planet in the solar system, yet discovered (Pluto is still under debate in some circles).

Neptune's Great Dark Spot. The next feature that is unique to Neptune is the Great Dark Spot (left). The Dark Spot 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. 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.