Structure and Atmosphere

In many ways Neptune is similar to Uranus, but unlike Jupiter and Saturn its structure is less layered. It likely has a very thick and roughly uniform composition of water, ammonia and methane ice mixed together with hydrogen, rock and small amounts of helium surrounding a small rocky core about the size of Earth. Since its surface temperature is higher than expected for a body so far away from the sun, it must have an internal heat source. Mysteriously, it also has a significant magnetic field, but it is tilted considerably from the planet's rotational axis and is off center.

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Typical of any gas giant, Neptune has bands of strong winds, but on Neptune they are the fastest in the solar system at around 2000 km/hr (1250 mph). Voyager photographed a Great Dark Spot of swirling gases in its southern hemisphere that is similar to Jupiter's Great Red Spot. Observations from the Hubble Space Telescope however, indicate that it has since disappeared. Late in 1994 Hubble discovered that another dark spot had appeared in its northern hemisphere.

The atmospheric composition is approximately 80% hydrogen, 19% helium and 1% methane. It is the methane that is responsible for Neptune's deep blue colour.