Atmosphere & Surface

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Uranus' atmosphere is about 83% hydrogen, 15% helium and 2% methane. There are also traces of water and ammonia. The planet's atmospheric details are very difficult to see in visible light. The methane gas above the cloud layers gives it a blue-green colour.

The core of Uranus is probably composed of liquid rock. Then, farther up, the liquid rock slowly gives way to an ocean made mostly of hydrogen, helium, and water, with small amounts of ammonia and methane. This "ocean" accounts for most of Uranus's bulk. Then, the water slowly thins out into the bland, almost featureless atmosphere. 

The fluid interior structure of Uranus means that it has no solid surface. The gaseous atmosphere gradually transitions into the internal liquid layers. However for the sake of convenience an oblate spheroid of revolution, where pressure equals 1 bar, is designated conditionally as a ‘surface’. It has equatorial and polar radii of 25,559 ± 4 and 24,973 ± 20 km, respectively. This surface will be used throughout this article as a zero point for altitudes.

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