In this article we journey further out into the Solar System and we are leaving our Sun far, far behind. The heat and light received from it is now very weak. This makes the temperature of the cloud layer we observe as we approach the next planet an extremely cold minus 214º Celsius. We have arrived at Uranus, the third of the gas giants.
Its atmosphere is mainly made up of hydrogen with about 15% helium and 2% methane. There are also some other trace gases present. To an observer, Uranus appears to be blue in colour. This is due to the methane in the atmosphere. It absorbs red light and reflects blue which gives it the colour we see. Uranus has a solid core of rock and ice. Surrounding this is a layer containing ammonia, methane and water in the form of ice. And above this comes the atmosphere.
An unusual aspect of the planet is its angle of tilt to the vertical axis. It is 98 degrees which means it looks as though it is orbiting on its side. We’re not certain why this is the case but some scientists believe that it came about after colliding with another object many millions of years ago. Seen from the Earth through a large enough telescope, an observer would see the moons seeming to orbit the poles rather than the equator. The same also applies to a ring system around Uranus but it is much fainter than the one encircling Saturn.
So far, twenty seven moons have been detected in orbit. One of them, called Ariel, is about a third of the size of our Moon. But not much else is known about it. The deep space-probe Voyager II has sent back some photographs of Ariel as it flew by. The surface appears to be icy with large fault lines cutting across it. But we know little else about it at the present.