The largest satellite of Saturn and the second-largest in the solar system after Ganymede. It was discovered in 1655 by Christiaan Huygens. Titan is surrounded by a thick atmosphere which consists mainly of molecular nitrogen but also contains methane. The surface pressure is 1.6 times greater than atmospheric pressure at the surface of the Earth. The action of sunlight on the methane and other atmospheric constituents such as carbon monoxide results in chemical changes, producing hydrocarbons and other molecules. These molecules condense in the cold atmosphere, forming a layer of opaque orange-coloured haze 200 kilometres 125 miles above the surface. It is believed that the conditions are such that liquid methane may exist on the surface, where the temperature is 95 K -178°C. There could even be methane rain falling from clouds in the lower atmosphere.