Triton's geysers. Artwork of erupting geysers (lower left & right) on Triton, with the planet Neptune seen at upper left. The geysers are composed of liquid nitrogen and methane. Prevailing winds carry the liquids (centre right) across the moons surface. This phenomenon was discovered following the Voyager 2 encounter with Neptune in August 1989. Triton is the coldest world in the solar system, with temperatures as low as -235 degrees Celsius. It is the only moon in the Solar System to orbit in an opposite direction to that of its parent planet's rotation (retrograde). Neptune's faint rings are seen. The Plough in Ursa Major is at upper right.