When the Voyager 2 probe flew past Neptune (top) in August 1989, it also imaged its largest moon Triton. The pictures showed that although Triton's surface is the coldest in the known Solar System at -240 Celsius (-400 Fahrenheit), it is still volcanically active. However, the volcanoes are cryonic – geysers of frigid nitrogen gas. The Voyager images showed, as does my own rendition here, how the geysers leave dirty streaks on the landscape as the nitrogen and other materials is blown across by high altitude winds.