Neptune from Triton. Artwork of a geyser (black) erupting on Triton, the largest moon of Neptune (upper right). Triton, among the coldest places in the solar system, has temperatures of -235 degrees Celsius. The nitrogen and methane atmosphere forms ice on the surface. The Voyager 2 space probe passed near Triton in August 1989. Pictures were beamed back of dark stains in the sunlit regions. These were explained as soot, ejected by geysers of nitrogen gas and blown into streaks by the prevailing winds. Triton orbits 355,000 kilometres from Neptune, the eighth planet, and has a gravity that is less than one tenth that of Earth.