Photograph courtesy NASA

The largest of Neptune's 13 moons, Triton is one of only three objects in the solar system known to have a nitrogen-dominated atmosphere. (The others are Earth and the Saturn moon Titan.) It's also one of the coldest objects in the solar system—its icy surface reflects so much of what little sunlight reaches it that its temperature is only about -400 degrees Fahrenheit (-240 degrees Celsius). It's so cold that most of its nitrogen condenses as frost, making it the only moon known to have a surface made mainly of nitrogen ice.