It could be argued that any planet or moon of significant size has an “atmosphere.” For example, a mist of oxygen atoms drifts along the surface of Jupiter’s moons Europa and Ganymede. Those oxygen atoms are the product of solar and Jovian radiation blasting oxygen from their water-ice surfaces. Other moons belch transient atmospheres into their black skies. Geysers on Saturn’s Enceladus toss 150 kg of water into the Saturnian system every second. The volcanic moon Io cocoons itself within a cloud of erupted sulfur, which it drags – comet-like – behind as it circles Jupiter. Out at Neptune, the moon Triton lets fly with its own geysers, columns of super-chilled nitrogen.