NASA's Cassini spacecraft has made a surprising discovery about Saturn's moon Enceladus: it has an atmosphere of water vapour. Cassini detected the atmosphere during both of its recent close flybys. Scientists are sure what's causing this cloud of water vapour, but it could be caused by volcanoes, geysers, or gasses escaping from the moon's interior. Enceladus' gravity is very small, so it wouldn't be able to hold onto an atmosphere for very long. This means there must be some continuous source replenishing it.