Earth is not the only orb with oceans. In 2005 Cassini, an American spacecraft, saw plumes of water shooting into space from cracks in the icy surface of Enceladus, one of Saturn’s moons (see picture). These suggest that Enceladus, too, has an ocean—albeit one completely covered by ice. The water in it, theory suggests, would be kept liquid by tides, which create internal friction and therefore heat. On April 3rd a team led by Luciano Iess of the University of Rome confirmed that the ocean exists, and also showed that, like Earth’s, it is not all-embracing. Dr Iess describes, in a paper in Science, how his team mapped Enceladus’s gravity by tracking Cassini’s orbit. The moon’s southern hemisphere is less massive than it would be were there no ocean, but its northern hemisphere is not. So the ocean covers only the southern part of the moon.