Saturn’s 500-km- (300-mile) diameter moon Enceladus is covered in ice which contains many long fractures or rifts, including fractures called tiger stripes. As the icy moon orbits Saturn, tidal forces cause the tiger stripe fractures to erupt tall geysers of water vapor; this suggests that there is liquid water somewhere beneath Enceladus’ icy surface and that there must be motion along the fractures. Planetary researchers expect that this motion...