The eruption activity varies, the scientists said: the plumes could only be seen when Europa was furthest from Jupiter in its orbit. Apparently, they said, at this time, tidal forces lead cracks in Europa’s icy crust to widen, letting out the vapors. Similar plumes have been discovered by the Cassini spacecraft on the Saturnian moon Enceladus, they noted, and the activities there are much like those on Europa during its orbit.