Enceladus’s Hot Pole

The Cassini spacecraft mission has found that one of Saturn’s moons, Enceladus, has a peculiarly ‘active’ south pole. There, water vapour appears to be streaming out of huge cracks in the ice that covers Enceladus’s surface, suggesting that there is liquid water beneath. Francis Nimmo and Robert Pappalardo explain in Nature this week why Enceladus’s south pole seems to be so warm.

Planetary scientists have assumed that the heat comes, in effect, from Saturn itself: because the moon is so near this giant planet, Saturn’s gravity tugs on the fabric of Enceladus and heats it. Jupiter’s moon Io is turned into a fiery, volcanic world by the same effect, called tidal heating. But why does this happen only at Enceladus’s south pole?

The researchers think that a plume of relatively warm ice at any point within the moon’s icy shell could have caused the moon to reorient itself so that the warm region becomes the new south pole. This happens because the warmer ice expands and becomes less dense, and the resulting uneven distribution of mass shifts the rotational axis of the moon.

But the warm, mushroom-shaped blob — or diapir — might not actually be within the ice shell at all: it could be in Enceladus’s rocky core. Or there could be diapirs in both regions. If the diapir is indeed in the ice, then Nimmo and Pappalardo say that the soft part of this shell, below a hard, frigid crust, would have to be at least 0.5 kilometres thick. If it’s in the rocky core, then there cannot be a subsurface layer of liquid water between this and the ice shell, since that would lubricate the interface and prevent reorientation from occurring. These and other predictions may make it possible to test the idea of reorientation once more observations of Enceladus have been made.