In July 2005, the Cassini space probe observed immense plumes of water vapour (pictured) spouting from an anomalously warm region near the southern pole of the saturnian moon Enceladus. That spectacular observation confirmed a long-held belief that Enceladus is the source of the icy grains that make up its home — the outer lane, or ‘E ring’, of Saturn's ring system.
But why are these plumes where they are? In this issue, Francis Nimmo and Robert T. Pappalardo (Nature 441, 614–616; 2006) show that the spout's polar location could, theoretically, be explained by the formation of a diapir, an upwelling region of low-density material, in Enceladus' interior. Simple gravitational considerations require that the moon swings round so that this less weighty, tectonically active region is aligned with the moon's axis of rotation — and so ends up at the pole.
There are caveats. The authors show that if the diapir is within Enceladus' subsurface ice shell, then the moon's outermost layer, or lithosphere, must be relatively rigid. Otherwise, the upwelling material would simply bulge out from the surface. That region would then tend to migrate not towards the poles, but towards the moon's equator.
A diapir within Enceladus' silicate core, on the other hand, would cause a poleward reorientation. But it would preclude the existence of a global subsurface ocean: such an ocean would decouple the core gravitationally from the ice shell, preventing reorientation.
Nimmo and Pappalardo's low-density diapir should be detectable through its effect on the moon's gravity field. Further analysis of data returned by radio instruments aboard Cassini should provide a conclusive test for its existence.