Washington University, St Louis, Missouri
A planetary scientist has big hopes for a little world.
Right now, the most exciting object in the Solar System is Saturn’s diminutive moon Enceladus. Its deformed south polar region emits copious amounts of heat along the length of several young, active ridges and fractures, as well as plumes of tiny ice particles, water vapour and other chemicals.
The Cassini spacecraft — equipped with plume-gas and particle analysers and clever imaging gadgetry — is currently in the neighbourhood. Seizing this opportunity, Gabriel Tobie of the University of Nantes in France and his colleagues have incorporated some of its recent measurements into theoretical models of tidal heat production on Enceladus (G. Tobie et al. Icarus 196, 642–652; 2008). Only the ebb and flow of tides could properly account for such prodigious geological activity on an icy moon that measures just 500 kilometres in diameter.
The authors start with the generally accepted idea that Enceladus has differentiated into a rock core and an icy mantle. They then show that the size of the tidal motion of the mantle is inadequate to generate the observed thermal emission, so there must be a fluid ocean sandwiched between the two solid layers. This is no great surprise, but Tobie et al. go further, showing that even if the mantle is made soft and deformable over the southern polar region (as the ice would be if it were relatively warm), a sandwiched, liquid ocean must reach at least as far as around the entire southern hemisphere.
The team imagines that, below Enceladus’s south pole, tidal heating concentrates in warm, upwelling, convectively mobile ice. This, in turn, causes the cold, brittle surface layer to rupture — and the exposed warm ice sublimates, releasing trapped gases. It is a compelling picture, and one that promises to help unlock the internal activity of other icy satellites.