Disappointment for the alien life hunters. The only plausible model explaining the arrangement of fractures and ridges documented by Cassini on Enceladus, Saturn's icy moon, shows no liquid water and thus an unlikely environment for life.

In June 30, 2004, the Cassini spacecraft has detected a south polar region of Enceladus with a complex pattern of fractures and ridges, high heat radiation and geyser-like plumes made of ice crystals and gases like methane, nitrogen and carbon dioxide.

The plumes sprouted from vents found in big fractures named "tiger stripes", at a discharge rate fitting that of the Old Faithful geyser in Yellowstone National Park. Named "Cold Faithful", the first explanation was the plumes tap into shallow pockets of liquid water in a water-ice shell, fact that boosted hopes of finding life forms thriving there.

But in 2006, a team at the University of Illinois led by geology professor and planetary scientist Susan Kieffer came with an alternate model, named "Frigid Faithful", explaining that plumes in the dissociation of some stiff ice forms, clathrates, which could cover Enceladus on layers of tens of kilometers. "Frigid Faithful gives a straightforward account of the measured composition, including the gases left unaccounted by Cold Faithful," said Kieffer, who holds a Charles R. Walgreen Jr. Chair at Illinois. "Perhaps more important, the plumes of Frigid Faithful could remain active far below the freezing point of water, under the frigid conditions that might be surmised inside a tiny, icy moon," said Kieffer.

Now, researchers at the same center have further deepened the model, taking into account both the tectonic traits and the heat transport in the southern hemisphere.

It appears that the deformation of a clathrate-rich shell harboring a mildly warm heat source located under the south pole makes possible a frigid, stiff Enceladus without a shifting interior (like plate tectonics on Earth) to form fractures and ridges, and transport heat at the accounted rhythm.

"As the heat source warmed at depth, it expanded and stretched the clathrate-rich shell above, giving rise to tensile stresses in the south polar cap. As a result, the shell cracked, forming the four 130 kilometer-long fractures known as tiger stripes." said lead author Gioia, mechanical science and engineering professor.

The team believes that the heat source could have been just 40° C warmer than the ice shell. "In this model, the tiger stripes are analogous to the cracks that form in the glazing of a porcelain vessel when the vessel is filled with hot tea," Gioia said.

The team also discovered that in the northwards of the south polar cap, the stresses became first from tensile to compressive, giving birth to the ring of ridges that surrounds the tiger stripes, and then turned back to tensile, originating the set of "starfish" fractures that go northward from the ring of ridges.

The tiger stripes could cut through the ice shell of Enceladus down to about 35 km. When the tiger stripes developed, the exposed clathrates on the cracked surfaces, which had absorbed heat from the source, experienced decompression and dissociated violently, attracting more clathrates to decompression, in a continuous phenomenon, still in process today.

The clathrates' dissociation gas carry heat to the surface (heat advection), where they form from time to time the plumes, and the Frigid Faithful works like a huge "advection machine". Unlike "heat conduction", where heat (in a metal bar, for example) goes from hot points to cold points, heat advection occurs at an almost uniform temperature.

The Cassini data shows that the surface temperature of Enceladus could be of -150° C. "This is indeed a frigid Enceladus. It appears that high heat fluxes, geyser-like activity and complex tectonic features can occur even if moons do not have hot, liquid or shifting interiors.", Gioia said.