New model shows life very unlikely on Saturns moon Enceladus

Researchers at the University of Illinois have developed a new model of Saturns icy moon Enceladus, which they say might help them decisively quell hopes of finding life there.

Washington, Aug 15 : Researchers at the University of Illinois have developed a new model of Saturn's icy moon Enceladus, which they say might help them decisively quell hopes of finding life there.

The NASA/ESA/ASI Cassini spacecraft, which has been orbiting Saturn since June 30, 2004 has revealed a south polar region of Enceladus with an elaborate arrangement of fractures and ridges (known as tiger stripes), radiating intense heat and emanating geyser-like plumes consisting of ice crystals and gases such as methane, nitrogen and carbon dioxide.

The plumes monitored by Cassini had a rate of discharge similar to Old Faithful geyser in Yellowstone National Park. Dubbed "Cold Faithful," the first model that was proposed to explain the plumes suggested the plumes tap into shallow pockets of liquid water in a water-ice shell.

Last year, University of Illinois geology professor and planetary scientist Susan Kieffer and colleagues proposed an alternate model, which they called "Frigid Faithful".

In this model, the plumes originate in the dissociation of certain stiff compounds of ice, called clathrates, which may cover Enceladus to a depth of tens of kilometres.

"Frigid Faithful gives a straightforward account of the measured composition, including the gases left unaccounted by Cold Faithful. 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, professor in the University's Center for Advanced Study.

Now, the team of Keiffer, mechanical science and engineering professor Gustavo Gioia, geology research associate Pinaki Chakraborty and geology professor and department head Stephen Marshak have expanded the model to account for both the tectonic features and the heat transport in the southern hemisphere.

The team examined the deformation of a clathrate-rich shell containing a mildly warm heat source buried under the south pole, and showed that it was possible for a frigid, stiff Enceladus without a shifting interior (such as plate tectonics on Earth) to develop fractures and ridges, and convey heat at the observed rate.

"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 kilometre-long fractures known as tiger stripes," said Gioia, lead author of the paper.

The researchers estimate the heat source could have been only 40 degrees warmer than the surrounding 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, adding that northwards of the south polar cap (in which the stresses are tensile), the stresses turned first from tensile to compressive - forming the ring of ridges that circles the tiger stripes - and then back to tensile - forming the set of "starfish" fractures that radiates northward from the ring of ridges.

Thus the model explains the formation of the entire arrangement of fractures and ridges observed by Cassini on the southern hemisphere of Enceladus, Gioia said.

The team believes the tiger stripes cut through the shell of Enceladus to a depth of about 35 kilometres.

After the tiger stripes formed, the clathrates exposed on the cracked surfaces of the tiger stripes were decompressed. Upon decompression, the exposed clathrates absorbed heat from the source at depth and dissociated explosively, exposing more clathrates to decompression, in a process that continues today, Gioia said.

The implication is that Frigid Faithful's shell remains close to the surface temperature to a depth of about 35 kilometres. According to the Cassini measurements, the surface temperature might be as many as 150 degrees below the freezing point of water, Gioia added.

The researchers describe the model in a paper due for publication in the Proceedings of the National Academy of Sciences (PNAS).

ANI