Washington: A new study has revealed that Saturn’s icy moon Enceladus’ decade-long frosty eruptions can be explained by a network of slots under the ice.
The Cassini spacecraft has observed geysers erupting on Saturn’s moon Enceladus since 2005, but the process that drives and sustains these eruptions has remained a mystery.
Now, the University of Chicago’s Edwin Kite and Princeton University’s Allan Rubin have pinpointed a mechanism by which cyclical tidal stresses exerted by Saturn can drive Enceladus’s long-lived eruptions.
Kite said that on Earth, eruptions don’t tend to continue for long, adding “When you do see eruptions that continue for a long time, they’ll be localized into a few pipelike eruptions with wide spacing between them.”
But Enceladus, which probably has an ocean underlying its icy surface, has somehow managed to sprout multiple fissures along its south pole. These “tiger stripes” have been erupting vapour and tiny frost particles continuously along their entire length for decades and probably much longer.
“It’s a puzzle to explain why the fissure system doesn’t clog up with its own frost,” Kite said. “And it’s a puzzle to explain why the energy removed from the water table by evaporative cooling doesn’t just ice things over.”
What’s needed is an energy source to balance the evaporative cooling. “We think the energy source is a new mechanism of tidal dissipation that had not been previously considered,” Kite said.
“I was very happy to see this new work by Kite and Rubin that brings to the fore a process that had escaped notice: the pumping of water in and out of the deep fractures of the south polar ice shell by tidal action,” said Carolyn Porco, head of Cassini’s imaging science team.
Enceladus, which Kite calls “an opportunity for the best astrobiology experiment in the solar system,” serves as a leading candidate for extraterrestrial life. Cassini data have strongly indicated that the cryovolcanic plumes of Enceladus probably originate in a biomolecule-friendly oceanic environment.
The Kite-Rubin model of the Enceladus plumbing system consists of a series of nearly parallel, vertical slots that reach from the surface down to the water below. They applied Saturn’s tidal stresses to their model on a desktop computer and watched what happened.
The only tricky part quantitatively is calculating the elastic interactions between the different slots and the varying water level within each slot as a response to the tidal stress, Kite explained. The width of the slots affects how quickly they can respond to the tidal forces. With wide slots, the eruptions respond quickly to tidal forcing. With narrow slots, the eruptions occur eight hours after the tidal forces reach their peak.
“In between there’s a sweet spot,” Kite said, where tidal forces turn water motion into heat, generating enough power to produce eruptions that match the observed five-hour lag. Porco called it “the best thing in my mind about this new work.”
Tidal pumping heats the water and the ice shell via turbulence. Kite and Rubin have proposed that new Cassini data can test this idea by revealing whether or not the ice shell in the south polar region is warm.
“If the new mechanism is a major contributor to the heat coming from the fractures, then the south polar ice in between the fractures may in fact be cold,” Porco said. “The jury is still on out on this until the results from the final Enceladus flybys of last year are fully analyzed.”
The study appears in the Proceedings of the National Academy of Sciences. (ANI)