New research explains sustained eruptions on an icy moon of Saturn.
Enceladus, Saturn’s sixth-largest moon, is full of promise. NASA calls it “one of the most scientifically compelling bodies in our solar system.” Since the discovery last year of a vast ocean beneath the moon’s icy shell, researchers believe it’s one of the best candidates for finding extraterrestrial life.
NASA’s Cassini spacecraft, which gathered the data confirming the underground ocean, has also observed geysers erupting on Enceladus’s southern pole since 2005. The process that drives and sustains these geysers has remained a mystery, but now scientists at the University of Chicago and Princeton University have pinpointed a mechanism that explains Enceladus’s long-lived eruptions.
“On Earth, eruptions don’t tend to continue for long,” says Edwin Kite, assistant professor of geophysical sciences at UChicago, who led the new research. “When you see eruptions that continue for a long time, they’ll be localized into a few pipelike eruptions with wide spacing between them.”
But Enceladus has somehow managed to sprout multiple fissures along its south pole. Known as “tiger stripes,” the fissures have been erupting vapor and tiny frost particles continuously along their entire length for decades and probably much longer.
Kite was puzzled by the behavior of these tiger stripes. Evaporative cooling from the eruptions continually removes heat and energy, yet the fissures don’t clog up with their own frost. Researchers knew some other source of heat and energy must be acting on the fissures to prevent icing over—they just didn’t know what the energy source was.
Kite and his coauthor Allan Rubin, from Princeton, think the energy source “is a new mechanism of tidal dissipation that had not been previously considered,” Kite says.
Saturn’s gravitational pull influences Enceladus’s tides, causing the fissures to flex and contract. This process forces water into and out of the fissures; the rise and fall produces enough heat to keep the water and fissures from freezing. Kite and Rubin’s explanation “brings to the fore a process that had escaped notice,” says Carolyn Porco, head of Cassini’s imaging science team at NASA and a leading scientist in the study of Enceladus.
Kite calls the moon “an opportunity for the best astrobiology experiment in the solar system.” Cassini data have strongly indicated that the icy plumes of Enceladus probably originate in a biomolecule-friendly oceanic environment, making it a possible home for extraterrestrial life.
Cryovolcanism, the eruption of ice volcanoes, also may have shaped the surface of Europa, one of Jupiter’s moons. “Europa’s surface has many similarities to Enceladus’s surface, and I hope this model will be useful for Europa as well,” Kite says.
One of the problems that attracted Kite and Rubin was the anomalous tidal response of the Enceladus eruptions: The eruptions are most powerful five hours after tidal stresses reach their peak. Scientists had previously suggested reasons for the lag, including a delay in the eruptions and a squishy, slowly responding ice shell.
“The new proposal is a way to [explain] a delay in the eruptions,” Porco says. “You really don’t need to propose any terribly squishy ice shell to do it.”
The Kite-Rubin model of the Enceladus plumbing system suggests the eruptions emanate from the tiger stripes, which reach from the surface down to the water below. They applied Saturn’s tidal stresses to their computer model 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 says.
The width of the slots affects how quickly they can respond to tidal forcing, the process by which Saturn’s gravitational fluctuations squeeze and stretch the fissures of Enceladus. With wide slots, the eruptions respond quickly; with narrow slots, the eruptions occur eight hours after the tidal forces reach their peak. “In between, there’s a sweet spot,” Kite says, 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.”
The new model also explains why Enceladus maintains a base level of cryovolcanic activity, even at the point in its orbit where the fissures should freeze shut and curtail the eruptions.
Key to the Kite-Rubin proposal is the idea that the tidal ebb and flow heats the water and the ice shell via turbulence. Kite and Rubin believe that new Cassini data can test this idea by revealing whether the ice shell in the south polar region is warm or cold. “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 says. This can be confirmed when the results of Cassini’s final Enceladus flybys of last year are fully analyzed.
Kite and Douglas MacAyeal, professor in geophysical sciences, are interested in studying an Earth analogue to the Enceladus geysers. A crack has formed across a section of the Ross Ice Shelf in Antarctica, partially breaking it away from the continent. “In that crack you have strong tidal flow, so it would be interesting to see what a real ice sheet does in an environment that’s analogous in terms of the amplitude of the stresses and the temperatures of the ice,” Kite says.
A version of this story originally appeared on the News Office website.