HOT SPOT: The plumes spewing out of Enceladus, imaged here by the Cassini spacecraft in December 2009, may mark a rare occurrence in the moon's geologic history. Image: NASA/JPL/Space Science Institute
Enceladus, Saturn's sixth-largest moon, is an icy bundle of contradictions. It is tiny in planetary terms—the entire moon could fit snugly inside the borders of New Mexico—and yet it hosts a level of geologic activity usually reserved for the big dogs of the solar system.
From Enceladus's south pole emanate geyserlike jets, watery plumes that spew outward from a region carved up by unusually warm gashes known as "tiger stripes". But by all rights, the moon is losing much more energy through this geologically active region than it has to spare. This disparity is "the big problem of Enceladus that sticks out like a sore thumb," says planetary scientist Craig O'Neill of Macquarie University in Australia.
In a paper published online Sunday in Nature Geoscience, O'Neill and planetary scientist Francis Nimmo of the University of California, Santa Cruz, propose a model to resolve the discrepancy between Enceladus's heat production and heat loss. The extreme heat loss at the moon's south pole, O'Neill and Nimmo say, can be explained by a model in which the kind of geologic eruptions now visible on Enceladus only occur every billion years or so. (Scientific American is part of Nature Publishing Group.)
As Enceladus orbits Saturn, the massive planet's gravitational pull causes the icy moon to flex, generating tidal heat on the satellite. "Because Enceladus's orbit isn't quite circular, as it goes round Saturn it's getting closer and farther away from Saturn, and so the gravitational force that it feels is changing slightly," Nimmo explains. "It's getting squeezed and stretched and squeezed and stretched. And just like if you have a rubber ball in your hand, if you squeeze and stretch and squeeze and stretch, it's going to heat up." Other contributors to Enceladus's heat inventory, such as radioactive decay in the moon's core, appear to be relatively minor players, Nimmo says.
But the mechanism of tidal heating would have to be several times more potent to generate the amount of heat Enceladus is blowing off from its south polar region. Unless, as O'Neill and Nimmo propose, what we are seeing from Enceladus now is somewhat anomalous behavior—a brief episode of tectonic activity that resurfaces a limited area of the moon. In the new model, "what's actually happening is that heat's being pent up inside over billions of years," O'Neill says. "It's building up to a critical level and then escaping in one big pulse of activity."
That pulse of activity would only last about 10 million years, which O'Neill describes as "a bit of a blink of the eye geologically." In other words, astronomers millions of years in the future might see a much calmer surface when they look to Enceladus. But if this periodic tectonic behavior is endemic to icy satellites with evidence of partial resurfacing throughout the solar system, perhaps those future astronomers would see eruptions on other moons that are currently dormant.
Norm Sleep, a professor of geophysics at Stanford University, calls the proposal an "excellent start" with the properties necessary for a successful model. Dave Stegman, a geophysicist at the Scripps Institution of Oceanography in La Jolla, Calif., says that "it appears that certain aspects of this model can explain some of the enigmatic observations that have not been previously addressed." At the same time, Stegman says, episodic tectonics arise in "an extremely narrow window" of parameters, so it will be interesting to see if future, finer-scale models uphold the proposal.
Nimmo acknowledges that "our model doesn't really address the details; all our model does is say, this amount of heat coming off this fraction of the surface is more or less what we see and more or less what our model gives." Trying to match up specific geologic features like the tiger stripes with the predictions of episodic tectonics will be an important test of the model's viability. "I think the next step will be to sort of zoom in," Nimmo says, "and try and see if the kind of behavior that the model produces relates to the geological features at the surface."