Since NASA's Cassini spacecraft spotted plumes of water spewing out of cracks in Enceladus' surface roughly a decade ago, Saturn's icy moon has captured the imaginations of scientists and layfolk alike who wondered whether liquid water – and how much of it – could lie hidden beneath the frigid exterior.
Now, through a careful analysis of more than seven years' worth of images, a team of scientists has found that Enceladus harbors a global ocean – far more extensive than the southern sea that researchers described last year.
The findings, released online by the journal Icarus, could shed light on the internal dynamics of one of the few worlds in our solar system with the potential to host life.
Enceladus is one of several icy "water worlds" in our solar system, such as Jupiter's moons Europa and Ganymede, that appear to host liquid water beneath their frozen shells. If they also contain the right chemical ingredients, they could, hypothetically speaking, be habitable for microbial life. Enceladus has been caught periodically squirting water out of slits in its southern pole since 2005, and since then, Cassini has also found signs of simple organic molecules in that plume and hydrothermal activity beneath the surface.
Last year, scientists studying the moon's gravitational field announced that they'd identified a lens-shaped sea beneath the southern pole. But this nearly 6.5-mile-thick layer of water was apparently only a regional phenomenon, hidden beneath nearly 22 miles of ice.
"Remember, the spacecraft flew only over the south pole, so it's much harder for them to be able to say what's on the other side of the moon," said study coauthor Carolyn Porco, leader of Cassini's imaging team at the Space Science Institute in Colorado.
For the new paper, researchers painstakingly mapped features such as craters and ridges on Enceladus and looked for tiny discrepancies in their position at different points in time, to see whether the moon had spun a little faster or slower than expected at various moments in its orbit around Saturn. The magnitude of this "wobble" in its spin could offer insight into how Saturn's massive gravitational pull affects the mass within Enceladus – and, thus, what Enceladus' interior looks like.
The researchers found a distinct wobble – one that, while exceedingly difficult to pick out, was still far higher than would be expected if Enceladus were solid all the way through. The wobble showed that Enceladus' icy shell must be separated from the core by a liquid layer – and that icy shell is more easily influenced by Saturn's gravity.
The wobble "is more pronounced if it's tugging on the ice shell because the ice shell is a lot less massive, so it's a lot easier to move around," Porco said.
This layer of ocean is probably quite thin, she said. But its very presence took planetary scientists by surprise because the presence of so much liquid water implies that Enceladus is much warmer than previously thought.
"There are other lines of evidence coming from different quarters suggesting that there may be a lot more heat produced either now or in the past in Enceladus than we had been thinking," Porco said. "And that's going to be really exciting once people work that out."
Until now, she added, "no one's really, like, rolled up their sleeves and really [been] diving into it."
For Enceladus, all the ingredients for a potentially habitable environment – water, simple organic molecules and potentially (at the hydrothermal hot spots) an energy source to draw from. But the question is, has that habitable environment been around long enough for microbial life to arise?
"Those conditions have to be around for a time greater than the origin of life," Porco said.
The problem with that standard, of course, is that we don't know how long it took for life to arise on Earth in the first place, she added. It could have been 100 million years or a mere million years; for now, that remains a mystery.
Porco says she and some colleagues are working on a proposal for a discovery-class NASA mission that would try to search more directly for evidence of life on Enceladus. This spacecraft's instruments would probe the chemistry of the moon's interior, looking for the presence of amino acids or fatty acids, among other clues.
But don't start packing your scuba gear just yet: Whether or not there are signs of life on Enceladus, that question won't be answered anytime soon. Such a mission, if approved, would probably be launched in 2022 and probably wouldn't arrive until about 2029, she said.
In the meantime, NASA is already working on plans to send a spacecraft to Europa sometime in the 2020s, which would study whether Jupiter's giant icy moon has all the right conditions to support life.