Scientists have found strong evidence of a watery sea beneath the icy surface of Enceladus, a moon that orbits Saturn and squirts jets of water vapor into one of the planet's rings.
The dramatic jets, which emerge from cracks in the moon's surface, have long tantalized scientists looking for signs of liquid water elsewhere in the solar system. Now, using gravitational data from NASA's Cassini spacecraft, the researchers have determined that Enceladus hosts a vast southern sea roughly the size of Lake Superior.
The discovery, described in Friday's edition of the journal Science, lends support to the idea that this tiny world could be friendly to life as we know it.
"This makes a good strong case," said William McKinnon, a planetary scientist at Washington University in St. Louis, who was not involved in the study. "There may indeed be a sea … but there may in fact be a [global] ocean as well."
A mere 313 miles across, Enceladus is a frozen dirt ball that's too small to stay warm on its own. But it is able to heat water to a liquid thanks to a gravitational love triangle that involves Saturn and another moon, Dione.
Both bodies tug on Enceladus' orbit, stretching its path around Saturn from a circle to an ellipse. The complex gravitational pull also squeezes and stretches the moon itself — and all that kneading from this tidal distortion heats it up, melting some of the water ice inside.
That's the theory, anyway. The search for this subsurface ocean warmed up after scientists discovered plumes of water vapor shooting out of a tiger-stripe pattern of cracks near the south pole. Still, they couldn't be sure that the plumes were fed by liquid water rather than ice on the moon's surface.
Scientists can't see through Enceladus' thick ice shell, but they can get a sort of window into what lies beneath by measuring its gravity in different spots. A stronger gravitational tug means that something very dense is beneath the surface; a weaker tug would signal the presence of something less dense.
During a series of close flybys, Cassini measured the moon's gravity using the Doppler method. Based on how the radio signal coming back from Cassini to Earth was squeezed and stretched, scientists could track how much the spacecraft was wobbling in response to the moon's varying gravitational tug.
As the spacecraft passed over the south pole, the scientists noticed that the tug from the moon was stronger than it ought to be. After all, there was a big dent in the bottom of the moon, which meant there should be less mass and thus the gravity should be weaker. Not so — which meant that there was something dense beneath the surface.
The researchers decided it had to be water, which is denser than ice. (That's why ice cubes float in your drink.)
"There's no other reasonable candidate," said Caltech planetary scientist David Stevenson, a coauthor on the study. "There is rock down there, but it gives the wrong signal."
Stevenson and his colleagues think the sea is roughly 6 miles deep, covered by a shell of ice that's perhaps 25 miles thick. The gravity data show the reservoir is at least a regional sea, and it could stretch far enough north to be a global ocean, scientists said.
Enceladus has been shown to have sodium and potassium salts as well as ammonia and methane in the plume of water vapor being spit out of its south pole. If there are organic molecules like methane and others in the liquid water, it could be a habitable environment, scientists say.
But McKinnon pointed out that ultimately, Jupiter's moon Europa would be a more likely place for potential life to take hold — it's more massive and probably had some volcanic activity in its past. Europa, too, has been caught emitting water from its south pole.
In fact, the liquid on Enceladus that exists now may just be a passing phase, McKinnon said. Most of that liquid water could refreeze on the order of a few million years. The cycle is tied to a complex gravitational story and may repeat over and over, but each time, the window for life to emerge may be relatively narrow.
"That's the implication — we are in fact enjoying a show that is not being put on all the time," he said.