A liquid-water ocean hides under the frozen surface of Saturn’s moon Enceladus, new evidence confirms. The presence of this water boosts Enceladus’ ranking among the top places in the solar system to look for extraterrestrial life, scientists say.
Enceladus has intrigued researchers since 2005, when NASA’s Cassini probe discovered water-rich plumes spewing from the moon’s south pole, raising the possibility that they were venting from a buried liquid sea. The same probe has now bolstered the sea hypothesis by measuring Enceladus’ gravitational field. Scientists carefully monitored how the moon deflected Cassini off course and determined that Enceladus must have more mass at its south pole than meets the eye. Because liquid water is denser than ice, a buried ocean could contribute this hidden mass, the investigators reasoned. “It’s very difficult to come up with any explanation for the data that does not involve a thick layer of liquid water beneath the ice,” says California Institute of Technology planetary scientist David Stevenson, a co-author of a paper reporting the finding that will appear in the April 4 Science. Although the gravity data provides no proof that the liquid is water and not something else, water is the likeliest explanation because it is plentiful on Enceladus (albeit mostly seen in the form of ice) and because rock in its place would not produce the gravitational pattern seen, Stevenson explains.
Although plumes could form from the melting of surface ice, a connection to an underground source of water is also likely. And the fact that Enceladus’s plumes stem from its south pole, the same location where the putative ocean resides, is another factor in favor of the water ocean explanation. “These new results are like the point in a detective story where finding the fingerprints confirms the hypothesis based on motive and opportunity,” says Larry Esposito, a planetary scientist at the University of Colorado Boulder who was not involved in the study. Stevenson himself admits to having been skeptical until now. “Before these results I personally thought it was not at all clear that Enceladus had an ocean,” Stevenson told reporters during a teleconference on Wednesday. “You can make water just by rubbing ice blocks together, so it wasn’t clear to me that you had a huge volume. We now know that we do.”
Cassini’s data imply a subsurface ocean about 10 kilometers deep that covers an area about the size of Lake Superior. It would be buried underneath roughly 50 kilometers of frozen ice. Such a reservoir could theoretically host some form of life, which is thought to rely on liquid water. “There are terrestrial organisms that would be perfectly comfortable in that environment,” said study co-author Jonathan Lunine, a planetary scientist at Cornell University. “It makes the interior of Enceladus potentially a very attractive place to look for life.”
Enceladus is not the only solar system body that may host an underground ocean. Jupiter’s moon Europa, another prime target for extraterrestrial life searches, is thought to contain a global ocean underneath its surface ice, and the other Jovian satellites, Callisto and Ganymede, also show evidence of subsurface seas. Whereas Ganymede’s ocean probably sits on a deeper layer of ice, the water on Enceladus would lie atop the moon’s silicate core. Because silicate could provide some of the chemicals necessary for life—such as salts, phosphorus and sulfur—this arrangement may offer the chance for those chemicals to mix with liquid water and brew life, Lunine said.
The latest findings required researchers to painstakingly track Cassini’s motion by monitoring minute changes in the frequency of the signal it sent back to Earth, called Doppler shifts. “It’s the same thing they’re using for the Malaysian plane but we can do it more accurately,” Stevenson says. After collecting data during three passes Cassini made near Enceladus, scientists could estimate the moon’s gravity field well enough to determine that some extra mass is beneath Enceladus’s surface. “If correct, it provides important new information about what might be going on well below the vents,” says University of Idaho planetary scientist Matthew Hedman, who was not involved in the research. “One important question that I think still needs answering is how such an ocean connects up with the surface to produce the plumes.” Another question is why Enceladus’s north pole so far shows no sign of plume activity or an ocean. Scientists think gravitational tides from Saturn could be heating the moon’s interior, melting ice to form the ocean. This heating would likely be greatest at the poles. “I don’t know why it is only at the south,” Stevenson says.
The new evidence and the questions it raises are only making scientists more eager to devote some of Cassini’s remaining time at Saturn to studying Enceladus. The spacecraft arrived at the ringed planet in 2004 and is due to die a spectacular death by plunging into the Saturnian atmosphere in 2017. Before then, Cassini has three more targeted flybys of Enceladus scheduled. Hopefully, more discoveries await.