NASA's Cassini orbiter has detected the faint signature of a hidden southern ocean beneath the ice of the Saturnian moon Enceladus, confirming past suspicions and sparking fresh speculation about extraterrestrial marine life.
"It makes the interior of Enceladus a very attractive potential place to look for life," said Jonathan Lunine, a planetary scientist at Cornell University and a member of the science team reporting the discovery in this week's issue of the journal Science.
Astrobiologists have had Enceladus on their list since 2006, when Cassini detected geysers of water spewing up from fissures in the southern hemisphere. However, it took much more subtle observations to confirm the source.
"It was not a surprise to find a water reservoir ... but the mass and geometry of this reservoir were unknown," Luciano Iess of Sapienza University of Rome, the Science study's lead author, told reporters during a teleconference.
Iess and his colleagues say the reservoir is a sea of liquid water, buried under 19 to 25 miles (30 to 40 kilometers) of ice. The sea is at least 6 miles (10 kilometers) deep and extends at least halfway up from the south pole toward the equator in every direction.
"This means that it is as large, or larger, than Lake Superior," said Caltech's David Stevenson, another co-author of the Science study.
How scientists know
It took masterful feats of observation and calculation to figure all that out.
Astronomers began by measuring slight variations in Cassini's velocity as it sped past the 310-mile-wide (500-kilometer-wide) moon on three occasions between 2010 and 2012. Those changes amounted to mere millimeters per second, and could be detected only by analyzing the Doppler shifts in the radio transmissions from the spacecraft. (A classic example of Doppler shift is the rise and fall in the pitch of a train's whistle as it zooms past you.)
The velocity variations were caused by anomalies in Enceladus' gravitational field — that is, regions of the moon that had more or less mass than average. Astronomers had already known about a huge depression in Enceladus' southern hemisphere, so they expected the mass concentration to be less in the south. But after taking everything they knew into account, researchers determined that the concentration was more massive than it should have been.
The best way to explain the extra mass was to assume the existence of a sea in the south, lying between Enceladus' rocky core and icy shell. Liquid water is denser than water ice, as illustrated by the ice cubes floating in a glass of water.
Planetary physicist William McKinnon of Washington University in St. Louis told Science that the interpretation made sense. "You could create a model without water, but people wouldn't find it satisfying," he said.
Looking for life
The gravity measurements mesh nicely with the presence of those water geysers spewing out from Enceladus' southern fissures, which are also known as "tiger stripes." Enceladus' core undergoes tidal flexing as it circles Saturn, and that flexing is thought to generate heat that's concentrated at the poles.
Astronomers suggest that there's enough heat at the south pole to melt the ice and push seawater up to the surface through the fissures.
That scenario is exciting for astrobiologists, because it means the sea could be in contact with organic-rich silicate material at the bottom, at the right temperature for sustaining life.
Earlier observations from Cassini have shown that the water in Enceladus' geysers contains salts as well as organic molecules such as methane and ethane. However, the spacecraft's instruments aren't designed to detect the heavier organic molecules that would constitute evidence for life, Lunine said.
The easiest way to check for life would be to send a probe with the right kind of instruments through Enceladus' geysers to look for the right chemicals.
Enceladus isn't the only game in town when it comes to the search for life, however. Scientists say that Europa, one of Jupiter's moons, also appears to have an ice-covered sea. Last December, researchers reported evidence that Europa is also spewing geysers of water into space. Such findings have led NASA to seek $15 million to start work on a mission to Europa.
The Europa mission alone would require years to plan and at least $1 billion in funding. Right now, Enceladus is a lower priority for future exploration, and it's not clear when the moon's southern ocean will get a closer look. But the latest findings suggest that places like Enceladus and Europa (and perhaps Ganymede and Callisto, two more ice-covered moons of Jupiter) could represent astrobiological frontiers at least as promising as Mars.
"I look at this as a cornucopia of habitable environments in the outer solar system," Lunine said.
In addition to Iess, Lunine and Stevenson, the authors of "The Gravity Field and Interior Structure of Enceladus" include M. Parisi, D. Hemingway, R.A. Jacobson, F. Nimmo, J.W. Armstrong, S.W. Asmar, M. Ducci and P. Tortora.