Oceans aren't just for Earth. A new study suggests that one of Saturn's moons boasts a large ocean sloshing atop a rocky seabed, offering some of the outer solar system's best real estate for living organisms.
The sea lies beneath the south pole of Enceladus, a tiny ice world orbiting close to the ringed planet. This newly discovered ocean rivals Lake Superior in size and, in some places, the most fathomless trenches of the Pacific in depth. The presence of a rocky seafloor gives the seawater an opportunity to leach sulfur, potassium and other elements out of the rock, providing the building blocks for molecules essential to life.
"There appears to be a liquid-water ocean, which has to be sandwiched between ice and a rocky core," says Cornell University's Jonathan Lunine, an author of the new study. "The configuration that we see … makes the base of the ocean very much like the base of our own ocean on the Earth."
Enceladus makes unlikely beachfront territory. A shell of glimmering ice covers the tiny moon, and surface temperatures hover at a bracing minus 300 degrees Fahrenheit. Geysers of ice particles and salty water vapor shoot hundreds of miles into space from Enceladus' "tiger stripes," long canyons raking the moon's surface at the south pole. Perhaps the geysers pull water from a hidden sea. Or perhaps an ocean on Enceladus "could not survive and would simply freeze out over time," Frank Sohl of Germany's Institute of Planetary Research says via e-mail.
To find out what lies beneath the ice, researchers sent the NASA spacecraft Cassini skimming close to Enceladus' poles three times and measured the minute pull on the spacecraft exerted by the moon's bulk. By comparing Enceladus' tug on Cassini with other information about the surface, they inferred that the moon's ice hides a liquid ocean and a central ball of rock, the researchers report in this week's Science.
"That rock has been in contact with water for a long time," says study co-author Francis Nimmo of the University of California-Santa Cruz. "You have a long-lived ocean, and you have a source of nutrients and a source of energy. So that's all looking pretty good for habitability" — the capacity to support living organisms.
Many scientists were already convinced that Enceladus had an internal ocean, but "this is really the first direct evidence we've had as to the configuration of the interior," says planetary scientist John Spencer of the Southwest Research Institute, who was not involved in the new research. "That's a really, really nice thing to have."
A sea in direct contact with rock "would substantially enhance the satellite's habitable potential," Sohl says via e-mail. Enceladus could harbor features similar to Earth's seafloor hydrothermal vents, where chemical-laden water supports rich communities of sea creatures.
Enceladus may not generate enough heat to sustain life. But if it does, it could be a long time before Earthlings find out. NASA's Cassini spacecraft lacks the sophisticated equipment to sniff out signs of living organisms, and NASA has no firm plans to send a spacecraft to the little moon.
The new data "makes the interior of Enceladus a very attractive potential place to look for life," Lunine says. And what it "tells us is that we need to be more aggressive in getting the next generation of spacecraft" to Saturn's moons and other places in the solar system that might harbor life, he says.
"We'll have to wait a long time before returning to Enceladus," says study co-author Sami Asmar of the Jet Propulsion Laboratory. "Hopefully, in our lifetime."