NASA/JPL/Space Science Institute
Jets of water vapor and ice escape the gravitational forces of Saturn's moon Enceladus and enter space.
Jets of water vapor and ice escape the gravitational forces of Saturn's moon Enceladus and enter space. NASA/JPL/Space Science Institute
NASA/JPL/Space Science Institute
Enceladus' tiger stripes emit jets of ice and water vapor that make up Saturn's outer ring.
Enceladus' tiger stripes emit jets of ice and water vapor that make up Saturn's outer ring. NASA/JPL/Space Science Institute
NASA Cassini-Huygens Collection
Scientists have traced the origins of salty ice grains in Saturn's outer ring to its moon Enceladus.
Scientists have traced the origins of salty ice grains in Saturn's outer ring to its moon Enceladus. NASA Cassini-Huygens Collection
A small, icy moon that orbits Saturn probably has an underground saltwater ocean, according to scientists who have found sodium in tiny ice grains that have spewed out of the moon and into Saturn's outermost ring.
Researchers have wondered if the moon Enceladus might contain a secret ocean ever since NASA's Cassini spacecraft flew by in 2005 and discovered giant plumes of fine, icy particles shooting up into space.
These jets of water vapor and ice appear to be coming from four 75-mile-long fissures in the terrain around the moon's south pole. These fissures are called "tiger stripes," because in images of the moon they look like long parallel slashes of blue in the smooth white expanse of fresh, clean ice that covers the surface of little Enceladus, which is just 300 miles across.
Forces inside the moon make plumes blast up from these fissures so fast that the water vapor and ice particles actually escape the weak gravity of the moon and continue on up into space, eventually forming Saturn's outer ring.
Now, scientists have analyzed the makeup of tiny ice particles in that ring using an instrument on the Cassini spacecraft, which is still orbiting Saturn. Frank Postberg, of the Max-Planck Institute for Nuclear Physics in Heidelberg, Germany, and colleagues report in the journal Nature that they found that some of the ice grains have small amounts of sodium salts.
These salty ice grains suggest that the interior of the moon may have liquid water that is washing salty minerals out of rock into a subterranean sea.
The scientists write that the presence of alkaline salt water, along with the organic compounds and thermal energy that have been observed at the south pole, "could provide an environment well suited for the formation of life precursors."
"Because they're seeing some salty grains, it's kind of hard to see how you would get those if the water is not, itself, salty," says John Spencer, of the Southwest Research Institute in Boulder, Colo., who wrote a commentary in Nature accompanying the report.
"Just having a place where we think liquid water is, another oasis out there in the solar system, and having now much more definitive evidence that that's true is very exciting," says Spencer.
But this salty water can't be close to the surface and just explosively boiling right up like a geyser into the vacuum of space, according to another team of researchers who recently made a separate set of sodium findings.
Nicholas Schneider, of the University of Colorado, Boulder, and his colleagues used telescope observations to look for sodium atoms in the vapor plumes coming out of Enceladus. They didn't detect any, they report in Nature.
Schneider says they should have seen more sodium there if there was water close to the surface that's violently exploding out of vents, like the famous geysers in Yellowstone National Park.
So he says that even though the plumes on Enceladus are often called geysers, that's probably not the right word for what the eruptions are like.
"If it really were an ocean oozing up into a crack very close to the surface and then violently erupting into space, we should see more sodium circling Saturn than we were actually able to see with our telescopic observations," Schneider says. "We don't think it's salt water erupting from right near the surface."
Still, he says, these findings don't rule out the possibility of a salty sea deep underground that is coming up in some other way.
For example, way below the icy crust, an ocean might be slowly evaporating, leaving the salt behind in the liquid just like the oceans on Earth evaporate to produce freshwater rain.
"So that's possible, that there might be some misty caverns underneath the icy crust of Enceladus," says Schneider. Some droplets of liquid might slosh into these chambers, and the water vapor and occasional droplets with salt could then leak out through narrow fractures that go up to the surface.
Scientists will learn more in November, when the Cassini spacecraft makes another flyby of Enceladus. This one will be the closest to the plumes yet, just 60 miles or so above them.
"And so that allows us to get a really good sample of the composition of those jets," says Spencer, "and we'll learn a lot from that."