Astronomers have found the strongest evidence yet for an ocean beneath the icy shell of Saturn's Enceladus, suggesting it could join the exclusive club of watery moons in our solar system.
The salty water is likely feeding jets of water-ice that spurt from the moon's south polar region.
Such plumes were first reported in 2005, and ever since, astronomers have suspected a liquid ocean might lie beneath the icy shell of Saturn's sixth largest moon.
The new finding, published in the June 25 issue of the journal Nature, could bump this diminutive world — measuring 310 miles (500 km) in diameter (about the width of Arizona) — into a class that includes Jupiter's Europa, Ganymede and Callisto.
In addition, the water and other key life ingredients such as organic material found in the plumes, could provide a suitable environment for life precursors, said lead researcher Frank Postberg of the Max-Planck Institute for Nuclear Physics in Heidelberg, Germany.
Four years ago, an analysis of data from NASA's Cassini spacecraft at Saturn revealed the water-ice jets that spurt from four fractures called tiger stripes, each extending some 75 miles (120 km) across Enceladus's south polar region.
The jets shoot thousands of miles into space, with some of the ice grains and water vapor escaping the moon's gravity and ending up in Saturn's outermost ring, the E ring.
In fact, some of the authors on the new paper reported last year in Nature that the water vapor jets blast out much faster than the dust particles, with the vapor reaching speeds rivaling a supersonic jet — about 650 to 1,100 mph (300 to 500 meters per second).
That finding suggested Enceladus had an ocean below its surface.
Now, evidence points precisely to such a salty body of water. The results come from data collected by the Cosmic Dust Analyzer instrument aboard Cassini, which showed sodium salts within ice grains of Saturn's E ring.
The composition of different sodium compounds and overall salt levels correspond with what the scientists would expect if there were an ocean beneath the moon's icy shell.
"If you have liquid water in contact with a rocky core, then salts would be the most abundant dissolved compounds," Postberg told SPACE.com. "The only way to get that much salt into water is to extract it from rock."
Not the Atlantic
While Postberg and his colleagues are not sure about the size of this ocean, even if it covered the southern hemisphere, the water body would be small compared with Earth's oceans. It would also be a little less salty than, say, the Atlantic or Pacific oceans, Postberg said.
And as far as swimming, a thick wetsuit would be in order, as the water would be close to freezing, he said. (That's warmer than the moon's surface, which reflects 100 percent of the sunlight striking it and plunges to minus 330 degrees Fahrenheit — minus 201 degrees Celsius.)
In another study published in the June 25 issue of Nature, researchers report results from ground-based observations of the vapor cloud in Saturn's E ring, rather than the ice grains. These observations didn't show any sodium in the vapor. The finding, however, doesn't exclude the possibility of an Enceladan ocean.
Instead, the team argues that if the plume vapor does come from ocean water, the evaporation must happen slowly deep underground, rather than as a violent geyser erupting into space. That's because a violent saltwater geyser would eject sodium into the vapor cloud, and the results show no such sodium.
"The original picture of the plumes as violently erupting Yellowstone-like geysers is changing. They seem more like steady jets of vapor and ice fed by a large water reservoir," Postberg said. "However, we can't decide yet if the water is currently 'trapped' within huge pockets in Enceladus' thick ice crust or still connected to a large ocean in contact with the rocky core."
Postberg and his colleagues say such steady jets are either fed directly by an ocean-like body of water or from reservoirs connected with that ocean.
Cassini flybys planned for the fall could glean more information on the ocean-geyser link, he said.
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