Does Saturn's moon have a salty ocean?

Posted by Emma Woollacott

Boulder, Colorado - There's a salty ocean on Saturn's moon Enceladus that could possibly harbor life - or there isn't. It depends which of two reports you prefer.

The Cassini spacecraft made a surprising discovery about Saturn's sixth largest moon, Enceladus, on its exploration of the giant ringed planet in 2005. Enceladus ejects water vapor, gas and tiny grains of ice  hundreds of kilometres into space.

First observed following a close flyby by NASA's Cassini spacecraft in July 2005, the jets inspired speculation by planetary scientists that they were geysers - violent explosions of water out of a vent caused by expanding bubbles of water vapor emanating from an ocean beneath the icy crust of Enceladus.

Scientists hypothesized that if such an ocean did exist, it might provide a suitable environment for primitive life forms, said Professor Nicholas Schneider of the University of Colorado's Laboratory for Atmospheric and Space Physics, who led one study. "We wondered if there was an ocean underneath that crust, and wondered if it is just spraying out through cracks like a geyser boiling away into space," he said.

To test the theory, Schneider and his colleagues performed experiments to find the relative content of sodium in the water vapor component of the jets. If the jets were geysers originating from an underground ocean, then the sodium content in the water vapor should be high.

The world's best telescopes can detect even a small number of sodium atoms orbiting Saturn. Observations by Schneider's team using the 10-meter Keck 1 telescope and the four-meter Anglo-Australian telescope, however, demonstrated that few if any sodium atoms existed in the water vapor.

However, a companion study has concluded there is at least some salt in the particles in the plume, suggesting the particles may have come from an ocean after all.

A team led by Nikolai Brilliantov of the University of Leicester studied data from the Cosmic Dust Analyzer (CDA) onboard the Cassini spacecraft and have combined this data with laboratory experiments. Their conclusion is that the icy grains in the Enceladus plume do indeed contain substantial quantities of sodium salts, implying a salty ocean deep below.

The scientists were able to relate the detected salt in the CDA with the likely concentration in the water vapour above the ocean, showing the consistency of the experimental data. The results of the study imply that the concentration of sodium chloride in the ocean can be as high as that of Earth's oceans.

One possible explanation for the contrasting results from the two studies is that deep caverns may exist where water evaporates slowly, said Schneider. When the evaporation process is slow the vapor contains little sodium, just like water evaporating from the ocean. The vapor turns into a jet because it leaks out of small cracks in the crust into the vacuum of space.

"Only if the evaporation is more explosive would it contain more salt," said Schneider. "This idea of slow evaporation from a deep cavernous ocean is not the dramatic idea that we imagined before, but it is possible given both our results so far."

But Schneider also cautions that several other explanations for the jets are equally plausible. "It could still be warm ice vaporizing away into space. It could even be places where the crust rubs against itself from tidal motions and the friction creates liquid water that would then evaporate into space," he said.

"These are all hypotheses but we can't verify any one with the results so far," said Schneider. "We have to take them all with, well, a grain of salt."

Both studies are published in Nature.

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