Possible salty ocean hidden in depths of Saturn moon

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 25 June, 2009

NASA’s Cassini spacecraft has provided strong evidence that Saturn’s moon Enceladus harbours a salty ocean below its icy shell, a discovery that offers exciting possibilities in the search for extraterrestrial life.

Following a flyby of Saturn’s enigmatic moon in 2005 that revealed jets shooting out of the moon’s so-called tiger stripes at the south pole, scientists have speculated about the presence of a water reservoir hidden at depth within the moon’s interior. The jets contain water vapour, gas and tiny grains of ice and dust, and shoot hundreds of kilometres into space.

Jets of water vapour, ice and dust grains leap hundreds of kilometres into space from fractures on the surface of Enceladus, but what is powering them – a global ocean or deep caves? Researchers in today's edition of the journal Nature present the evidence. Image: Cassini Imaging Team/SSI/JPL/ESA/NASA.

European researchers, including Juergen Schmidt of the University of Potsdam, Nikolai Brilliantov of the University of Leicester and Frank Postberg of the University of Heidelberg and the Max Planck Institute for Nuclear Physic, analysed data from Cassini’s Cosmic Dust Analyzer (CDA), revealing that the instruments tasted sodium salts among the dust ejected in the plumes. Combined with laboratory experiments, the research team are confident that the findings point to a salty ocean that must exist at some depth beneath the moon’s icy shell.

“Some estimates based on the heat flux from the surface of the satellite suggest that the ocean should not be less than a few hundred meters and not more than a few kilometres,” says Brilliantov.

Moreover, the salt content could be as high as that found in Earth’s oceans, some 0.1-0.3 moles of salt per kilogram of water. “We believe that the salty minerals deep inside Enceladus washed out from rock at the bottom of a liquid layer,” says Postberg, lead author of the paper that appears in today’s issue of the journal Nature.

In a second paper also featured in today’s edition of Nature and lead by Nicholas Schneider of the University of Colorado at Boulder, astronomers used the 10-metre Keck telescope and the four-metre Anglo Australian telescope to look for sodium in the plumes. Sodium is detectable by space- and ground-based telescopes by the same yellow light that is emitted by street lights, but the team did not find any indication of sodium at Enceladus, contesting the idea of a global salty ocean.

The jets emanate from the so-called tiger stripe fractures carved into the south pole region of Enceladus. Image: NASA/JPl/SSI.

Brilliantov says that since Cassini detected a low abundance of salt anyway, “the predicted amount of sodium is much too small to be detected by the Keck Telescope.”

Schneider suggests that instead of explosive geysers – violent
explosions of water emanating from a vent and caused by expanding bubbles of water vapour rising up from an ocean – deep caverns may feed the plumes. “Only if the evaporation is more explosive would it contain more salt,” says 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.”

He also speculates on other explanations for the jets, such as warm ice vaporising 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 says. “These are all hypotheses but we can’t verify any one with the results so far. We have to take them all with, well, a grain of salt.”

Aside from Earth, Mars and Jupiter’s moon Europa, Enceladus is one of the only places in the Solar System for which there is direct evidence for the presence of water. Furthermore, the three main ingredients for life – liquid water, energy and suitable chemical building blocks – are all present on Enceladus. Combined with the latest results, scientists can take another tantalizing step closer in the search for life elsewhere in our Solar System.

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