Plumes of material erupt from the long 'tiger stripe' fractures on the south pole of one of Saturn's moons, Enceladus.
Credit: NASA
SYDNEY: Saturn's icy moon Enceladus harbours a salty ocean under the surface, astronomers have discovered, making it a favourable place for alien life.
Two studies released in Nature today looked for evidence for salt in the particles ejected from a plume of water vapour and ice crystals stemming from Enceladus' south pole. The plume was first discovered by NASA's Cassini spacecraft in November 2005.
The first study, led by Frank Postberg from the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, used Cassini's Cosmic Dust Analyser to check the composition of ice and dust particles. The particles had been spread through Saturn's outermost 'E' ring, where Enceladus orbits, by the plume.
Ocean beneath the ice
The team found that most grains were pure water ice, about 6% were very salty and mixed with carbonates such as soda, in concentrations that fit with the predicted composition of an Enceladus ocean.
The salt is thought to have been washed out of an inner rocky core by a body of water locked beneath Enceladus's icy crust and ejected in plumes thousands of kilometres into space. It provides the first direct confirmation of the presence of water.
"We believe that the salty minerals deep inside Enceladus washed out from rock at the bottom of a liquid layer," said Postberg.
"If the liquid source is an ocean, that, coupled with the heat measured near the moon's South Pole and the organic compounds found within the plumes, could provide a suitable environment on Enceladus for the formation of life precursors."
'Tiger stripes' also point to ocean
Enceladus is a small, icy moon with a radius of just 247 km and a surface temperature of minus 201 degrees Celcius. Astronomers previously theorised that features on Enceladus, including the plumes and dark, 'tiger stripes' of fresh ice, were consistent with the presence of a subsurface ocean beneath the ice. What wasn't clear was whether the water was still present today, and in what form.
The body of water can't be directly below the surface, however, because there isn't enough sodium in the plume itself, discovered Nicholas Schneider, from the University of Colorado, in Boulder. Along with colleagues, he published the second study on Enceladus' ocean, also in Nature.
Schneider's team directly observed the plume using ground-based spectrometry from one of the twin 10-metre Keck telescopes in Hawaii and the four-metre Anglo-Australian telescope in Coonabarabran, NSW.
Astronomers previously thought the plume spurted out from cracks in Enceladus' icy surface, fed by a hot spot below, in the same way that geothermal activity on Earth creates geysers of superheated water.
Instead of a geyser, the lack of sodium in the plume's vapour suggests a "wide variety of alternative eruption sources, including a deep ocean, a freshwater reservoir or ice," Postberg and his colleagues wrote.
"The original picture of the plumes as violently erupting Yellowstone-like geysers is changing. They seem more like steady jets of vapour and ice fed by a large water reservoir.
"There could be something living there"
"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," said Postberg.
Astrobiologist Malcolm Walter, director of the Australian Centre for Astrobiology at the University of New South Wales, Sydney, said the find was "very significant" as it indicated at some stage there was a salty ocean on Enceladus.
"It doesn't prove it, but it is consistent [with a salty ocean] because the salt had to come from somewhere, probably the rocky core of Enceladus," he said.
The ground-based observations may not have detected sodium, but there could still be salt in the plume, because the ground-based spectrographs were searching for atomic sodium, not sodium locked up in salt, he said, "whereas the Cassini spacecraft can directly detect the salt."
"If there is a salt ocean, then there could be something living there."
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