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April 3, 2014 11:47 pm
An ocean of liquid water, potentially capable of sustaining simple life, is sloshing around inside Saturn’s ice-encrusted moon Enceladus.
US and Italian scientists used instruments on Nasa’s Cassini spacecraft, which has been orbiting Saturn since 2004, to deduce that the southern hemisphere of Enceladus contains an ocean perhaps 10km deep beneath 30km to 40km of ice. The results appear in the journal Science.
The water, sitting on an ocean floor consisting of silicate rocks, is kept liquid by tidal heating as Enceladus moves around Saturn.
“The contact between water and silicates makes this environment suitable for rich and complex chemical reactions that, in the presence of an energy source, might create prebiotic conditions” favourable for life, said Luciano Iess of Sapienza University in Rome, the project leader.
Enceladus, a small moon just 500km in diameter, joins Europa, a larger moon of Jupiter that also has an ice-bound ocean, on scientists’ list of bodies in the solar system that could sustain microbial life.
No one knew there was liquid water on Enceladus until 2005 when Cassini sent pictures back to Earth showing geysers spewing water vapour and ice from fractures in its icy surface, known as tiger stripes. But that observation was also explicable by local melting of the ice rather than the presence of a deep ocean.
Confirmation of the ocean comes from geophysical measurements of its internal structure during three subsequent fly-bys of Enceladus. Variations in gravitational field produced small variations in Cassini’s velocity as it skimmed past the moon.
When mission scientists analysed these gravitational changes, in conjunction with the surface topography, they concluded that a liquid ocean sandwiched between ice and a rocky core was the only viable explanation. It is less clear whether the ocean is global or lies only beneath the moon’s southern hemisphere.
Cassini scientists had already published intriguing evidence of the chemical signature of the water plumes emerging from the tiger stripes.
“Material from Enceladus’ south polar jets contains salty water and organic molecules, the basic chemical ingredients for life,” said Linda Spilker, Cassini’s project scientist.
“Their discovery expanded our view of the ‘habitable zone’ within our solar system and in planetary systems of other stars,” she added. “This new validation that an ocean of water underlies the jets furthers understanding about this intriguing environment.”
Cassini is scheduled to fly close to Enceladus on at least two more occasions before the mission ends in 2017, for imaging and radar observations that may provide more clues about its ice and oceans but no more gravity measurements are planned.
The ocean discovery will encourage planetary scientists who want Nasa or the European Space Agency to fund another mission to explore Enceladus more closely for signs of life but none is on the drawing board. However, both agencies are planning similar missions to Europa.
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