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Ocean of water found on Saturnian moon

Water world An ocean of liquid water exists below the frozen surface of Saturn's icy moon Enceladus, according to new data from the Cassini spacecraft.

The discovery, reported in the journal Science, explains the source of the huge plumes of water seen spewing from geyser-like fractures called tiger stripes.

According to the study's lead author, Professor Luciano Iess of Sapienza University in Rome, the new finding allows scientists to quantify the amount of water, and its distribution under the surface.

"Everybody was expecting to find liquid water, but we didn't know if it was a huge subsurface ocean, or in tiny pockets layered below the surface, in which case it could have been a lot less," says Iess.

Iess and colleagues compared the newly discovered subsurface ocean to North America's Lake Superior, which contains about 12,000 cubic kilometres of water, and is the second largest lake on Earth.

The Enceladus ocean is between eight and ten kilometres deep, and located about 30 to 40 kilometres below the moon's southern polar region.

"Our data is consistent with an ocean extending up to about 50° south latitude, but we cannot exclude that the ocean continues up to the north pole, which would be a global ocean in this case," says Iess.

The water is kept liquid by a process called tidal heating, which occurs as Enceladus is gravitationally stretched and pulled during its orbit around Saturn.

Gravitational anomalies

Iess and colleagues used Doppler data from three of Cassini's flybys, which brought the spacecraft within 100 kilometres of the moon's surface, to determine the gravity field of Enceladus and to explore the asymmetry between its northern and southern hemispheres.

"We measured tiny changes in the spacecraft's velocity as it flew past the moon ... especially at the south pole," says Iess.

"Enceladus isn't exactly round, there's a depression in the southern polar region of about a kilometre, and when you have missing mass, you see less gravity."

"However, the negative gravity anomaly wasn't as strong as expected from the missing mass, so there must be some denser material there, and liquid water which is about seven per cent denser than water ice is the most likely explanation."

Chemical soup

The Cassini data also confirm that Enceladus is a differentiated body, with a 200-kilometre radius low-density rocky core beneath its subsurface ocean.

Earth and the Jovian moon Europa are the only other places in the solar system where liquid water is in direct contact with rock

This raises the possibility of a chemical soup developing with mineral nutrients mixing into the water, which wouldn't happen if the subsurface ocean was only surrounded by ice.

"There is a rich chemistry likely taking place, which is much more exciting than if you just had liquid water and ice with no chemical reactions, which may be the case for Ganymede, Callisto and Titan," says Iess.

"The prospect of a chemical reaction between rock and water makes Enceladus a very exciting and interesting object from the point of view of astrobiological research."