Photograph of Saturn's moon Enceladus as seen by NASA's Cassini Equinox spacecraft. The 503 km-wide moon is venting water ice from its south pole, generating four defined plumes.
Credit: NASA / Cassini
LONDON: The source of mysterious ice plumes that emanate from Saturn's moon Enceladus is very likely an ocean, according to a new study. And in one important respect this ocean is similar to those on Earth: the water is salty.
The plumes were discovered in 2005 by the space probe Cassini, which has been criss-crossing the Saturn system since 2004. An instrument on board Cassini can measure the composition of small particles that crash into a metal plate mounted on the space probe.
Publishing in the current issue of Nature, astrophysicist Frank Postberg from the University of Heidelberg in Germany, used this to determine the composition of the ice particles in the plumes during three flybys of Enceladus, at different distances.
Feeding Saturn's ring
Postberg's results show that the ice contains salts, mostly the regular table salt sodium chloride and also some sodium bicarbonate and potassium salts.
He found that the ice that was intercepted closer to the surface of the moon was saltier than ice further out, as these heavier particles tend to be slower.
Light, salt-poor particles, in contrast, easily escape the gravitational pull of Enceladus and feed the so-called 'E-ring' of Saturn.
Sweeping ice particles into space
These measurements support the view that the plume particles start out as aerosols over the surface of an ocean, around which water droplets form and freeze, said Postberg. "If it was just evaporated water from ice that formed the particles, they would be salt-poor."
The saltiness of the ocean has the same explanation as the saltiness of sea water on earth: the salts are dissolved out of rock. Enceladus is thought to have a rocky core.
The aerosols are initially expelled from the water by the bubbling up of water vapour and some other gases, like CO2 and methane. These same gases sweep the ice particles out into space.
The ocean itself can't be seen by Cassini, as Enceladus is covered in a thick layer of ice. But it's not altogether surprising that underneath that ice liquid water might exist. The southern hemisphere of the moon shows no sign of cratering, which means the surface must be very young - and keeping a surface young requires energy.
"The surface of the northern hemisphere is several billion years old, almost as old as the moon itself. The surface of the southern hemisphere is no older than a few thousand years," said Postberg.
