A ‘global’ ocean containing a vast liquid water reservoir lies beneath the icy crust of Saturn’s geologically active moon Enceladus, according to new research using data from NASA’s Cassini mission.
Researchers found the magnitude of the moon’s very slight wobble, as it orbits Saturn, can only be accounted for if its outer ice shell is not frozen solid to its interior, meaning a global ocean must be present.
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The finding implies the fine spray of water vapour, icy particles and simple organic molecules Cassini has observed coming from fractures near the moon’s south pole is being fed by this vast liquid water reservoir.
Previous analysis of Cassini data suggested the presence of a lens-shaped body of water, or sea, underlying the moon’s south polar region.
However, gravity data collected during the spacecraft’s several close passes over the south polar region lent support to the possibility the sea might be global.
The new results – derived using an independent line of evidence based on Cassini’s images – confirm this to be the case.
“This was a hard problem that required years of observations, and calculations involving a diverse collection of disciplines, but we are confident we finally got it right,” said Peter Thomas, a Cassini imaging team member at Cornell University, New York, and lead author of the paper.
Cassini scientists analysed more than seven years’ worth of images of Enceladus taken by the spacecraft, which has been orbiting Saturn since mid-2004.
They carefully mapped the positions of features on Enceladus – mostly craters – across hundreds of images, in order to measure changes in the moon’s rotation with extreme precision.
As a result, they found Enceladus has a tiny, but measurable wobble as it orbits Saturn.
Because the icy moon is not perfectly spherical – and because it goes slightly faster and slower during different portions of its orbit around Saturn – the giant planet subtly rocks Enceladus back and forth as it rotates.
The team plugged their measurement of the wobble, called a libration, into different models for how Enceladus might be arranged on the inside, including ones in which the moon was frozen from surface to core.
“If the surface and core were rigidly connected, the core would provide so much dead weight the wobble would be far smaller than we observe it to be,” said Matthew Tiscareno, a Cassini participating scientist at the SETI Institute, Mountain View, California, and a co-author of the paper.
“This proves that there must be a global layer of liquid separating the surface from the core,” said Tiscareno.
The mechanisms that might have prevented Enceladus’ ocean from freezing remain a mystery.
Thomas and colleagues suggest the possibility that tidal forces due to Saturn’s gravity could be generating much more heat within Enceladus than previously thought.
The research was published in the journal Icarus.