Planetary researchers with NASA’s Cassini mission have found a large body of water beneath the surface of Saturn’s moon Enceladus.

The team, led by Dr Peter Thomas of Cornell University, found the magnitude of the Enceladus’ 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.

The finding implies the fine spray of water vapor, icy particles and simple organic molecules the Cassini spacecraft has observed coming from fractures near the moon’s south pole is being fed by this huge reservoir of liquid water.

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 – prove that that regional sea is a widening of a global ocean.

“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 Dr Thomas, lead author of a paper published in the journal Icarus.

Dr Thomas and co-authors analyzed more than seven years’ worth of images of Enceladus taken by Cassini.

They carefully mapped the positions of features on the moon 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 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 researchers plugged their measurement of the wobble, called a libration, into different models for how the moon 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. This proves that there must be a global layer of liquid separating the surface from the core,” said co-author Dr Matthew Tiscareno of the SETI Institute.

The mechanism that might have prevented the ocean from freezing remains a mystery.

“This exciting discovery expands the region of habitability for Enceladus from just a regional sea under the south pole to all of Enceladus,” Dr Tiscareno said.

“The global nature of the ocean likely tells us that it has been there for a long time, and is being maintained by robust global effects, which is also encouraging from the standpoint of habitability.”

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P.C. Thomas et al. Enceladus’s measured physical libration requires a global subsurface ocean. Icarus, published online September 11, 2015; doi: 10.1016/j.icarus.2015.08.037