'Global ocean' discovered on Saturn's moon

Cassini data continues to prompt new discoveries about Saturn and its moons. The latest revelation is of a global ocean lying beneath Enceladus's icy crust.

Annotated diagram showing the global ocean lying beneath the moon's surface
Credit: NASA/JPL-Caltech

A vast, global ocean lies underneath the icy crust of Saturn’s moon Enceladus, according to data sent back by NASA’s Cassini mission.

The discovery was made by researchers who determined the ocean’s existence due to the magnitude of the moon’s wobble as it orbits its host planet.

This movement can only be accounted for if its outer ice shell is not frozen solid, leading to the conclusion that a vast ocean lies beneath its surface.

Cassini had previously observed a spray of water vapour and icy particles emerging from fractures near Enceladus’s south pole. This latest study suggests that these phenomena are being fed by the underlying ocean.

The spacecraft’s previous analysis of the moon had already implied the presence of a body of water under the south polar region, but new gravity data collected by Cassini during its close passes suggested this could in fact be a global ocean. This has now been confirmed.

The conclusion was reached by scientists working on the Cassini mission who studied over seven years’ worth of images taken by the spacecraft, which has been orbiting Saturn since 2004.

"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," says Peter Thomas, a Cassini imaging team member at Cornell University and lead author of the paper.

The study revealed Enceladus has a tiny wobble as it orbits the ringed planet. The moon is not perfectly spherical and, because it speeds up and slows down during different sections of the orbit, Saturn rocks the icy moon back and forth as it rotates.

"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," says Matthew Tiscareno, a Cassini participating scientist at the SETI Institute and a co-author of the paper. "This proves that there must be a global layer of liquid separating the surface from the core."

However, it remains a mystery how Enceladus’s ocean has not frozen completely. A few suggestions have been posited, including the possibility that tidal forces caused by Saturn’s gravity could be generating heat within Enceladus that is preventing a solid freeze.

On 28 October, Cassini will make a close flyby of Enceladus, passing just shy of 50km above the moon’s surface.


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