Ten years ago the Cassini space probe first discovered mysterious geysers of water erupting from the polar regions of one of Saturn’s moons, the ice-covered Enceladus. After much analysis, last year NASA scientists said there was evidence that this water was coming from a sub-surface ocean beneath the pole. But today, they have revised that conclusion – and now believe that there is a global ocean beneath the icy crust of the Saturnian moon.
In a paper published online this week in the journal Icarus, the researchers outline how a very slight wobble in the movement of Enceladus can only be accounted for if the outer, visible ice shell is not frozen solid to the moon’s core:
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, Ithaca, New York, and lead author of the paper.
Cassini scientists analyzed 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.”
The mystery that remains is how this ocean remains in liquid form, rather than freezing. The researchers think it’s possibly due to heat from tidal forces generated by Saturn’s massive presence nearby.
And the question that is posed by the discovery is: if there is a massive water ocean beneath the surface of Enceladus, could there be alien life living there, out of sight of our prying eyes?
Cassini will make a super-close flyby of Enceladus in late October, passing a mere 30 miles/50 km above the moon’s surface through the icy plumes erupting from within.