Global Ocean Makes Saturn’s Moon Wobble

Planetary scientists have discovered that a global ocean lies beneath the moon’s thick icy crust, by investigating tiny wobbles of Saturn’s moon Enceladus, whose cosmic quavers are detectable only in high-resolution images taken by NASA’s Cassini spacecraft.

Researchers analyzed more than seven years worth of Enceladus images taken by the spacecraft, which has been orbiting Saturn since mid-2004. Peter Thomas, a research scientist at Cornell University, says:

“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.”

Enceladus, a geologically diverse moon, vigorously vents vapor and liquid water from fractures in its icy crust at its south polar region, as discovered early in Cassini’s exploration of the Saturn system. However, space scientists were uncertain about the extent of the subsurface water source.

Libration Was Giveaway

With each Cassini photographic pass, the scientists painstakingly pinpointed and measured the moon’s topographic features, about 5,800 points in all, by hand.


Illustration of the interior of Saturn’s moon Enceladus showing a global liquid water ocean between its rocky core and icy crust. Thickness of layers shown here is not to scale. (Credit: NASA/JPL-Caltech)

A slight wobble, about a tenth of a degree, was detected, but even this small motion, called a libration, is far larger than if the surface crust were solidly connected to the satellite’s rocky core. Thus, the scientists determined that the satellite must have a global liquid layer, far more extensive than the previously inferred regional liquid “sea” beneath the south pole.

Matthew Tiscareno, who left Cornell this summer to join the SETI Institute, says:

“If the surface and core were rigidly connected, the core would provide so much dead weight that 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.”

Adds Joe Burns, professor of engineering and of astronomy:

“We’re just at the start of learning that Enceladus is incredibly interesting. Thanks to great spacecraft like Cassini and exquisitely fine measurements, we’re seeing things not possible 20 years ago.”

P.C. Thomas, R. Tajeddine, M.S. Tiscareno, J.A. Burnsa,  J. Joseph, T.J. Loredo, P. Helfenstein, C. Porco
Enceladus’s measured physical libration requires a global subsurface ocean
Icarus doi:10.1016/j.icarus.2015.08.037

Top Illustration: NASA/JPL/Space Science Institute