By examining tiny wobbles of Saturn's moon Enceladus—whose cosmic quavers are detectable only in high-resolution images taken by NASA's Cassini spacecraft—planetary scientists have discovered that a global ocean lies beneath the moon's thick icy crust.
For a new study, researchers analyzed more than seven years worth of Enceladus images taken by the spacecraft, which has been orbiting Saturn since mid-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 research scientist at Cornell University.
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.
With each Cassini photographic pass, the scientists painstakingly pinpointed and measured the moon's topographic features—about 5,800 points in all—by hand.
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.
"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," says Matthew Tiscareno, who left Cornell this summer to join the SETI Institute in Mountain View, California. "This proves that there must be a global layer of liquid separating the surface from the core."
"We're just at the start of learning that Enceladus is incredibly interesting," says Joe Burns, professor of engineering and of astronomy. "Thanks to great spacecraft like Cassini and exquisitely fine measurements, we're seeing things not possible 20 years ago."
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency, and the Italian Space Agency. NASA's Jet Propulsion Laboratory in Pasadena, California, manages the mission.