Ever since 2005, when NASA’s Cassini Orbiter found plumes of water vapor venting out of cracks in the south pole of Saturn’s moon Enceladus, scientists have been looking to find out what lies below its icy crust. Now, we think we know: Enceladus is covered in a vast, liquid-water ocean.

The key to the discovery lies in the moon’s wobble, or “libration.” Enceladus moves slightly faster and slightly slower in different parts of its orbit around Saturn, suggesting that the crust is disconnected from its rocky interior, essentially floating atop a subterranean sea. “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,” explains team member Matthew Tiscareno. “This proves that there must be a global layer of liquid separating the surface from the core.”

Led by Peter Thomas, a Cassini imaging team member at Cornell University, the researchers analyzed hundreds of high-resolution photographs taken by the spacecraft since its launch in 2004. By carefully mapping the positions of Enceladus’ key features, they were able to measure the moon’s movements with extreme precision. “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 Thomas.

Plenty of other moons – most notably Jupiter’s sixth moon Europa – are thought to have water hiding miles below their mysterious surfaces, but that doesn’t make the discovery any less exciting. The team has now turned their attention to the bigger question: is anything swimming in this ocean?  “This is a major step beyond what we understood about this moon before, and it demonstrates the kind of deep-dive discoveries we can make with long-lived orbiter missions to other planets,” they say.

Cassini is scheduled to do a bit of moisture farming during a flyby of Enceladus next month. The probe will soar just than 49 kilometers (30 miles) above the moon’s surface, sleuthing for chemical clues as to what lies beneath.

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IMAGES: NASA, NASA Jet Propulsion Laboratory JPL-Caltech