Scientists have long suspected that Saturn’s tiny moon Enceladus harbored a subsurface ocean. A new study published in the journal Icarus and based on data collected by NASA’s Cassini spacecraft, indicates the ocean covers the entire globe.

Over the past decade, the Cassini spacecraft has been busy studying the Saturnian system, providing unprecedented data and images of the gas giant and its many moons. Prior to its arrival, scientists predicted Enceladus to be an inactive, somewhat boring world comprised mainly of rock and ice. However, when the spaceprobe got its first look at Enceladus, scientists realized the tiny moon was anything but boring. Hydrothermal vents were a surprising discovery in 2005, and just last year Cassini was able to link them to a subsurface ocean in the moon’s south polar region.

For the new study, researchers analyzed over seven years’ worth of Cassini images, meticulously measuring changes in the moon’s rotation. They determined that Enceladus wobbles slightly as it orbits Saturn. Different models were used to determine the source of the wobble – officially known as a libration – and the team determined that the only explanation was that a global ocean separated the moon’s rocky core from its outer icy crust.  

"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 co-author of the paper, in a statement. "This proves that there must be a global layer of liquid separating the surface from the core."

This means that the plumes spewing from Enceladus’ south pole are being sustained by a vast, global ocean. Despite previous examination of Cassini data suggesting that a subsurface south polar sea was feeding the jets, gravity data collected during several of Cassini’s close flybys of the area hinted that the ocean might be more widespread. The new data confirms it. 

"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 lead author Peter Thomas.

With temperatures reaching lows of minus 201 degrees Celsius (minus 330 degrees Fahrenheit), one would expect any water on the tiny moon to freeze solid, yet all evidence points to the presence of liquid water. How is this possible? The researchers aren't sure, but suggest tidal forces from Saturn’s massive gravity could be generating enough heat within the moon to keep the ocean from freezing completely.

"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," said co-author Carolyn Porco. "Cassini has been exemplary in this regard."

Cassini is set to end its mission in 2017, but not before completing the closest flyby yet of the tiny moon. Next month, on October 28, Cassini is expected to fly just 49 kilometers (30 miles) above Enceladus’ south pole, right through the heart of the plumes. The data collected during this deep dive will hopefully tell us more about the geological processes taking place.

Despite being a frigid world, Enceladus may be a good place to look for life beyond Earth. Finding a global subsurface ocean is a big deal. Enceladus’ core could be rich in molecules like phosphorus and sulphur, and having direct contact with water allows all sorts of chemical reactions – maybe even the kind that lead to life – to potentially take place.