Ever since NASA’s Cassini spacecraft returned startling pictures of geyers shooting into space from Enceladus, scientists have wondered just how much water is buried beneath the moon’s icy surface.

“We’ve known for some time that a liquid layer is present, but not how extensive it is. Well now we do,” said Cassini imaging team chief Carolyn Porco, with the Space Science Institute in Boulder, Colorado.

By measuring tiny wobbles in Enceladus’ orbit, scientists have figured out that Enceladus has a global ocean.

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“How did they do it? By looking for a libration … a small, cyclical, back-and-forth deviation from uniform rotation … and finding that it is present and much too large to be a libration of the entire body. The conclusion: It is a libration in the thin, outer ice shell only, indicating that ice shell and rocky core are decoupled and separated by a liquid layer,” Porco posted on Facebook.

Previously, scientists suspected that Enceladus had some pockets of water beneath its surface, but no evidence the ocean may be global, similar to what is believed to exist within Jupiter’s moon Europa.

The first hint of widespread subsurface water came from gravity measurements taken in 2013 and 2014 as Cassini passed close to Enceladus’ south pole.

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That data provided evidence of a south polar sea, about 22 miles below the surface and about six miles thick, and perhaps connected to a thinner global ocean, Porco said.

Now an independent analysis, matching Enceladus’ slight wobble with computer models, confirms the theory.

The computer models show that a layer of liquid must separate the moon’s solid core from its icy surface, scientists wrote in an article to be published in the journal Icarus.

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“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,” Cassini scientist Matthew Tiscareno, with the SETI Institute in Mountain View, California, said in a statement.

“This proves that there must be a global layer of liquid separating the surface from the core,” he said.

An artist’s rendering of Enceladus’ solid core, liquid subsurface ocean and icy shell, along with hydrothermal vents in the southern pole from which geysers flow. NASA/JPL-Caltech

“This was a hard problem that required years of observations and calculations … but we are confident we finally got it right,” added Cassini scientist Peter Thomas, with Cornell University in New York.

How the ocean has managed to remain liquid is not yet known. One idea is that tidal forces from Saturn’s gravity could be generating much more heat within Enceladus than previously thought.

Cassini is scheduled to pass right through an active plume on Enceladus on Oct. 28, coming as close as 30 miles from the moon’s surface.

The geyers – 101 have been found so far — reach thousands of miles into space and contain water vapor, carbon dioxide and methane, along with a possible trace of ammonia. The gas in the plume is either carbon monoxide or nitrogen.

“These findings point to the solar system’s most accessible extraterrestrial watery environment — a habitat — within Enceladus where, perhaps, a second genesis has taken hold,” noted Porco.