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.