Enceladus’ Global Subsurface Ocean Confirmed

For years there has been talk of a subsurface ocean present within Saturn’s icy moon Enceladus.  Many have simply assumed it to be fact, but the reality is, something so complex on a world so far away is very difficult to prove conclusively.  But now, using data from over a decade of observations by the Cassini spacecraft, mission scientists have shown conclusively that Enceladus must have a global ocean beneath a surface of ice.

Illustration of the interior of Saturn’s moon Enceladus showing a global liquid water ocean between its rocky core and icy crust. Thickness of layers shown here is not to scale. Credit: NASA/JPL-Caltech

Previous data analysis suggested that there was a lens-shaped liquid ocean beneath the south polar region of the planet, giving a source for the observed plumes of water vapour shooting out from the Moon.  With the new result, the entire planet contains a liquid ocean, encased by a thick layer of ice frozen by the cold of space.

“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 Peter Thomas, a Cassini imaging team member at Cornell University, Ithaca, New York, and lead author of the paper.

By observing over a decade’s worth of images from the Cassini spacecraft, researchers looked closely at surface features on the surface of Enceladus, and used them to carefully calculate the moon’s precise rotation and look for any subtle changes.

The hard work paid off, as they found that Enceladus has a tiny wobble as it orbits around Saturn.  This is because the moon is not quite spherical, nor does it have a circular orbit.  Based on this observed wobble, the team could produce simulations of Enceladus while varying its composition, trying out every possibility for the moon’s interior, including a solid ball of ice.

“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 Cassini participating scientist at the SETI Institute, Mountain View, California, and a co-author of the paper. “This proves that there must be a global layer of liquid separating the surface from the core.”

This is a triumph for the Cassini mission, and it provides a solid site for future searches for life within the solar system.  The next steps will be to look at the data and determine how a subsurface ocean could form, and how thick the layer of water truly is, though estimates suggest that Enceladus has more water than the Earth.

On October 28th, Cassini will celebrate by diving deeply through the plumes of icy material shooting out of the South pole of Enceladus, at an altitude of only 50 Km above the surface.  What will the next decade of discoveries bring for Cassini?

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