Vast Ocean Underlies Ice on Saturn's Moon Enceladus
September 18, 2015
A global ocean lies beneath the icy crust of Saturn's moon Enceladus, according to new research using data from NASA's Cassini space probe.
Researchers concluded that the moon's distinctive, tiny wobble must be due to a body of liquid sitting beneath the outer ice shell, all the way around.
This ocean layer provides a squishiness that enables the moon's icy crust to wobble somewhat independently of its hard core, they argue; it also feeds a spray of vapor, icy particles and simple organic molecules that burst from cracks near the south pole.
The research is presented in a paper published online this week in the journal Icarus.
Previous analysis of Cassini data suggested a lens-shaped body of water might underlie the south polar region.
But gravity data collected during the spacecraft's several close passes over that area suggested the sea might be global. The new results-derived using an independent line of evidence based on Cassini's images-confirm this, the investigators argued.
“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 in Ithaca, New York, and lead author of the paper.
Cassini scientists analyzed more than seven years' worth of images of the geologically active moon, snapped by the spacecraft, which has been orbiting Saturn since mid-2004. They mapped the features on Enceladus-mostly craters-across hundreds of images, in order to measure changes in the moon's rotation very exactly.
The result showed that Enceladus wobbles very slightly as it orbits Saturn. Because the icy moon isn't perfectly round-and because it goes slightly faster and slower during different parts of its orbit around Saturn-the giant ringed planet subtly rocks Enceladus back and forth as it spins.
The team plugged their measurement of the wobble, called a libration, into different models for how Enceladus might be arranged on the inside, including ones in which the moon was frozen all the way to the core.
But if that were the case, “the core would provide so much dead weight the wobble would be far smaller,” 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.”
The mystery is why the ocean isn't frozen, he added. Thomas and his colleagues suggest a few ideas, including the surprising possibility that tidal forces due to Saturn's gravity could be generating much more heat within Enceladus than previously thought.
Scientists first detected signs of the moon's icy plume in early 2005, and followed up with a series of discoveries about the material gushing from warm fractures near its south pole. They announced strong evidence for a regional sea in 2014, and this year, they shared results that suggest hydrothermal, or heating, activity is taking place on the ocean floor.
Cassini is scheduled to make a close flyby of Enceladus on Oct. 28, in the mission's deepest-ever dive through the moon's active plume of icy material. The spacecraft will pass just 30 miles (49 kilometers) above the moon.
The Cassini-Huygens mission is a project of NASA, the European Space Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory in Pasadena, Calif., manages the mission for the agency's Science Mission Directorate in Washington.
Source : http://www.world-science.net
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