[meteorite-list] Cassini Finds Global Ocean in Saturn's Moon Enceladus

[Ron Baalke]

September 15, 2015

RELEASE 15-188

Cassini Finds Global Ocean in Saturn's Moon Enceladus

A global ocean lies beneath the icy crust of Saturn's geologically active 
moon Enceladus, according to new research using data from NASA's Cassini 
mission.

Researchers found the magnitude of the moon's very slight wobble, as it 
orbits Saturn, can only be accounted for if its outer ice shell is not frozen 
solid to its interior, meaning a global ocean must be present.

The finding implies the fine spray of water vapor, icy particles and simple 
organic molecules Cassini has observed coming from fractures near the moon's 
south pole is being fed by this vast liquid water reservoir. The research is 
presented in a paper published online this week in the journal Icarus.

Previous analysis of Cassini data suggested the presence of a lens-shaped 
body of water, or sea, underlying the moon's south polar region. However, 
gravity data collected during the spacecraft's several close passes over the 
south polar region lent support to the possibility the sea might be global. 
The new results -- derived using an independent line of evidence based on 
Cassini's images -- confirm this to be the case.

"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.

Cassini scientists analyzed more than seven years' worth of images of 
Enceladus taken by the spacecraft, which has been orbiting Saturn since 
mid-2004. They carefully mapped the positions of features on Enceladus -- 
mostly craters -- across hundreds of images, in order to measure changes in 
the moon's rotation with extreme precision.

As a result, they found Enceladus has a tiny, but measurable wobble as it 
orbits Saturn. Because the icy moon is not perfectly spherical -- and because 
it goes slightly faster and slower during different portions of its orbit 
around Saturn -- the giant planet subtly rocks Enceladus back and forth as it 
rotates.

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 from surface to core.

"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," he said.

The mechanisms that might have prevented Enceladus' ocean from freezing 
remain a mystery. Thomas and his colleagues suggest a few ideas for future 
study that might help resolve the question, including the surprising 
possibility that tidal forces due to Saturn's gravity could be generating 
much more heat within Enceladus than previously thought.

"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 
imaging team lead at Space Science Institute (SSI), Boulder, Colorado, and 
visiting scholar at the University of California, Berkeley. "Cassini has been 
exemplary in this regard."

The unfolding story of Enceladus has been one of the great triumphs of 
Cassini's long mission at Saturn. 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 more recently, in 
2015, they shared results that suggest hydrothermal 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 a mere 30 miles (49 kilometers) above the moon's 
surface.

The Cassini-Huygens mission is a cooperative project of NASA, ESA (European 
Space Agency) and the Italian Space Agency. NASA's Jet Propulsion Laboratory 
(JPL) in Pasadena, California, manages the mission for the agency's Science 
Mission Directorate in Washington. JPL is a division of the California 
Institute of Technology in Pasadena, California. The Cassini imaging 
operations center is based at Space Science Institute.

For more information about Cassini, visit:

http://www.nasa.gov/cassini

and

http://saturn.jpl.nasa.gov

-end-


Dwayne Brown / Laurie Cantillo
Headquarters, Washington
202-358-1726 / 202-358-1077
dwayne.c.brown at nasa.gov / laura.l.cantillo at nasa.gov

Preston Dyches
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-7013
preston.dyches at jpl.nasa.gov