NASA’s Cassini team has discovered a global ocean beneath the frozen surface of Saturn’s moon Enceladus.

Credits: NASA/JPL-Caltech
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

Back in 2005 the Cassini spacecraft observed geysers jetting into space from the moon’s south pole, and scientists had more recently confirmed the presence of a sub-surface sea in that region.  This discovery indicates that the ocean of liquid water encapsulates the moon’s rocky core.

The discovery is the result of analysis of years of imagery taken by the Cassini spacecraft.  Scientists carefully mapped Enceladus’ surface features across hundreds of images in order to make extremely precise measurements of the moon’s rotation.

They observed the moon had a small wobble in its orbit around Saturn, called a libration. Models showed the libration’s characteristics could only be explained if the moon’s icy surface was not moving in sync with its rocky core (which comprises most of its mass), and therefore a moon-wide liquid ocean must be separating the two.

“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.” – Peter Thomas, Cornell University

Enceladus is one of a handful of bodies in the Solar System which are believed to have oceans of liquid water.  Jupiter’s moon Europa is the best-known – scientists think it might have a large subsurface saltwater ocean beneath its icy crust, warmed by the giant planet’s gravity squeezing the moon, heating it in the process.

A free-moving liquid water ocean (as opposed to one where isolated pockets of water are sandwiched in between layers of ice) opens up the possibility that the moons could potentially support life (though whether life actually exists on Europa or Enceladus is extremely uncertain).

Jupiter’s large moons Ganymede and Callisto may also have trapped seas of liquid water, as may two more of Saturn’s Moons, Titan and Mimas, and the dwarf planet Ceres.

The Cassini mission launched from Cape Canaveral in October of 1997.  It took nearly seven years to reach the Saturnian system, where it has been operating now for 11 years.  In that time the truck-sized spacecraft has launched a European-made probe, named Huygens, which parachuted into Titan’s thick atmosphere, landing on the surface and capturing the first and only images from the huge moon’s surface.

Cassini has also taken measurements and imagery of Saturn and its dozens of moons, and even captured a sequel to Voyager 1’s famous ‘Pale Blue Dot‘ portrait of Earth: NASA’s Cassini spacecraft has captures the majesty of Saturn’s rings and Earth (indicated by arrow) in the same frame. Credit: NASA/JPL-Caltech/Space Science Institute Dramatic plumes, both large and small, spray water ice out from many locations along the famed “tiger stripes” near the south pole of Saturn’s moon Enceladus. Credit: NASA/JPL/Space Science Institute

The Cassini mission has been extended several times since its four-year primary mission was completed on 30 July 2008.  The current ‘Cassini Solstice’ mission began in 2010 and is expected to last until 2017.