Enceladus, one of Saturn's smaller moons, can join the ranks of outer solar system moons that appear to have liquid water beneath their surfaces, researchers say. New gravity data from the Cassini spacecraft, which has been exploring Saturn's moons for 10 years, suggests that Enceladus houses a vast ocean of water beneath 18 to 24 miles (30 to 40 kilometers) of ice.
This discovery should help astronomers to understand the formation of planetary bodies and the distribution of water in the solar system. It may also shed some light on potentially habitable environments within the solar system, if this subsurface ocean inside Enceladus happens to be in contact with a rocky core, as the researchers who analyzed the Cassini data suggest.
Luciano Iess, a professor of aerospace engineering at the Sapienza Università di Roma in Rome, Italy, and a team of Italian and American scientists used Cassini to study the gravity field of Enceladus, intrigued by the notable asymmetry that the moon exhibits between its northern and southern hemispheres. Their findings  appear in the 4 April issue of Science.
Iess and his colleagues looked at Doppler data from three of Cassini's flybys, which brought the spacecraft within 62 miles (100 kilometers) of Enceladus' surface. They filtered the data in order to single out the moon's specific influence on the spacecraft and found that the southern polar region of Enceladus simply doesn't have enough mass at its surface to account for that hemisphere's gravity field.
After weighing a number of options, the researchers suggest that something dense below the surface of Enceladus—most likely liquid water—must be compensating for the absence of mass.
"This [underground] ocean may extend halfway or more toward the equator in every direction," said David Stevenson, a Marvin L. Goldberger Professor of Planetary Science at Caltech, during a 2 April press teleconference. "This means that it is as large, or larger, than Lake Superior [in the United States]."
The team's findings help to explain the mineral-rich jets of water vapor that were first observed in 2005, flowing from long, distinctive fractures called tiger stripes in the moon's southern polar region.
They also imply that Enceladus is a differentiated body with a low-density core and a separate mantle and crust. "There appears to be a liquid water ocean…sandwiched between ice and a rocky core," said Jonathan Lunine, from Cornell University in Ithaca, New York. "[But] we haven't [actually] discovered a rocky core. We infer its presence from the density of Enceladus."
"The configurations that we seem to see from the gravity data makes the base of this ocean very much like the base of our own oceans on Earth," he continued, "where we have liquid water and rock beneath it—and that liquid water can circulate through the rock, become warmer and pick up nutrients."
An important future goal for the Cassini mission is to "continue its tour of the Saturnian satellites up to 2016, and then in the final month before being dumped into Saturn's atmosphere, it will carry out important measurements near Saturn," said Iess.
The mission is an international collaboration between NASA, the European Space Agency (ESA), and the Italian Space Agency (ISA). "The international collaboration here has really been key to returning some of these results," said Linda Spilker, a Cassini-Huygens project scientist with the Jet Propulsion Laboratory. "Having a mission as long as Cassini has been in orbit around Saturn, for 10 years, gives us the opportunity for multiple gravity passes of Enceladus and a chance to really thoroughly study the system."