Enceladus—one of Saturn's smaller satellites—has joined the ranks of Titan and Europa as a moon that appears to have liquid water splashing around inside of it, researchers say. New gravity data from the Cassini spacecraft, which has been exploring the planet's moons for 10 years, reveal that Enceladus harbors an ocean of water beneath 18 to 24 miles (30 to 40 kilometers) of ice at its surface.

A team of Italian and American scientists led by Luciano Iess at Sapienza Università di Roma in Rome, Italy investigated the moon's gravity field and the notable asymmetry it exhibits between northern and southern hemispheres to reach these conclusions. Their results appear in the 4 April issue of Science.

"Using geophysical measurements, we have been able to confirm that there is a large ocean beneath the surface of Enceladus' south-polar region," said Science co-author David Stevenson, a Marvin L. Goldberger Professor of Planetary Science at Caltech and an expert in studies of the interior of planetary bodies. "This provides a possible source for the water that Cassini has seen spewing from the geysers in this region."

The researchers analyzed Doppler data from three of Cassini's flybys, which brought the spacecraft within 62 miles (100 kilometers) of Enceladus' surface, and found that the southern polar region of the moon doesn't have enough mass at its surface to account for the hemisphere's gravity field. So, Iess and his team suggest that something dense below the surface of Enceladus—probably liquid water—must be compensating.

"This water ocean… may extend halfway or more towards the equator in every direction," Stevenson added. "This means that it is as large—or larger—than Lake Superior [in the United States]."

The researchers' findings imply that Enceladus is a differentiated body with a low-density core and a separate mantle and crust. They also help to explain the mineral-rich jets of water vapor that were first observed flowing from long, distinctive fractures in the moon's southern polar region—called tiger stripes—in 2005.

In addition, Iess and the other researchers posit that this sub-surface ocean on Enceladus sits atop silicate rock instead of ice, which means that the environment there is ripe for complex chemical reactions, including some that—with the help of an energy source—might create conditions like those on the early Earth.

"Enceladus shows some similarity to Europa—a much larger moon of Jupiter—which, like Enceladus, has an ocean that is in contact with underlying rock," said Stevenson. "In this respect these two bodies are of particular interest for understanding the presence and nature of habitable environments in our solar system."