Gravity measurements suggest the Saturnian moon Enceladus has a subsurface ocean lurking 30 kilometres below its icy exterior.
The tiny moon, which is just 500 kilometres across, is Saturn's sixth largest satellite and was discovered in 1789 by the English astronomer William Hershel. Its shiny veneer of ice means that it reflects almost all of the light that lands upon it, making it relatively easy to spot.
But what singles out this tiny icy marble as special are the plumes of salty water periodically seen issuing from the south pole of the satellite and ascending thousands of kilometres into the sky and even contributing to Saturn's rings.
Dubbed the largest geysers in the solar system, these phenomena have attracted attention because they point to the possible existence of a liquid water ocean inside Enceladus. Scientists have speculated that, as the moon orbits Saturn, it receives a gravitational "massage", triggering frictional heating of the interior and powering the plume emissions.
However, there was no direct data to support the presence of such an internal reservoir of water, until now. Luciano Less, from Sapienza University in Rome,writing in Science describes how he and his colleagues have used data from close fly-bys of Enceladus by the Cassini probe, which reached Saturn in 2004.
By tracking the trajectory of the spacecraft as it skimmed the surface of Enceladus several times at altitudes as low as 100 kilometres, the scientists were able to deduce the gravity field around the moon and thus what its internal composition must be.
The analysis shows a strong anomaly in the moon's south pole that can be explained if a large body of water, some 10 kilometres deep, is present 30-40 kilometres below the surface.
The presence of a liquid water ocean, and an energy source, could also mean Enceladus might be more hospitable to alien life that we might otherwise have believed...