Galileo scientists reported Saturday that a salty ocean capped by perhaps 105 miles (170 kilometers) of ice best explains scientific data gathered by the spacecraft when it flew past Ganymede on May 20.

"We have now, after months and months of wrestling with the data, reached a point where we feel confident that there is a layer of water under Ganymedes icy surface," said Margaret Kivelson, a University of California, Los Angeles planetary scientist and the principal investigator for Galileos magnetometer.

Jupiter bathes Ganymede, and fellow Galilean moons Europa and Callisto, in its own magnetic field, which steadily rocks back and forth as the planet rotates.

Given the presence of an electrically conductive layer, such as salty water, within any one of the three moons, that rocking would set up a changing internal magnetic field that would flip-flop with measurable regularity.

Indeed, previous flybys by Galileo have found just that in both Europa and Callisto, providing the best evidence they hide salty oceans beneath their icy crusts.

In the case of Ganymede, however, the moon sports a strong, permanent field of its own.

That made it far more difficult to sort out any small directional changes that a salty ocean would enable in response to the rocking of Jupiters magnetic field.

However, Kivelson said the Galileo magnetometer team ultimately was able to discern a slight rocking of Ganymedes poles as Jupiters own field shifted, a result that was consistent with the presence of a global subsurface ocean capable of conducting electricity.

The ocean would stay liquid, sandwiched between layers of ice, thanks to heat from natural radioactivity in Ganymedes rocky interior.

Other evidence gathered by Galileo during its May flight within 503 miles (809 kilometers) of the solar systems largest moon supports the notion of a liquid ocean.

The flyby produced some of the highest resolution images of the moon, which show an icy surface surprisingly dominated by faulting and fracturing consistent with tectonic activity. Previously, scientists looked to volcanism to explain Ganymedes variegated surface.

The new evidence strongly resembles what Galileo has already shown to be the case on Europa, where cracks open up in the ice crust, only to be filled with new, slushy material welling up from below.

"Were starting to see some major similarities to Europa," said James Head III, a Brown University geologist.

Furthermore, recent study of the salt minerals that splotch Ganymedes mottled surface suggests that sometime in the remote past briny water may have breached the ice that now caps the moon, said Thomas McCord, a University of Hawaii planetary scientist.

Unlike the case of Europa, where water may still lurk within miles (kilometers) of its surface, Ganymedes deep-seated ocean today presents far less of a likely place to go prospecting for life, said Dave Stevenson, a Caltech planetary scientist.

However, Galileo has proven Ganymede to be a far more intriguing place than scientists believed even a few short months ago something the spacecrafts namesake foresaw upon first peering upon Jupiters largest moons.

"It really is, as Galileo proclaimed back in 1610, another planetary system," Stevenson said.

Galileo will swing past Ganymede again on Dec. 28 as it enters its sixth year in orbit about Jupiter.