Saturn's largest moon, Titan.

Saturn's largest moon, Titan, may have an ocean under its surface. Photo: AP/NASA

Scientists have reported on the strongest sign yet that Saturn's giant moon may have a salty ocean beneath its chilly surface.

If confirmed, it would catapult Titan into an elite class of solar system moons harbouring water, an essential ingredient for life.

Titan boasts methane-filled seas at the poles and a possible lake near the equator. And it has long been speculated that Titan contains a hidden liquid layer, based on mathematical modelling and electric field measurements made by the Huygens spacecraft that landed on the surface in 2005.

The latest evidence is still indirect, but some scientists said it probably was the best that could be obtained short of sending a spacecraft to drill into the surface. The research looked convincing, said Gabriel Tobie of France's University of Nantes. ''If the analysis is correct, this is a very important finding.''

The finding by international researchers was released by the journal Science. The scientists pored over data from the orbiting Cassini spacecraft, which flew by Titan six times between 2006 and last year and took gravity measurements for a glimpse of its interior.

They found Titan was squeezed and stretched depending on its orbit around Saturn, suggesting the presence of a buried ocean. If Titan were solid rock and ice, such deformations would not occur.

''Titan is quite squishy,'' noted Jonathan Lunine of Cornell University, part of the research team.

Previous estimates suggested the ocean could be 48 to 99 kilometres deep with traces of ammonia.

Titan is one of the few worlds in the solar system with a significant atmosphere, and an underground ocean could help explain how Titan replenishes methane in its hazy atmosphere.