Scientists have reported 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 harboring water, an essential ingredient for life.

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

The latest evidence is still indirect, but outside scientists said it's probably the best that can be obtained short of sending a spacecraft to drill into the surface — a costly endeavor that won't happen anytime soon.

The research looks convincing, said Gabriel Tobie of France's University of Nantes.

"If the analysis is correct, this is a very important finding," Tobie said in an email.

Squeezing, stretching suggests buried ocean

The finding by an international team of researchers led by astronomer Luciano Iess of Sapienza University of Rome was released online Thursday by the journal Science.

The scientists pored over data from NASA's orbiting Cassini spacecraft, which flew by Titan half a dozen times between 2006 and last year and took gravity measurements for a glimpse of its interior.

This artist's concept shows a possible scenario for the internal structure of Titan with a global subsurface ocean sitting above a subsurface layer of high-pressure ice and a water-infused silicate core. (A. Tavani/NASA)

They found Titan got 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, who was part of the research team.

Scientists did not delve into the characteristics of the ocean, but previous estimates suggested it could be 50 to 100 kilometres deep and contain traces of ammonia. Titan is one of the few worlds in the solar system with a significant atmosphere, and the presence of an underground ocean could help explain how Titan replenishes methane in its hazy atmosphere.

Having an internal body of water would also make Titan an attractive place to study whether it would be capable of supporting microbial life. Other moons on the shortlist: Jupiter's Europa, where an underground ocean is thought to exist and another Saturn moon, Enceladus, where jets have been seen spewing from the surface.

"Any environment that has liquid water needs to be investigated carefully," said planetary scientist Jean-Luc Margot of the University of California, Los Angeles, who had no role in the research.