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Pluto's giant moon Charon could be hiding a once warm ocean under the surface, Nasa has claimed.

Researchers analysing cracks on the surface of the frozen moon say it points to the existence of strong, warm, tidal oceans beneath.

The discovery could help in the hunt for alien life, for which the presence of  water is key.

This artist concept shows Pluto and some of its moons, as viewed from the surface of one of the moons. Pluto is the large disk at center. Charon is the smaller disk to the right - and could have large underground oceans.

If the surface is cracked, analysis of the fractures could reveal if its interior was warm, perhaps warm enough to have maintained a subterranean ocean of liquid water, according to a new Nasa-funded study.

'Our model predicts different fracture patterns on the surface of Charon depending on the thickness of its surface ice, the structure of the moon's interior and how easily it deforms, and how its orbit evolved,' said Alyssa Rhoden of NASA's Goddard Space Flight Center in Greenbelt, Maryland.

'By comparing the actual New Horizons observations of Charon to the various predictions, we can see what fits best and discover if Charon could have had a subsurface ocean in its past, driven by high eccentricity.'

Other moons around the gas giant planets in the outer solar system have cracked surfaces with evidence for ocean interiors – Jupiter's moon Europa and Saturn's moon Enceladus are two

This is a mosaic of images showing cracks in Saturn's moon Enceladus taken by the Cassini spacecraft during its close flyby on March 9 and July 14, 2005. Researchers say the cracks also hint at underground oceans, which could harbour life

As Europa and Enceladus move in their orbits, a gravitational tug-of-war between their respective parent planets and neighboring moons keeps their orbits from becoming circular.

Instead, these moons have eccentric (slightly oval-shaped) orbits, which raise daily tides that flex the interior and stress the surface. It is thought that tidal heating has extended the lifetimes of subsurface oceans on Europa and Enceladus by keeping their interiors warm.

In Charon's case, this study finds that a past high eccentricity could have generated large tides, causing friction and surface fractures.

The moon is unusually massive compared to its planet, about one-eighth of Pluto's mass, a solar system record. It is thought to have formed much closer to Pluto, after a giant impact ejected material off the planet's surface.

The material went into orbit around Pluto and coalesced under its own gravity to form Charon and several smaller moons.

This is a close-up view of the fractures on Enceladus taken by Cassini during its flyby Nov. 21, 2009. The area, about 504 kilometers (313 miles) across, focuses on Baghdad Sulcus, a fracture in the south polar region.

'Depending on exactly how Charon's orbit evolved, particularly if it went through a high-eccentricity phase, there may have been enough heat from tidal deformation to maintain liquid water beneath the surface of Charon for some time,' said Rhoden.

'Using plausible interior structure models that include an ocean, we found it wouldn't have taken much eccentricity (less than 0.01) to generate surface fractures like we are seeing on Europa.'

Since liquid water is a necessary ingredient for known forms of life, the oceans of Europa and Enceladus are considered to be places where extraterrestrial life might be found.

However, life also requires a useable energy source and an ample supply of many key elements, such as carbon, nitrogen, and phosphorus.

It is unknown if those oceans harbor these additional ingredients, or if they have existed long enough for life to form.

The same questions would apply to any ancient ocean that may have existed beneath the icy crust of Charon.