Saturn's icy moon Enceladus harbours a vast ocean of liquid water beneath an icy crust that may be capable of supporting life, NASA scientists say.
Gravity measurements suggest a large, possibly regional, ocean about 10 kilometres deep, beneath an ice shell about 30 to 40 kilometres thick.
NASA's Cassini spacecraft and Deep Space Network uncovered that Enceladus is home to a large underground ocean of liquid water, furthering scientific interest in the moon as a potential home to extraterrestrial microbes.
Researchers theorised the presence of an interior reservoir of water in 2005 when Cassini discovered water vapour and ice spewing from vents near the moon's south pole.
The new data provide the first geophysical measurements of the internal structure of Enceladus, consistent with the existence of a hidden ocean inside the moon.
"As the spacecraft flies by Enceladus, its velocity is perturbed by an amount that depends on variations in the gravity field that we're trying to measure," said Sami Asmar of NASA's Jet Propulsion Laboratory in Pasadena, California, a coauthor of the paper.
"We see the change in velocity as a change in radio frequency, received at our ground stations here all the way across the solar system," said Asmar.
The subsurface ocean evidence supports the inclusion of Enceladus among the most likely places in our solar system to host microbial life.
Before Cassini reached Saturn in July 2004, no version of that short list included this icy moon, barely 500 kilometres in diameter.
"This then provides one possible story to explain why water is gushing out of these fractures we see at the south pole," said David Stevenson of the California Institute of Technology, Pasadena, one of the paper's co-authors.
The technique of analysing a radio signal between Cassini and the Deep Space Network can detect changes in velocity as small as less than one foot per hour.
With this precision, the flyby data yielded evidence of a zone inside the southern end of the moon with higher density than other portions of the interior.
The south pole area has a surface depression that causes a dip in the local tug of gravity.
However, the magnitude of the dip is less than expected given the size of the depression, leading researchers to conclude the depression's effect is partially offset by a high-density feature in the region, beneath the surface.
"The Cassini gravity measurements show a negative gravity anomaly at the south pole that however is not as large as expected from the deep depression detected by the on-board camera," said the paper's lead author, Luciano Iess of Sapienza University of Rome.
"Hence the conclusion that there must be a denser material at depth that compensates the missing mass: very likely liquid water, which is seven per cent denser than ice. The magnitude of the anomaly gave us the size of the water reservoir," said Iess.
The findings are published in the journal Science.
