'Best evidence' for Ganymede ocean

There is further, compelling evidence that Ganymede - the largest moon in the Solar System - has an ocean of water beneath its icy crust.

The new data comes from the Hubble Space Telescope, which has been studying how auroral lights dance around the satellite of Jupiter.

The presence of a sub-surface ocean would heighten interest in Ganymede as a potentially habitable world.

Europe's robotic Juice probe is being sent to orbit the moon in the 2030s.

Nasa's Galileo mission returned information in the early 2000s that suggested the 5,300km-wide moon had a hidden sea. The new insights from Hubble deepen that impression.

Damping effect

Ganymede's great distinction among moons - apart from its size - is that it has its own magnetic field.

Hubble has managed to track that field's behaviour by watching how it draws in and excites space particles, generating a glow of ultraviolet light around the satellite's north and south poles.

But this intrinsic magnetic field also interweaves with Jupiter's, and the aurora "rock" back and forth as a result of the interplay.

It is by modelling the expected rocking against what is observed by Hubble that scientists can infer something about the internal structure of the moon. And they now say a salty ocean at depth is the best explanation for what they see.

That is because Jupiter's field induces a secondary field in the salt water, and this tries to counterbalance the big planet's influence.

The end result is that the aurora rock only by two degrees over time when without the presence of the ocean, they should be rocking by six degrees.

Magnetic interplay

"The ocean cannot be deeper than 330km; anything deeper would not explain the data," said lead scientist Joachim Saur of the University of Cologne in Germany.

"The data are consistent with an ocean of a 100km thickness with a certain salt content of about 5g per one litre of water. But it could equally well be an ocean of only 10km but with 10 times more salt."

The idea that a sub-surface ocean exists on Ganymede is exciting because wherever you have liquid water, you have one of the main ingredients for life.

You need much more, of course - not least a source of energy and some complex carbon chemistry. But understanding the ocean will be one of the primary objectives of the European Space Agency's billion-pound Juice robot when it arrives at Ganymede in 2030.

The orbiter will have a very sensitive magnetometer instrument to study Ganymede's magnetic field in more detail, as well as a radar instrument to look beneath the icy crust. But other types of observations, like gravity measurements, should also elicit additional insights.

'Soggy' Solar System

Heidi Hammel, from the Association of Universities for Research in Astronomy, Washington DC, said the Hubble information represented the "best evidence yet" for the Ganymede's hidden ocean. Although the Galileo orbiter had strongly suggested the water volume was present, it was possible still to interpret its magnetometer information in different ways, she explained.

"There was some ambiguity, which is why the word 'putative' was still used [in relation to the ocean]. But this result takes us out of the realm of ambiguity," she told reporters.

Ganymede is just one of a large list of objects in the Solar System now thought to hide an ocean deep below the surface. These include the dwarf planets Pluto and Ceres; other Jupiter moons - Europa and Calisto; Saturn's moons Enceladus, Titan and Mimas; and possibly Neptune's moon, Triton.

"The Solar System is now looking like a pretty soggy place," joked Jim Green, the US space agency's director of planetary science.