'Underground' ocean confirmed on Jupiter's largest moon

© NASA, ESA, G.Bacon (StScl)

An artist's impression of aurora on Ganymede.

Scientists using the Hubble Space Telescope have confirmed that the Jupiter-orbiting moon Ganymede has an ocean beneath its icy surface.

The finding resolves a mystery about the largest moon in the solar system after NASA's now-defunct Galileo spacecraft provided hints that Ganymede has a sub-surface ocean during exploration of Jupiter and its moons from 1995 to 2003.

Scientists say it took some detective work to confirm the discovery.

Like Earth, Ganymede has a liquid iron core that generates a magnetic field, though Ganymede's field is embedded within Jupiter's magnetic field. That sets up an interesting dynamic with telltale visuals -- twin bands of glowing aurora around Ganymede's northern and southern polar regions.

As Jupiter rotates, its magnetic field shifts, causing Ganymede's aurora to rock. Scientists measured the motion and found it fell short. Using computer models, they realised that a salty, electrically conductive ocean beneath the moon's surface was counteracting Jupiter's magnetic pull.

"Jupiter is like a lighthouse whose magnetic field changes with the rotation of the lighthouse. It influences the aurora," says geophysicist Joachim Saur, with the University of Cologne in Germany.

"With the ocean, the rocking is significantly reduced," says Saur.

Scientists ran more than 100 computer models to see if anything else could be having an impact on Ganymede's aurora. They also repeated the seven-hour, ultraviolet Hubble observations and analysed data for both belts of aurora.

"This gives us confidence in the measurement," says Saur.

Astounding demonstration

NASA Planetary Science Division director Jim Green says the finding is "an astounding demonstration."

"They developed new approach to look inside a planetary body with a telescope," says Green.

Ganymede joins a growing list of moons in the outer solar system with sub-surface water.

Earlier this week scientists reported that Saturn's moon Enceladus may have hot springs beneath its icy crust. Other water-rich worlds include Jupiter moons Europa and Callisto.

Scientists estimate Ganymede's ocean is 100 kilometres thick, 10 times deeper than Earth's oceans, and is buried under a 150-kilometre crust of mostly ice.

"It is one step further toward finding that habitable, water-rich environment in our solar system," says astronomer Heidi Hammel of the Association of Universities for Research in Astronomy.