Auroras wrapped around Jupiter’s moon Ganymede, the solar system’s largest moon, have confirmed that a salty ocean hides beneath the world’s surface.

View Images Ganymede, as seen by NASA’s Galileo spacecraft in 1996. (NASA/JPL)

The observations, made with the Hubble Space Telescope, add Ganymede to a long list of places in the solar system where water is abundant. They also suggest a way for scientists to remotely look for water on distant exoplanets. Described today in the Journal of Geophysical ResearchDescribed today in the Journal of Geophysical Research, the results are a slick observational feat, and the first time auroras have been used to remotely peer inside a world.

“We aren’t at Jupiter. Hubble is at the Earth, and yet it can probe the internal structure of this moon, remotely,” said Heidi Hammel of the Association of Universities for Research in Astronomy. “That’s a really powerful tool.”

Alien Auroras

View Images Auroras circling Ganymede (shown in blue) can be used to probe the moon’s interior. (NASA/ESA/J/Saur)

Finding evidence for a buried sea on Ganymede isn’t exactly surprising – scientists have suspected the moon hosted a subsurface ocean for decades. Data collected by NASA’s Galileo spacecraft, which orbited Jupiter between 1995 and 2003, strongly suggested that an ocean lived beneath the moon’s surface. But it wasn’t a done deal.

“It still was 50-50,” said study author Joachim Saur, a geophysicist at Germany’s University of Cologne.

Then, in 2010 and 2011, Hubble took a close look at the moon. More specifically, it looked at the auroral bands ringing Ganymede’s poles. Hubble studied the auroras in the ultraviolet, but Saur said the shimmering lights would be visible to human eyes.

“If somebody could be standing on Ganymede looking up into the night sky, it would appear as red aurora, to you,” he described.

The locations of Ganymede’s auroras depend on the moon’s magnetic field. But as Jupiter rotates, its immense magnetic field tugs on Ganymede’s auroras; physics predicts the auroras should wobble around the poles by about 6 degrees as Jupiter spins. Yet multiple hours of Hubble observations suggested the auroras weren’t rocking as much as they should – an effect that could only be explained by a counterbalancing, electrically conductive (i.e., salty), buried ocean.

“The aurora in all these cases moved by approximately 2 degrees, only,” Saur said. “Our new HST observations provide the best evidence to date for the existence of an ocean on Ganymede.”

View Images Jupiter’s magnetic field tugs on Ganymede’s auroras, causing them to rock around the moon’s poles. But the amount of rocking depends on whether an electrically conductive ocean is buried beneath the moon’s surface (blue lines). (NASA/ESA/A Feild)

The Hubble observations don’t offer much detail about the ocean itself, but scientists say it has to be within 330 kilometers of the surface. The ocean’s depth remains a mystery, but it could contain more water than Earth’s oceans. What’s more, observations of Ganymede’s surface terrain suggest that at some point long ago, the ocean spilled onto the moon’s surface. “We believe that there have been times when that ocean may have communicated with the surface of Ganymede in the distant past,” said Jim Green, NASA’s director of planetary sciences.

Ocean Worlds and Exo-Worlds

The new Hubble observations suggest that in the future, similar strategies and powerful space telescopes could be used to infer the presence of water on distant exoplanets. The data also officially add Ganymede to the A-list of places in the solar system in which water is abundant. It’s an esteemed list, headlined by the planet Mars, Jupiter’s moon Europa and the Saturnian satellites Titan and Enceladus. Indeed, it seems the more we look, the more water we find – particularly in the outer solar system.

“The solar system is now looking like a pretty soggy place,” Green said. “Water is really of an enormous abundance.”

View Images Ganymede’s ocean is likely sandwiched between two layers of ice, limiting the amount of chemical nutrients it might host. (NASA/ESA/A Feild)

But Ganymede’s ocean isn’t quite like the better-studied seas lying beneath the surfaces of Europa and Enceladus, both of which are tantalizing targets in the search for life beyond Earth.

Unlike those oceans, Ganymede’s sea is probably sandwiched between two layers of ice, says Robert Pappalardo, of NASA’s Jet Propulsion Laboratory. That makes it a less intriguing astrobiological target because it’s harder for rock-derived chemical energy to leach into the sea. Ganymede is also less geologically active than either Europa or Enceladus, which makes it harder for chemical nutrients from the surface to work their way into the ocean.