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.
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 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.”
At 5,200 kilometers across, Ganymede is bigger than the planet Mercury. It’s the only moon in the solar system that generates its own magnetic field, propelled by a rotating liquid iron core.
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.”
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.”
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.
“Ganymede’s ocean is lesser on the probability scale of where we would expect life to exist or where we could search for it,” Pappalardo says. “Nevertheless, understanding Ganymede is very important in comprehension of the range of icy satellite ocean environments that are possible, and to our understanding of how icy satellites and their oceans form and evolve.”