A Blog by

Auroras Reveal Buried Ocean on Jupiter’s Moon Ganymede

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.

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

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

(NASA/ESA/A Feild)
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.”

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

“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.”

12 thoughts on “Auroras Reveal Buried Ocean on Jupiter’s Moon Ganymede

  1. and lets not Jupiter’s other moon Callisto which is said to have a ocean as well and Saturn’s moon Titan. Uranus’s moon Titania. and Neptune’s moon Triton and even Pluto itself and beyond so there’s water everywhere the question is what kind of life is in them ? a new undiscovered type of fish or other marine life ? its endless what we might find.

  2. I’d like to understand why minerals from rocky layers couldn’t migrate through the ice to overlying liquid water – wouldn’t convection happen in the ice layers?

  3. sommes-nous vraiment certains que les planètes ont un soleil intérieur et abritent des civilisations avancées qui se protègent du cosmos , comme l’Agartha dans les régions inférieures de la Terre

  4. Google ought to get into the business of Space Travel and make it possible for everyone to do a bit of spacefaring and see exotic alien life that might lurk beneath the surface of Ganymede, and in other “soggy” planets.

  5. With the advance of our technology, we’ll be stunned to know that we on Earth are not he only life form. How incredible are the species that exist in our solar system. Then we think of what lies ahead in our Milky Way and beyound!

  6. If, as the diagram suggests, Ganymede has liquid water, would it more likely be in contact with the rocky mantle and the ice on the outside of this?
    As on Earth, ice would “float” on the denser liquid water. The rocky mantle would likely have volcanic activity and therefore a potential source of life.

  7. I have a question that no teacher would or could answer for me. In school I was taught that the planets we see are so many light years away that they appear to us as they were millions of years ago. Does that mean the closer we get to these planets they change appearance to be as they actually are today?

  8. Becky: The planets are not that far away. Jupiter ranges from 20 light-minutes to 40 light-minutes, depending on where we are in our orbit. Even Pluto is only about 5.5 light-hours. (The moon, on the other hand, is 1.3 light-seconds away.) That means we see Jupiter as it was between 20 and 40 minutes ago, Pluto as it was about 5.5 hours ago, and the moon as it was 1.3 seconds ago.

    Even the stars individually observable by the naked eye (i.e., excluding galaxies like Andromeda, where you can’t see the individual stars) are at most a few thousand light-years away, and almost all of them are much closer than that. xkcd has a good comic on this: https://xkcd.com/1342/ (And explain xkcd has some more detail: http://www.explainxkcd.com/wiki/index.php/1342)

    But to your larger point the answer is “Yes”. When New Horizons set out towards Pluto, for example, it was seeing Pluto as it was five hours ago. Over the past decade, as it has moved closer and closer, the light from Pluto has had less distance to travel to reach New Horizons and so therefore New Horizons is seeing a progressively more “up-to-date” image of Pluto. Given the relative speed of New Horizons, however (five light-hours covered in 10 years), this “fast forwarding” of Pluto would be imperceptible.

    The same thing happened to the astronauts who landed on the moon. When they left Earth, they saw the moon as it was 1.3 seconds before. By the time they reached the moon, three days later, they were seeing it almost instantaneously.

    (Note: This time delay *always* exists. It takes about 2 billionths of a second for the light from your computer monitor to reach your eyes, so you’re seeing your computer monitor as it *was*, not as it *is*. 2 billionths of a second might not sound like much but that is probably four to eight clock cycles for your computer’s CPU, which can probably complete 2-3 instructions per clock cycle. In other words, your computer’s CPU can do useful work in the time it takes the light from your monitor to reach your eyes.)

Leave a Reply

Your email address will not be published. Required fields are marked *