Yesterday’s discussion of hydrothermal activity inside Saturn’s moon Enceladus reminds us how much we can learn about what is inside an object by studying what is outside it. In Enceladus’ case, Cassini’s detection of tiny rock particles rich in silicon as the spacecraft arrived in the Saturnian system led to an investigation of how these grains were being produced inside Enceladus through interactions between water and minerals. If correctly interpreted, these data point to the first active hydrothermal system ever found beyond Earth.

Now Ganymede swings into the spotlight, with work that is just as interesting. Joachim Saur and colleagues at the University of Cologne drew their data not from a spacecraft on the scene but from the Hubble Space Telescope, using Ganymede’s own auroral activity as the investigative tool. Their work gives much greater credence to something that has been suspected since the 1970s: An ocean deep within the frozen crust of the moon.


Image: NASA’s Hubble Space Telescope observed a pair of auroral belts encircling the Jovian moon Ganymede. The belts were observed in ultraviolet light by the Space Telescope Imaging Spectrograph and are colored blue in this illustration. They are overlaid on a visible-light image of Ganymede taken by NASA’s Galileo orbiter. The locations of the glowing aurorae are determined by the moon’s magnetic field, and therefore provide a probe of the moon’s interior, where the magnetic field is generated. The amount of rocking of the magnetic field, caused by its interaction with Jupiter’s own immense magnetosphere, provides evidence that the moon has a subsurface ocean of saline water. Credit: NASA, ESA, and J. Saur (University of Cologne, Germany). Ganymede Globe Credit: NASA, JPL, and the Galileo Project

The early work on a Ganymede ocean grew out of computer models of the interior, but the Galileo spacecraft was able to measure the moon’s magnetic field in 2002, offering enough evidence for an ocean to keep the idea in play. The problem was that the Galileo measurements were too brief to produce an overview of the field’s long-term cyclical activity.

It was Saur’s idea to look at the idea afresh. Given that Ganymede is deeply embedded in Jupiter’s magnetic field, the aurorae that are produced in its polar regions are going to be influenced by any changes to that field, changes that produce a ‘rocking’ movement in the aurorae. These movements, Saur reasoned, would be a useful marker, one that, like the silica grains near Enceladus, could tell a story about activity deep below the surface. Says Saur:

“I was always brainstorming how we could use a telescope in other ways. Is there a way you could use a telescope to look inside a planetary body? Then I thought, the aurorae! Because aurorae are controlled by the magnetic field, if you observe the aurorae in an appropriate way, you learn something about the magnetic field. If you know the magnetic field, then you know something about the moon’s interior.”

The ‘rocking’ of the aurorae on Ganymede depends upon what’s inside the moon, and by the researchers’ calculations, a saltwater ocean would create a secondary magnetic field that would act against Jupiter’s field, tamping down the motion of the aurorae. The Hubble data show us that this is happening, for Saur’s models indicate the auroral activity is reduced to 2 degrees as opposed to the 6 we would expect if an ocean were not present. Ganymede thus joins Europa and Enceladus as an outer planet moon with increasing evidence for an ocean.

Does the likelihood of an ocean now mean we’ll shift more resources toward Ganymede as a possible venue for life? Remember that the European Space Agency’s JUICE mission (Jupiter Icy Moons Explorer) is still on track for a possible 2022 launch. Current planning calls for flybys of Callisto and Europa followed by an extended period of orbital operations around Ganymede. The mission would reach Jupiter in 2030, if these plans come to fruition.

For all its interest, though, Ganymede’s ocean seems less accessible than Europa’s, as it’s evidently sheathed in a crust of rock and ice that is 150 kilometers thick. Beneath that crust is an ocean scientists believe to be as much as 100 kilometers deep. Ganymede is a world that may well hold more water than all the water on the surface of our planet. But even if we could get to it, it’s also an ocean still thought to be trapped between two layers of ice, meaning the interesting interactions with the rocky core (as at Enceladus) would not be occurring.


Image: This is an illustration of the interior of Jupiter’s largest moon, Ganymede. It is based on theoretical models, in-situ observations by NASA’s Galileo orbiter, and Hubble Space Telescope observations of the moon’s aurorae, which allows for a probe of the moon’s interior. The cake-layering of the moon shows that ices and a saline ocean dominate the outer layers. A denser rock mantle lies deeper in the moon, and finally an iron core beneath that. Credit: NASA, ESA, and A. Feild (STScI).

What excites us about Enceladus is the prospect that hydrothermal vents at the ocean floor could be producing an environment with enough sources of energy and nutrients to make life possible. We know that the dark seafloor vents on our own planet are now considered a serious candidate for the place where life originated. If our current models are correct, Ganymede would lack the ability to develop this kind of ecosystem, but we still have much to learn about all these icy moon environments and the oceans they apparently conceal.

And as a draft of the paper on this work notes, whether Ganymede is life-bearing or not is of less consequence than the method employed here, which has implications not only within our own Solar System but elsewhere:

The method introduced here can also be applied to other planetary moons and planets to study their electrical conductivity structure. If these bodies are exposed to time-variable magnetic fields and exhibit auroral emissions, then their auroral patterns will be modified by any electrically conductive layers. Observations of the auroral emission responses combined with appropriate models for the responses will provide valuable information about these conductive layers, such as subsurface oceans. The method might one day even be applicable to exoplanets (and exomoons), once appropriate objects and associated magnetic fields are observationally confirmed.

The paper is Saur et al., “The Search for a Subsurface Ocean in Ganymede with Hubble Space Telescope Observations of its Auroral Ovals,” accepted at the Journal of Geophysical Research. Full publication information and links as soon as I have them. Meanwhile, this news release is helpful.