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Aurora clue points to giant ocean on Jupiter's largest moon

Amy Tyndall, News reporter
Mar 13, 2015, 3:20 UTC

Sen—The Hubble Space Telescope has found strong evidence of a saltwater ocean under the surface of Ganymede, Jupiter's largest moon, by observing its aurorae. 

Ganymede is the only moon in the Solar System to possess its own global magnetic field, but it is also embedded with that of its parent planet, Jupiter. Just as on Earth, highly energetic, electrically charged particles accelerate along the magnetic field lines and collide with gas atoms, which results in the emission of light—an aurora.

By observing the aurorae on Ganymede, which take the form of two auroral "ovals" over the northern and southern mid-latitudes, Hubble was able to detect a "rocking" (oscillating) motion of 2°, caused by the influence of Jupiter's magnetic field, that is only possible if there is a liquid, electrically conducting, saltwater ocean underneath the moon's icy crust.

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Illustration showing the magnetic field lines around Ganymede, which are generated in the moon's iron core. Image credit: NASA, ESA, and A. Feild (STScI)

"We have developed a new approach to look inside a planetary body with a telescope," explained Joachim Saur, Professor for Geophysics at the University of Cologne, Germany, in a statement.

Although the Hubble images were taken in ultraviolet light, Saur said that the aurora would in fact appear red if you were to stand on the surface of Ganymede. The aurorae are controlled by the magnetic field, so by observing them information is gained about the magnetic field itself.

Subsequently, in learning about the magnetic field, clues are revealed about the composition of the interior of the object in question.

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The Hubble Space Telescope observed a pair of auroral belts using the Space Telescope Imaging Spectrograph (STIS). The belts were observed in ultraviolet light 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. Image credit: NASA, ESA, and J. Saur (University of Cologne, Germany). Ganymede Globe Credit: NASA, JPL, and the Galileo Project

The magnetic field of Jupiter changes direction within a period of ten hours, corresponding to the planet's rotational period. In this way, Jupiter acts like a electromagnetic "lighthouse" with the magnetic field pointing towards and then away from Ganymede every five hours. The result is that this lighthouse effect causes Ganymede's aurorae to rock by 6°.

However, models suggested that when an ocean under the surface is taken into account, it counterbalances this effect by inducing a secondary magnetic field to reduce the rocking to just 2°.

This shift of 2° is exactly what Hubble observed, providing the best evidence to date of the presence of a sub-surface ocean. "Four independent measurements of the 2° rocking gives us confidence in the measurement," said Saur during a NASA teleconference on the findings. "We ran over one hundred models to rule out other influences, but all of them gave this 6 degree rocking until we included the ocean."

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This image illustrates the motion of the auroral belts on Ganymede. When Jupiter's magnetic field changes the aurorae on Ganymede also change, "rocking" back and forth. This amount of rocking is inhibited if the moon has a subsurface ocean. Image credit: NASA, ESA, and A. Feild (STScI)

The presence of an ocean on Ganymede has been in question since as far back as the 1970s. Just six close "fly-bys" of Ganymede were performed by the Galileo spacecraft during its time in orbit between December 1995 and September 2003, and in 2002 it detected the intrinsic magnetic field of moon itself, generated by its iron core. Each fly-by lasted for just 20 minutes, which was too short to resolve the effects of the ocean on the aurorae without ambiguity.

The new Hubble observations, equating to seven hours worth of data, provided the team with the evidence they desired. "There is no ambiguity anymore," Saur stated. The data suggests that the ocean cannot be more than 330km deep, with 100km being the most likely depth. Although they can say with confidence that the ocean is indeed liquid, at this stage they cannot say just how salty it is, or what its precise temperature might be—two parameters that could constrain the formation of life.

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An illustration of the interior of Ganymede, based on theoretical models, in-situ observations by NASA's Galileo orbiter, and Hubble Space Telescope observations of the moon's aurorae. 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. Image credit: NASA, ESA, and A. Feild (STScI)

"This is a really great example of using a remote sensing technique to study a moon that is in orbit around Jupiter, yet be able to make inferences about the interior of that moon just by looking at it from the outside ... Hubble is at Earth, yet we can probe the internal structure of this moon remotely," said Heidi Hammel, executive vice president for the Association of Universities for Research in Astronomy, Washington.

These new results from Hubble almost complement those recently released about the Cassini spacecraft detecting oceanic hydrothermal events on Saturn's moon, Enceladus. "Every mission that we send to places in the Solar System is taking us one step closer to that truly habitable water environment," said Hammel.

"One of the questions NASA has at the top of its list is if there are water oceans on exoplanets,” Hammel continued. "It won't be easy!... It may require a much larger space telescope than Hubble, but it's a tool we now have that we didn't have prior, and now we can think about using it to find water on planets around other stars."

"It demsontrates the benefits of having serviced the telesope," said Jennifer Wiseman, Hubble senior project scientist at NASA Goddard Space Flight Center in Maryland. "Thanks to the fact we've had multiple astonaut missions going up to the telescope... we have the instrument [STIS] working very well and that has allowed the detection of these precise oscillations of the aurorae." 

"The Hubble Space Telescope is celebrating its 25th anniversary," said Hammel. "Even 25 years in, we are getting brand new, innovative results." Wiseman agreed. "Hubble is really still at the peak of its scientific capaibilities and [scientific] return."

Now the team is looking forward to the launch of the James Webb Space Telescope (JWST) in 2018. JWST will be sensitive to infrared light, and as some of the aurorae emisson falls in this wavelength it will give astronomers the ability to study them in different ways.

The European Space Agency's JUICE probe (JUpiter ICy moons Explorer) is due to arrive in the Jovian system in 2030 and will study Ganymede, as well as two of Jupiter's other large moons, Callisto and Europa.