Ganymede and its auroras, with Jupiter in the background.
Illustration: NASA/ESA

Twenty-two years ago, the Galileo spacecraft made its first flyby of Ganymede, Jupiter’s largest moon. Scientists with NASA have taken a new look at the data collected during that historic encounter, providing tantalizing new details about this strange celestial object, its unique magnetic shield, and its unusually bright auroras.

We don’t talk very much about Ganymede, but it’s actually a really cool moon. Not only is it Jupiter’s largest moon, it’s the largest moon in the Solar System, and the ninth largest object in orbit around the Sun. Its diameter is about 3,200 miles (5,200 km) in length, making it slightly larger than Mercury. Fascinatingly, it features a subsurface ocean, with potentially more water stored within it than exists on Earth’s surface.

Ganymede
Image: NASA

On June 26, 1996, NASA’s Galileo spacecraft made the first of six flybys around the moon, and it made a rather surprising discovery: Ganymede has a magnetic shield, a feature also known as a magnetosphere. This caught astronomers off guard, as they previously thought only planets, such as Earth, Mars, and Jupiter, are capable of generating and sustaining magnetic fields. But there it was, Ganymede with its faint magnetosphere nestled within Jupiter’s much larger, and far more powerful, magnetic shield. The going explanation for this unexpected feature is that it’s being generated by convection within the moon’s liquid iron core.

Studying magnetospheres, whether it’s the one around our planet or the one around some distant moon, is important. It’s very unlikely that Ganymede can host life, but by learning more about this moon’s protective magnetic bubble we stand to learn more about this critically important physical phenomenon. Without a magnetosphere, Earth would likely be a sterile rock, with the Sun’s damaging radiation perpetually bombarding the surface. It’s safe to say that without a magnetosphere, the chances for life are slim to none. So by studying the magnetospheres within our Solar System, we could learn about the environments around other potentially habitable worlds elsewhere in the galaxy.

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Among Galileo’s many instruments was the Plasma Subsystem (PLS), which measured the density, temperature, and direction of plasma ( i.e. excited, electrically charged gas) flowing around Ganymede. The spacecraft diligently collected this information during the 1996 flyby, but the data was never published. A study led by Glyn Collinson from NASA’s Goddard Space Flight Center, published this week in Geophysical Research Letters, has taken a fresh look at this old data, offering new details about Ganymede’s unique magnetosphere.

An infographic describing Ganymede’s magnetosphere.
Image: NASA’s Goddard Space Flight Center/Mary Pat Hrybyk-Keith

The new findings are painting a portrait of a truly alien world.

Normally, magnetospheres are forged by the pressure of the Sun’s incoming winds, but in this case, it’s being shaped by slow-moving plasma wafting around the Jovian system. This gives Ganymede’s magnetosphere a strange, horn-like shape that extends ahead of the moon in the direction of its orbit.

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At the same time, Jupiter’s high-energy plasma particles are raining down on the polar regions of Ganymede’s icy surface, causing highly charged, water-based particles to careen back into space.

“There are these particles flying out from the polar regions, and they can tell us something about Ganymede’s atmosphere, which is very thin,” said Bill Paterson, a co-author of the study at NASA Goddard, in a statement. “It can also tell us about how Ganymede’s auroras form.”

Indeed, Ganymede features auroras similar to Earth’s northern and southern lights. But unlike on Earth, where our auroras are produced by incoming solar particles, the auroras on Ganymede are being generated by the plasma surrounding Jupiter. Using data previously collected by the Hubble Space Telescope, and by corroborating it with Galileo’s PLS data, the NASA scientists were able to characterize and pinpoint the location of the auroras on Ganymede. Further research could explain why Ganymede’s auroras are so freakishly bright, a strange observation given the weakness of the moon’s magnetosphere.

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Excitingly, this research isn’t over. Future analysis of PLS data from Galileo’s historic encounter could teach us about the subsurface ocean previously determined to exist within the moon. It’s thought that this subterranean ocean, through the forces of friction, is creating a secondary magnetic field that’s interacting with Jupiter’s field.

Ganymede doesn’t steal the spotlight like many of the Solar System’s other moons, but that’s poised to change. In 2022, the European Space Agency will launch JUICE—JUpiter’s ICy Moons Explorer—an interplanetary spacecraft that’s scheduled to visit Europa, Callisto, and Ganymede in the early 2030s.

[Geophysical Research Letters]

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