Ganymede: The Solar System's Mightiest Moon

Jupiter's moon Ganymede is virtually a planet in its own right. Not only is this distant world the largest of the solar system's moons, but it's also even larger than the planet Mercury. Ganymede has captured the imaginations of people on Earth for decades, making it a popular setting for science fiction, from a novel by Robert A. Heinlein to the anime series Cowboy Bebop. But, hiding oceans of water and a host of secrets, the truth of Ganymede may just be even more exciting than fiction.

Ganymede was officially discovered in 1610 by the Italian astronomer Galileo. There's a possibility though, that it may first have been observed far earlier. The "Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures" mentions an ancient Chinese astronomer, Gan De, whose accounts suggest he may have spotted the moon as early as 365 B.C.E. In his writings, he mentions seeing a "small reddish star" in "an alliance" with Jupiter. Remarkably, De made his observations without using a telescope; while Ganymede is theoretically bright enough to be seen with the naked eye, it's usually outshone by Jupiter. De, evidently, had exceptional eyesight.

In April 2023, ESA launched a spacecraft to Jupiter. Named the Jupiter Icy Moons Explorer (Juice), Ganymede is both the primary target and final destination of its 500 million-mile journey. Once it arrives, it will give humanity a much closer look at this fascinating little world.

Ganymede rotates a lot more slowly than Earth. A single day there lasts a little over a week back here on Earth. The reason why Ganymede's days are so slow is that it's tidally locked to Jupiter. Just like Earth's moon, the same side of Ganymede is constantly gazing at its parent planet, while its other side always faces away. In other words, a day on Ganymede coincides with its orbit. In Ganymede's sky, Jupiter is a permanent fixture that never moves from its one spot.

Tidal locking happens with any moon orbiting its planet closely enough. When the gravitational forces on a moon are strong enough, the closer side feels a stronger pull than the side facing outward. The difference in gravitational pull causes the moon's rotation to slow until it synchronizes so that it completes only one spin per orbit. But tidal locking isn't perfect, and a moon in this situation often rocks slightly back and forth on its axis, in an orbital quirk called libration. Earth's moon rocks this way, meaning that over the course of a month, slightly more than half of its surface can be seen from down here on Earth's surface. Ganymede also seems to librate (oscillate) as it orbits Jupiter. According to a study published in the journal Geosciences, one of the Juice spacecraft's many tasks when it arrives is to use lasers to accurately measure Ganymede's libration to find out both how much and how quickly it rocks.

Earth's iron core is what generates our planet's magnetic field, protecting its surface — and us — from damaging cosmic radiation. Because Ganymede also has an iron core, it too has a magnetic field. In fact, as NASA explains, it's the only moon in the solar system to have a magnetic field like a planet does. Ganymede's field is most likely generated by its core, but some observations suggest something else in the moon which affects its magnetism. Saltwater. Observations of Ganymede's magnetic field suggest that it could be hiding a vast amount of liquid water under its surface, influencing the shape of its magnetic field.

Unlike Earth though, Ganymede's magnetic field is quite weak. Even at its strongest, it's still weaker than the flimsy magnetic field of Mars. In turn, Mars has a magnetic field about 40 times weaker than Earth's. While Earth's magnetism keeps its surface shielded, Ganymede's magnetic field is seemingly far too weak to offer the same kind of comfort. It likely wouldn't do much to protect any future space travelers who might land there. But then, that probably wouldn't make much difference anyway. Jupiter, the largest planet in the solar system, has an exceptionally strong magnetic field. Ganymede's orbit is well within Jupiter's influence, meaning that its faint field is like a drop in a magnetic ocean.

[Featured image by NASA, ESA, and A. Feild (STScI) | Cropped and scaled | CC BY 4.0]

With both a magnetic field and a thin atmosphere, Ganymede is the first moon to have aurorae detected in its skies. Aurorae are caused by space weather, as energetic particles fall down magnetic field lines and collide with atmospheric molecules, making them emit light. On Ganymede, the aurorae are caused by interactions between the magnetic fields of Ganymede and Jupiter. Unlike on Earth though, Ganymede's aurorae don't hang over the moon's poles but closer to the equator, near its tropical regions.

The light emitted by Ganymede's aurorae is also in the visible range. As Keck Observatory explains, it's very similar to the green aurorae seen here on Earth, caused by oxygen molecules. The much lower pressure of Ganymede's atmosphere, though, means that Ganymede is less likely to have the bright green of Earth's aurorae. Instead, the glow in Ganymede's skies would most likely be a deep red color, with an undertone of infrared. It's uncertain whether any future spacefarers would be able to see the aurorae on Ganymede with their own eyes though. The atmosphere may be too thin for much to be easily visible, and human eyes aren't very good at seeing red colors in dim light anyway. With cameras though, this moon's dark skies are probably well-lit. A lot of the light from Ganymede's aurorae is also visible in the ultraviolet.

Ganymede's sister moon Europa is famous as a place where scientists expect to find plentiful amounts of liquid water. With all the buzz surrounding Europa's water though, Ganymede is often overlooked as another potential water world, and there's a good chance that Ganymede too contains a vast subsurface ocean. While all eyes have remained mostly on Europa, scientists have long suspected that Ganymede might also contain a secret ocean. Its presence has now been all but confirmed by watching the motion of Ganymede's aurorae. Saltwater can carry an electric charge, meaning that watery currents deep inside Jupiter's moon affect its magnetic field and, in turn, affect the aurorae dancing in its skies.

Ganymede has so much water that it may contain more than all of Earth's oceans combined! Below about 95 miles of hard, icy crust, NASA mentions that Ganymede's oceans are expected to be 10 times deeper than the oceans here on Earth. There's so much water hiding inside the moon that there's a possibility it forms a continuous layer surrounding a rocky core deep inside the icy moon. If this is the case, Ganymede's crust might even be totally decoupled from its interior, meaning that the world's core and crust could rotate at different speeds.

Another way in which Ganymede is similar to Earth is its surface. The uneven terrain of Jupiter's oversized moon is patchy and covered with unusual grooved terrain. According to a study in the journal Icarus, it shows signs of strong tectonic activity in the past. Ganymede's surface is a jumbled patchwork of lighter streaks and darker, much more ancient terrain, and is seemingly covered in the same kind of slip-strike faultlines which cause earthquakes here on our own planet. Ganymede has so many of these faultlines that the scientists studying them were taken completely by surprise. It seems Jupiter's large moon may have seen some cataclysmic activity in the past, likely including cryovolcanoes — volcanoes that erupt ice instead of lava. However, unlike Earth, and even neighboring Europa, Ganymede's tectonic activity seems to have long since seized up.

Ganymede's surface now is frozen and silent, its motion having long since stilled. Hints at its past activity can still be seen though, by looking at craters. The older dark terrain is full of unusually shallow craters, which give clues about the moon's past. One explanation for the shallow craters is that Ganymede experienced episodes of heating in the past, partially melting these craters and flattening them out. Because Ganymede's surface is such a mixture of newer and older terrain, its surface is like a forensic crime scene. By looking at the moon's surface today, scientists can build up a picture of what happened in the past, and when.