Ganymede: A Water World at Jupiter

This artist's concept of the moon Ganymede shows it in orbit around the giant planet Jupiter. NASA's Hubble Space Telescope observed aurorae on the moon that are controlled by Ganymede's magnetic fields. Two auroral ovals can be seen over northern and southern mid-latitudes. Hubble measured slight shifts in the auroral belts due to the influence of Jupiter's own immense magnetic field. This activity allows for a probe of the moon's interior. The presence of a saline ocean under the moon's icy crust reduces the shifting of the ovals as measured by Hubble. As on Earth, Ganymede's aurorae are produced by energetic charged particles causing gases to fluoresce.ield in the ocean that would counter Jupiter's field. This "magnetic friction" would suppress the rocking of the aurorae. This ocean fights Jupiter's magnetic field so strongly that it reduces the rocking of the aurorae to 2 degrees, instead of 6 degrees if the ocean were not present. NASA/STScI

When you think about the Jupiter system, you think of a gas giant planet. It has major storms whirling around in the upper atmosphere. Deep inside, it's a tiny rocky world surrounded by layers of liquid metallic hydrogen. It also has strong magnetic and gravitational fields that could be obstacles for any kind human exploration. In other words, an alien place. 

Jupiter just doesn't seem like the kind of place that would also have tiny water-rich worlds orbiting around it.  Yet, for at least two decades, astronomers have suspected that the tiny moon Europa had subsurface oceans. They also think that Ganymede has at least one (or more) oceans as well. Now, they have strong evidence for a deep saline ocean there. If it turns out to be real, this salty subsurface sea could have more than all the water on Earth's surface.

Discovering Hidden Oceans

How do astronomers know about this ocean? The latest findings were made using the Hubble Space Telescope to study Ganymede. It has an icy crust and a rocky core. What lies between that crust and core have intrigued astronomers for a long time.

This is the only moon in the entire solar system that is known to have its own magnetic field. It's also the largest moon in the solar system. Ganymede also has an ionosphere, which is lit up by magnetic storms called "aurorae". These are mainly detectable in ultraviolet light.  Because aurorae are controlled by the moon's magnetic field (plus the action of Jupiter's field), astronomers came up with a way to use the motions of the field to look deep inside Ganymede. (Earth also has aurorae, called informally the northern and southern lights). 

Ganymede orbits its parent planet embedded in Jupiter's magnetic field. As Jupiter's magnetic field changes, the Ganymedean aurora also rock back and forth. By watching the rocking motion of the aurorae, astronomers were able to figure out that there's a large amount of salt water beneath the crust of the moon.The saline-rich water suppresses some of the influence that Jupiter's magnetic field has on Ganymede, and that is reflected in the motion of the aurorae. 

Based on Hubble data and other observations, scientists estimate the ocean is 60 miles (100 kilometers) deep. That's about ten times deeper than Earth's oceans. It lies under an icy crust that's about 85 miles thick (150 kilometers).

Beginning in the 1970s, planetary scientists suspected the moon might have a magnetic field, but they didn't have a good way to confirm its existence. They finally got information about it when the Galileo spacecraft took brief "snapshot" measurements of the magnetic field in 20-minute intervals. Its observations were too brief to distinctly catch the cyclical rocking of the ocean's secondary magnetic field.

The new observations could only be accomplished with a space telescope high above Earth's atmosphere, which blocks most ultraviolet light. The Hubble Space Telescope Imaging Spectrograph, which is sensitive to ultraviolet light given off by the auroral activity on Ganymede, studied the aurorae in great detail.  

Ganymede was discovered in 1610 by astronomer Galileo Galilei. He spotted it in January of that year, along with three other moons: Io, Europa, and Callisto. Ganymede was first imaged up-close by the Voyager 1 spacecraft in 1979, followed by a visit from Voyager 2 later that year. Since that time, it has been studied by the Galileo and New Horizons missions, as well as Hubble Space Telescope and many ground-based observatories.The search for water on worlds such as Ganymede is part of a larger exploration of worlds in the solar system that could be hospitable to life.  There are now several worlds, besides Earth, that could (or are confirmed) to have water: Europa, Mars, and Enceladus (orbiting Saturn). In addition, the dwarf planet Ceres is thought to have a subsurface ocean.