Jupiter's Largest Moon Ganymede May Have Huge Subterranean Ocean

Posted on March 13, 2015

Aurorae on Ganymede

There may be a huge subterranean ocean on Ganymede, Jupiter's largest moon. Astronomers believe the underground saltwater ocean on Ganymede may have more water than all the water on Earth's surface. The ocean is estimated to be 60 miles (100 kilometers) thick. The artist's concept above shows an aurorae observed using the Hubble Space Telescope.

Ganymede is the largest moon in our solar system. It is also the only moon known to have its own magnetic field. Ganymede is close to Jupiter and also gets embedded in Jupiter's magnetic field. The auroral ovals on Ganymede change when Jupiter's magnetic field changes and "rock" back and forth. Scientists led by Joachim Saur of the University of Cologne in Germany were able to use observations of this rocking motion to determine that a large amount of saltwater exists beneath Ganymede's crust. A larger version of the above image can be found here.

John Grunsfeld, associate administrator of NASA's Science Mission Directorate at NASA Headquarters, says in a statement, "This discovery marks a significant milestone, highlighting what only Hubble can accomplish. In its 25 years in orbit, Hubble has made many scientific discoveries in our own solar system. A deep ocean under the icy crust of Ganymede opens up further exciting possibilities for life beyond Earth."

The scientists say that if saltwater were present then Jupiter's magnetic field would create a secondary magnetic field in the ocean that would counter Jupiter's field. The scientists say the ocean exists and it fights Jupiter's magnetic field so strongly that it reduces the rocking of the aurorae to 2 degrees. If the ocean did not exist the rocking of the aurorae would be 6 degrees.

Saur says in a statement, "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."

Ganymede interior diagram


Photo: NASA/ESA