It’s long been suspected that Jupiter’s giant moon Ganymede may harbor a subsurface ocean of liquid water beneath its icy yet hard-as-rock crust, and now some ingenious observations with the Hubble Space Telescope are making an even more convincing case for it!
Ganymede is not only Jupiter’s largest moon but also the largest moon in the entire Solar System. At 3,273 miles (5,268 km) across it’s even larger than Mercury and Pluto, and about 3/4 the size of Mars. Ganymede is the only moon found to generate its own magnetosphere, the result of a liquid iron core, which is even strong enough to hold its own within Jupiter’s powerful and far-reaching magnetic field.
Because of its magnetic field, Ganymede possesses aurorae like those found above the high latitudes on Earth as well as Jupiter, Saturn, and other planets with magnetospheres that interact with the solar wind. (Ganymede doesn’t have an atmosphere like Earth’s but it does have a thin exosphere of hydrogen and oxygen ions.)
As Ganymede’s magnetosphere interacts with Jupiter’s it causes its auroral belts to tilt or “rock.” Using observations made in ultraviolet wavelengths with Hubble, researchers have found that Ganymede’s belts tilt far less than they would if there were no global ocean inside it – a deep salty sea that creates a magnetic friction against the sliding of the magnetosphere.
“I was always brainstorming how we could use a telescope in other ways,” said Joachim Saur of the University of Cologne in Germany, who came up with the idea of using Hubble to learn about what may be happening inside Ganymede. “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.”
Based on these findings as well as on data acquired through NASA’s Galileo mission to Jupiter, Ganymede may have more water beneath its crust than what Earth has over its entire surface.
With a magnetic field protecting it from the Sun’s and Jupiter’s radiation, salty liquid water, organic molecules (also discovered by Galileo), and potential heat sources from tidal flexing and radioactive decay at its core, Ganymede is earning a spot on the short list of places to look for life beyond Earth.