Jupiter, besides some remarkable weather patterns, isn’t super interesting to us – it’s a gas giant, after all. It’s moons are a different story, though. Case in point: Ganymede, Jupiter’s largest moon, recently came into the Hubble Space Telescope’s cross hairs, where it was discovered that the moon is home to an underground saltwater ocean that may have once been home to alien life.
“This discovery marks a significant milestone, highlighting what only Hubble can accomplish,” said John Grunsfeld, associate administrator of NASA’s Science Mission Directorate at NASA Headquarters, Washington. “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.”
Ganymede may be a moon, but it’s far from an insignificant planetary body. Ganymede is the largest moon in the solar system and the only moon to have its own magnetic field. Because of that field, Ganymede has its own aurorae – bands of electrified gas at the poles, similar to those on Earth. When Jupiter’s magnetic field changes, so goes Ganymede’s, and the aurorae “rock” in response to changes.
By observing the rocking of Ganymede’s aurorae, astronomers determined that there must be a saltwater ocean present below the surface. Ordinarily, scientists would expect Ganymede’s aurorae to rock about six degrees in response to Jupiter’s magnetic field. In actuality the aurorae only tilt about two degrees, indicating a saltwater ocean beneath Ganymede’s surface creates a powerful magnetic field of its own, counteracting Jupiter’s influence.
“I was always brainstorming how we could use a telescope in other ways,” said Joachim Saur of the University of Cologne in Germany. “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.”
The ocean is believed to be enormous – about 60 miles deep, making it 10 times deeper than Earth’s oceans at their deepest point. The ocean is in no danger of surfacing, as it’s buried under 95 miles of ice.
The researchers say that Hubble is the only device that could have confirmed these suspicions, held by scientists since the 1970s. NASA’s Galileo was able to measure Ganymede’s magnetic field in 2002, though the snapshots it captured were too brief to illustrate the rocking motion. It took Hubble, positioned high above Earth’s atmosphere, to capture the UV light that revealed the rocking.