After reaching a deeper understanding of the subsurface ocean of Enceladus just yesterday, a stunning discovery has just been made about the largest Moon in the solar system. The largest moon of Jupiter, Ganymede, contains a subsurface ocean of it’s own. The discovery was made with the Hubble space telescope and a careful study of aurora on the giant moon.
That’s right Ganymede has auroral activity. This is because it is the only moon in the solar system with a magnetic field. The magnetic field funnels radiation from the Sun toward the north and south poles, where it ionizes molecules in the atmosphere, creating colourful veils in a circular pattern. This phenomena occurs on every planet with a magnetic field and atmospheric molecules. It has easily been observed on Earth throughout history.
Because Ganymede is so close to Jupiter, it’s own magnetic field is influenced by small changes in Jupiter’s. As the magnetic field of Jupiter changes, the aurorae on Ganymede respond with a subtle rocking. If Ganymede has a salt water ocean, Jupiter’s magnetic field would induce a secondary magnetic field, causing a sort of ‘magnetic friction’ that would suppress the rocking of the aurorae. By using the Hubble Space Telescope to look at the changes in the aurora on Ganymede over time, a team from the University of Cologne in Germany was able to determine that Ganymede’s aurorae were showing 2 degrees of rocking, less than the six degrees expected if there was no salt water. This result suggests that there is more salt water on Ganymede than on the Earth!
In fact Ganymede would be a very fine planet if it orbited the Sun rather than Jupiter. All aurorae aside, Ganymede is actually larger than the planet Mercury. It has a thing Oxygen-rich atmosphere and a protective magnetic field.
From this point on, Ganymede will be thrust into the spotlight as a potential host for the second location of life in the solar system. Europa and Enceladus are the other contenders, but with continuing research one of the three will emerge as a frontrunner, paving the way for a future mission to give a detailed study and search for the seeds of life.
What if it has a molten core?
That would also give it a magnetic field. But based on it’s orbit and it’s age, the core would have cooled a long time ago. There also wouldn’t be enough tidal friction from Jupiter and the other moons to keep it liquefied.
Great Question!