NASA’s trusty Juno orbiter has been hanging around Jupiter for many years now. Its observations have taught us a lot about the gas giant and its intense storms. It revealed that the storms stretch farther into the planet than previously thought, and Juno is our primary eye in the sky when it comes to observing the swirling vortices that make Jupiter so iconic. But Jupiter isn’t Juno’s only target, and the spacecraft has also spent a great deal of time studying several of Jupiter’s moons, including Io and Europa.
Now, for the first time since the year 2000, Juno is going to make a close flyby of Ganymede, the largest moon of Jupiter and also the largest moon in the entire solar system. It’s even larger than Mercury by volume, but less than half as massive. What makes Ganymede so special isn’t just its size, but also the fact that it has its own magnetic field. As the only moon in our solar system with a magnetic field, it may be better protected against certain threats from space, though without a significant atmosphere it’s unclear whether life could exist there.
We tend to think of planets as needing substantial atmospheres in order to support life as we know it. This makes sense since, without an atmosphere surrounding us here on Earth, we’d suffocate in about two seconds and be completely wiped out. However, moons like Ganymede may have found a way around this would-be prerequisite.
Ganymede is an icy moon that is thought to contain a significant amount of water. A good portion of that water is in the form of ice that covers it, but it’s possible that liquid water exists closer to the center of the moon. Some estimates suggest that there is more liquid water deep within Ganymede than exists on the entirety of Earth.
We’ve never found life anywhere but Earth, so it’s entirely unclear whether life can indeed exist in vast sub-surface oceans like the one that may be lurking within Ganymede. We know that life can exist in the absence of sunlight since there are many Earthly life forms that do just that, but such aquatic life would need some source of energy. If there is significant heat within the Moon’s core, that could be enough to allow some form of life to thrive.
Moons like Ganymede were once thought to be completely frozen with no liquid water present. Now we understand that due to the pull of gravity, tides within these worlds generate heat and ensure that liquid water doesn’t freeze. Other moons like Europa and Saturn’s Enceladus may also contain life, though we have yet to prove or disprove these theories.
Juno’s flyby of Ganymede won’t answer that question for us, but it will provide us with our best look yet at the massive moon’s surface, and make us long for a mission to explore its vast ocean.