Jupiter’s moon Ganymede is already famous for being the largest moon in the solar system. At more than 3,200 miles wide, it is even bigger than the planet Mercury.
But what makes Ganymede truly unusual is that it is the only moon known to have its own magnetic field, something scientists believe is created by a hidden ocean of moving liquid metal deep inside the moon.
Now, a new study from researchers at California Institute of Technology suggests that Ganymede’s metal core may still be forming today, billions of years after the moon was born.
The study, published in Science Advances, proposes a completely different explanation for how Ganymede’s interior evolved. Instead of forming with a hot metal core early in its history, the moon may have started out cold and slowly heated up over time.
Scientists are especially interested in Ganymede because of its magnetic field. Magnetic fields are usually created by a “dynamo,” which happens when liquid metal inside a planet or moon moves and churns around. Earth’s magnetic field works this way, thanks to its partly molten iron core.
For years, researchers believed Ganymede formed hot and developed its metal core shortly after it was created about 4.5 billion years ago. In that traditional picture, the moon’s core has slowly cooled ever since, and the movement of liquid metal inside it still powers the magnetic field seen today.
But the new research paints a very different picture. The scientists used computer simulations showing that Ganymede may have formed as a cold mixture of ice, rock, and metal. Over billions of years, heat from radioactive elements and internal processes may have slowly warmed the moon from the inside. According to this idea, the metal core may still be growing and changing even now.
Lead author Kevin Trinh says this could explain why Ganymede has a magnetic field while another large moon, Callisto, does not. Both moons formed around Jupiter and are similar in size and density, yet they behave very differently internally.
The findings could change how scientists think about magnetic fields across the solar system. Many worlds, including Earth, Mercury, and even ancient Mars, either have or once had dynamos. This new model suggests that some of those magnetic fields may not come only from cooling cores, but also from cores that are still forming.
Researchers hope to learn more when the European Space Agency’s Jupiter Icy Moons Explorer spacecraft reaches Jupiter’s moons in 2031. The mission is expected to gather detailed data about Ganymede’s magnetic field and hidden interior, which could help reveal whether the moon is still warming up deep below its icy surface.
Source: California Institute of Technology.
