Ganymede Might Still Be Forming Its Metal Core Today

Jupiter's moon Ganymede is the largest moon in the solar system; at over 3,200 miles across, it is larger than the planet Mercury. It is also the only moon with its own intrinsic magnetic field, produced by a churning central core of electrically conductive metal called a dynamo. But how Ganymede's unique dynamo works is still a mystery.

Models to explain Ganymede's formation and dynamo have been developed, but they are at odds with each other. Now, a study led by Caltech proposes a new mechanism to reconcile the two—one that suggests Ganymede's interior may still be warming up.

A paper describing the research appears in the journal Science Advances on May 6.

"Dynamos are one of the few ways we can understand what's happening deep in a body's interior with spacecraft data," says lead author Kevin Trinh, the Melza M. and Frank Theodore Barr Foundation Postdoctoral Scholar Research Associate in Planetary Science. Curiously, Ganymede is the only moon in the solar system that has a dynamo, which begs the question: why? "It formed in the same disk of material as Jupiter's other moons," Trinh says. "Callisto, for example, is similar in size and density, but it has no obvious evidence of a dynamo. Why are they so different?"

Moons, like planets, form by coalescing out of a disk of material around whatever object they are orbiting—for Ganymede, that is Jupiter. Bigger objects accrete more mass and generate more heat, which might trigger the melting of metal and the formation of an interior core that is differentiated from the body's outer layers. Radioactive isotopes and tidal forces will further heat the moon over billions of years. Over time, as the core cools off, liquid metal may start circulating in a process called convection and produce a magnetic field. Ultimately, the core becomes too cold, completely solidifies, and the dynamo shuts off. (Earth's core is only partially solidified, leading to our own present-day dynamo.)

Current models predict that Ganymede's hot core formed early in the moon's 4.5 billion year history and is currently in a stage of cooling off, producing the dynamo that we detect today. But the new model proposes that Ganymede formed as a cold mixture of ice, rock, and metal that slowly heated up over billions of years—and may still be heating up today.

"Our study highlights the need to reevaluate conventional dynamo mechanisms for Ganymede," Trinh says. "The 'cold start' simulations showed us a new mechanism that hadn't been identified before, challenging the notion that core-hosted dynamos necessarily arise from a cooling reservoir of constant size. Many bodies in the solar system have or had a dynamo—Earth, Mercury, ancient Mars; our Moon probably once did too. This study opens a new angle to how dynamos work. Our results do not rule out a cooling-driven dynamo at Ganymede. However, we introduce a new dynamo mechanism that is aligned with the idea that Ganymede started out cold and without a metal core. More work is needed to identify the most likely mechanism to explain Ganymede's dynamo today."

The new model will be compared to observational data when the European Space Agency's Juice mission (JUpiter Icy Moons Explorer) arrives at the Jovian moon system in 2031.

The paper is titled "Powering Ganymede's Dynamo with Protracted Core Formation." In addition to Trinh, co-authors are Flavio Petricca and Steven D. Vance of the Jet Propulsion Laboratory, which Caltech manages for NASA; and Douglas J. Hemingway of the University of Texas at Austin. Funding was provided by Caltech's Melza M. and Frank Theodore Barr Fellowship and the NASA Solar System Workings program.