The problem
Ganymede has an intrinsic magnetic field, but what supplies the liquid-metal motion that sustains its dynamo? In one cooling-core model, iron crystallizes near the core’s top, sinks and remelts deeper down. The remelting adds dense iron-rich liquid above lighter liquid, driving compositional convection below a stable upper layer. This iron-snow mechanism is a proposed energy source, not a direct observation of crystals inside the moon.[1]
Scope
The energy source and interior evolution sustaining Ganymede’s present magnetic dynamo.
What remains open
A 2026 cold-start model instead supplies dense iron melt from the mantle to a growing core. Stirring is possible under assumed iron–sulfur composition and thermal history; other compositions can favor stable layering. Neither model uniquely establishes what happened. Core composition, heat loss and the timing of metal separation remain uncertain.[1] [2]
Why it matters
Its field helps reveal the history hidden beneath the ice.[2]
Differentiation—the separation of metal from rock—can precede a long cooling history or continue while a magnetic field is generated. Those possibilities make the observed field a test of how an icy moon assembled and evolved, rather than a direct clock for when its core finished forming.[2]
Definitions
3 terms
dynamo
Generation and maintenance of a magnetic field by motion of electrically conducting fluid. Ganymede’s field is attributed to such motion in its metal core.[1] [2]
compositional convection
Fluid motion driven by density differences caused by composition. In the iron-snow model, remelting iron produces dense liquid above lighter liquid.[1]
differentiation
Separation of initially mixed materials into interior layers, such as metal collecting into a core while rock remains above. Its timing in Ganymede is uncertain.[2]
Sources
- The Fe snow regime in Ganymede’s core: A deep-seated dynamo below a stable snow zone
Notes on The Fe snow regime in Ganymede’s core: A deep-seated dynamo below a stable snow zone
Core-evolution model proposes iron crystallization and remelting as a compositional driver.
- Powering Ganymede’s dynamo with protracted core formation
Notes on Powering Ganymede’s dynamo with protracted core formation
Published model proposes ongoing core formation as another possible energy source; its accretion and thermal assumptions remain conditional.