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Let's go to Mars
c. 4.55 billion years ago

Iron sinks and a dynamo switches on

Molten iron drained to the centre in a few million years, building a core roughly half the planet's radius — how much has been re-argued twice since 2021 — and its churning gave Mars a magnetic field.

An illustration of Mars cut open to show a red core beneath the mantle, with white field lines looping from pole to pole around the planet.
A cutaway of the interior with the field a core dynamo produces — the arrangement Mars had, and then lost. · Image: NASA/JPL/GSFC

The hafnium–tungsten measurement that dates Mars's accretion is dating the separation of the core. Accretional impacts and the decay of short-lived aluminium-26 left the young Mars hot enough to melt through, and in a global magma ocean the iron and nickel — denser than everything around them, and chemically inclined to drag tungsten along — drained to the centre. It took a few million years at most.

What that left at the centre is still being sized, and the number has moved twice since seismology first reached it in 2021. Through every revision the shape of the answer has held: the core is metal, close to half the planet's 3,390 km radius, and too light to be pure iron. A 2025 Nature study of InSight data reports a solid inner core about 613 km in radius. A great deal of sulfur went down with the metal. Sulfur lowers the melting point, and a core that stays molten longer can go on stirring.

A large, hot, liquid iron core convecting hard is a dynamo, and Mars had one. Its signature is frozen into the oldest crust in the southern highlands, magnetised more strongly than anything comparable on Earth. The field is likely to have switched on early, while the heat of accretion and core formation was still driving convection hard. The onset is not pinned: documented dynamo activity runs from about 4.3 to 3.6 billion years ago.

A dynamo runs on heat escaping the core, and how fast a planet sheds heat is set by how big it is. Mars is small: less heat to spend than Earth, and proportionally more surface to lose it through.