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.
The hafnium–tungsten measurement that dates Mars's accretion is really dating something more violent: the moment the planet turned itself inside out. 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. Mars was a layered planet almost as soon as it was a planet at all.
What that left at the centre is still being sized, and the number has moved twice since seismology first reached it in 2021 — a fight that belongs to the lander doing the listening, not to this entry. What has held through every revision is the shape of the answer: the core is liquid metal, it takes up something close to half of the planet's 3,390 km radius, and it is too light to be pure iron. A great deal of sulfur went down with the metal. That matters more than it sounds, because sulfur lowers the melting point, and a core that stays molten longer is a core that can go on stirring.
Stirring is the whole point. 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 must have switched on early — within the first few tens of millions of years, while the heat of accretion and core formation was still bleeding out of the interior and driving convection hard.
And it was doomed from the start by the same arithmetic that built it. A dynamo runs on heat escaping the core, and how fast a planet can shed 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. The field came on early, and the clock on it started running down in the same instant.