How an asteroid differentiates

Most meteorites are chondrites from asteroids that never melted. But iron meteorites, stony-irons and many achondrites come from bodies that melted and sorted themselves into layers, the same process that gave Earth its core, mantle and crust. Geologists call it differentiation. Here is how a small world goes through it, and how we come to hold pieces of every layer.

1. Step 1 of 7

   A body of primitive dust and rock

   Planetesimals began as loose piles of the disc's solid ingredients: silicate minerals, chondrules, CAIs and small grains of iron–nickel metal and iron sulphide, all evenly mixed. A chondrite is a sample of that starting material.

   Whether such a body later melted depended mainly on when it formed. Those that grew in the first million or two years after the first solids contained enough short-lived radioactive heat to melt; those that formed later only warmed up, and became the parent bodies of the chondrites.

2. Step 2 of 7

   Heating from inside

   The main heat source was aluminium-26, a radioactive isotope that was present in the early Solar System and decays with a half-life of about 720,000 years. Every decay releases a little energy, and in a large enough body that heat cannot escape quickly: rock is a good insulator, so the interior warms while only the surface radiates heat away to space.

   Because aluminium-26 fades so fast, timing was everything. A body that assembled within the first million years or so could reach well over 1,000 °C inside. One that assembled a few million years later found most of the heat source already gone.

3. Step 3 of 7

   Metal and sulphide melt first

   The first thing to melt was not rock but a mixture of iron metal and iron sulphide, which together melt at about 950 to 1,000 °C, lower than either on its own. Films of this liquid formed between the grains and gradually joined up into veins.

   Rocks caught at this stage survive as primitive achondrites, such as the acapulcoites and lodranites. They still look much like chondrites in chemistry, but their textures show they were partly melted and lost some metal and sulphide melt before cooling again.

4. Step 4 of 7

   Metal sinks to make a core

   Iron–nickel metal is more than twice as dense as silicate rock, so once it is molten and the rock around it has softened or partly melted, the metal drains downwards, rather as water soaks down through sand. It gathers at the centre as a core.

   Iron meteorites carry a clock that times this. The hafnium-182 to tungsten-182 system, which has a half-life of about 9 million years, shows that the metal in many iron meteorites separated from rock within about 1 to 3 million years of the first solids. Some of the very first planetesimals had cores before many chondrules had formed.

5. Step 5 of 7

   Mantle and crust

   As the rock itself melts, it separates as well. Magma rich in the ingredients of basalt is lighter than the rock it comes from, so it rises and erupts or freezes near the surface to form a crust. What it leaves behind, rich in olivine, forms the mantle. Some bodies may have melted so much that they briefly had a global ocean of magma.

   The asteroid Vesta is the best-known example. The eucrites are its basalt lavas, the diogenites are rocks rich in orthopyroxene from deeper down, and the howardites are rubble mixed from both on its battered surface. The Dawn spacecraft, which orbited Vesta in 2011 and 2012, confirmed a layered world with a metal core.

6. Step 6 of 7

   A slow freeze

   With the heat source fading, the body began to cool, but slowly, because the mantle insulated the core. Metal cores cooled by roughly a few degrees to a few hundred degrees per million years. That slow cooling is what lets the Widmanstätten pattern grow in iron meteorites.

   As a core freezes, the stirring of its remaining liquid can generate a magnetic field, as Earth's core does today. Faint magnetism recorded in some meteorites, including pallasites and some achondrites, suggests that a number of small asteroid cores did just that, billions of years ago.

7. Step 7 of 7

   Smashed open: pieces of every layer

   Most of the bodies that differentiated did not survive whole. Collisions in the crowded early Solar System broke many apart, scattering fragments of their cores, mantles and crusts. Vesta is one of the few survivors, and even it was blasted by impacts that produced its family of small asteroids.

   That is why we hold samples of layers we could never reach on Earth. Iron meteorites are pieces of metal cores, from dozens of different parent bodies. Pallasites, with olivine crystals set in metal, were long pictured as coming from the boundary between core and mantle; some scientists now think impacts mixed the two. Basaltic achondrites are crust. The NASA spacecraft Psyche, launched in 2023, is travelling to a metal-rich asteroid that may be an exposed core.

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