Worlds that melted

Some early asteroids melted and separated into core, mantle and crust, then were shattered. Iron meteorites, pallasites and the HED meteorites from Vesta are their pieces.

Melting and layering

Bodies that formed early trapped plenty of aluminium-26, a short-lived radioactive isotope that heated them from the inside. Many melted. Dense metal sank to form cores, and silicate rock formed mantles and crusts: the same layering, called differentiation, that Earth underwent. Later collisions broke many of these bodies apart, scattering samples of every layer.

Cores and their boundaries

Iron meteorites are iron–nickel alloy from cores. They are classified into chemical groups, each probably from a different parent body; dozens of distinct parent bodies are represented. When cut, polished and etched, many show the Widmanstätten pattern, interlocking plates of two iron–nickel minerals that could grow only during cooling over millions of years.

Pallasites are olivine crystals set in metal. They were once thought to come from the core–mantle boundary; some may instead have formed where molten metal from an impactor mixed with an asteroid's mantle. Mesosiderites are a jumble of crustal rock and metal, probably mixed by a collision.

Vesta

The HED meteorites, howardites, eucrites and diogenites, are crustal and deeper rocks from one body. Telescope spectra matched them to the large asteroid Vesta in 1970, and NASA's Dawn spacecraft, which orbited Vesta in 2011 and 2012, confirmed the match. Dawn also mapped a huge impact basin near the south pole, Rheasilvia, about 500 km across, the probable source of many HEDs. Vesta is the only intact differentiated asteroid we can match to meteorites with such confidence. This site takes its name from the howardites.

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