Thermal alteration of asteroids: evidence from meteorites

The worldʹs meteorite collections contain meteorites from at least ∼27 primitive, chondritic and ∼108 partially or totally melted asteroids. Detailed studies of these meteorites have shown that all of their asteroidal parent bodies have been thermally altered by internal heating to some degree. These alterations range from low temperature, aqueous processes (∼0 to <∼300°C), to thermal metamorphism (∼400 to ∼950°C), to partial melting and formation of unfractionated (∼980–1050°C) and fractionated (∼1000 to >1250°C) residues, to partial and complete melting, differentiation and fractional crystallization of asteroids (∼1150 to ⪢1250°C). The most likely heat source was the decay of short-lived radionuclides, notably 26Al. These thermal alterations took place penecontemporaneously on all asteroids of which we have samples, and in the first few Ma of solar system history. The asteroid 4 Vesta, the likely parent body of the HED meteorites, is a highly differentiated object, may have a metal core, and can be viewed as the smallest of the terrestrial planets. It accreted, was heated, was partially to completely melted, and formed an extrusive basaltic crust, all within a few Ma of formation of CAIs and the dawn of the solar system.