Thermal alteration of asteroids: evidence from meteorites

The world's meteorite collections contain meteorites from at least similar to 27 primitive, chondritic and similar to 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 (similar to0 to < <similar to>300 degreesC), to thermal metamorphism (similar to 400 to similar to 950 degreesC), to partial melting and formation of unfractionated (similar to 980-105 degreesC) and fractionated (similar to 1000 to >1250 degreesC) residues, to partial and complete melting, differentiation and fractional crystallization of asteroids (similar to 1150 to much greater than 250 degreesC). The most likely heat source was the decay of short-lived radionuclides, notably (26)Al. 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. (C) 2000 Elsevier Science Ltd. All rights reserved.