Online Astronomy eText: Asteroids, Comets, and Interplanetary Debris
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(brief notes from lecture of July 12, 2007)
Differentiated Meteorites (pieces of asteroids):
     Stones 90% (rocks, similar to upper mantle rocks on Earth)
     Irons 10% (crystalline nickle-iron (mostly iron), similar (?) to core of Earth)
     Stony-irons 0.1% (mixture from mantle-core boundary?)

"Primitive" Meteorites:
      A small portion of the meteorites are not pieces of differentiated or undifferentiated asteroids. They are ‘primitive’ things that have, based on their chemical and physical structure, have NEVER been inside an object that melted and differentiated. Most of these are stony objects which were formed at temperatures in excess of 200 Celsius (~ +400 Fahrenheit).
     A small proportion of these are carbon-rich objects (sometimes even with hydrates or icy compounds) which formed at temperatures well below that. (carbonaceous chondrites, e.g.) It is from THESE that we get the radioactive-age-dating value for the Solar System of 4.55 billion years.

     Asteroids come in three types (based on their reflection/spectrum):
     Carbonaceous (C-type), stony (S-type), metallic (M-type)
     C-type predominate beyond 2.5 AUs from the Sun
     S-type predominate inside 2.5 AUs
     M-type are “common” inside 2.5 AUs, but not as common as S-type
     In rare cases, we can associate specific asteroids or groups of asteroids with a particular type of meteorite. For instance, Vesta is believed to be the source of the howardites and euchrites (brecciated ("broken") basaltic material)