The number ratio of carbonaceous to ordinary chondrites (the CC/OC ratio) is mass dependent. It is somewhat high for large meteorites (0.20), very high for the largest fireball-producing meteoroids (30), low for most meteorite falls (0.04-0.05), and extremely high for micrometeorites (86) and interplanetary dust particles (IDPs) (>>100). The high CC/OC ratio among small particles reflects the predominance of C asteroids beyond 3 AU; these particles spiral into the Inner Solar System (and reach the Earth) via the Poynting-Robertson effect. The high CC/OC ratio among large objects results from the seasonal Yarkovsky effect, which transfers asteroids (mainly the abundant C asteroids from the Outer Solar System) into Near-Earth Asteroid (NEA) orbits.

The study of the largest (D ≳100 km) Main Belt asteroids is not only important because of the clues it delivers regarding the formation and evolution of the Main Belt itself but also because many of these bodies are likely “primordial” remnants of the early Solar System, that is their internal structures have likely remained intact since their formation. Thus, many of these bodies offer, similarly to Ceres and Vesta detailed in the present book, invaluable constraints regarding the processes of planet formation over a wide range of heliocentric distances. Here, we review the current knowledge regarding these objects derived from Earth-based spectroscopic and imaging observations, with an emphasis on D >200 km bodies including Ceres and Vesta. Our motivation is to provide a meaningful context for the two largest main belt asteroids visited by the Dawn mission and to guide future in-situ investigations to the largest asteroids.

Compositions and classification of chondritic and differentiated meteories and of interplanetary dust particles