Asteroids classification

Before considering the chemistry of asteroids, we will briefly review their physical properties, spectral classifications, and distributions in space. This information will provide some context for the chemical data to follow. 

We note, however, that this classification parameter does not generally reflect the oxidation state of individual chondritic components, e.g., CAIs in all chondrite groups formed under highly reducing conditions (see Chapter 1.08), and magnesian (type-I) and ferrous (type-II) chon-drules (both of which occur in the same meteorites) require formation under different redox conditions (see Chapter 1.07). In addition, the role of nebular and asteroidal processes in establishing of the oxidation states of chondrites remains controversial (e.g., Krot et aL, 2000a). 

Chondrites are classified into petrologic (or petrographic) types 1-6 to provide a guide to the extent and nature of asteroidal processing. This classification scheme, which has been invaluable (Anders and Kerridge, 1988), nevertheless... 

The C-type asteroids have a bluish color, and their reflection spectra indicate the presence of carbonaceous material at their surfaces. The S-type asteroids, on the other hand, are more reddish in color and their reflection spectra indicate that presence of surface silicate material. Other classifications include M-type, indicating the presence of surface metals, and R-type, indicating a deep, dark red color. Observations have revealed that the S-type asteroids tend to reside in the inner main belt, near the orbit of Mars. The C-type asteroids, in contrast, tend to reside toward the outer edge of the main belt nearer to Jupiter s orbit. The S-type asteroids are thought to be the primary source of stony and stony-iron meteorites, while the M-type asteroids are the most likely source of the iron meteorites. 

Besides the planets and their satellites, the Solar System harbors a large number of smaller objects, ranging from hundreds of kilometers in size down to dust particles. If they consist of solid material and have at least the size of small boulders, they are called asteroids. If they enter the atmosphere of Earth and reach the surface, they are named meteorites. If they are very small and bum up on entry, they are referred to as meteors. If the bodies contain a substantial fraction of ices and develop tails as they come closer to the Sun, they qualify as comets. However, the classifications are not very consistent for example, remnants of the dust tails of comets cause meteor showers, and older comets, once they have expended most of their volatile matter during many passes near the Sun, may not be distinguishable from asteroids. This section is devoted to comets and the next section (7.3) deals with asteroids. 

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