The Origin and Evolution of the Solar System

34 The structure of the Solar SystemFigure 1.19. Comparison of the reflectivity of the calcium-rich achondrite meteorite Kapoetaand the asteroid Vesta.an example of a match is shown in figure 1.19. On the basis of their spectralcharacteristics asteroids have been divided into six types. The two most commontypes, which together account for 80% of the spectrally observed asteroids, aredesignated as C, associated with carbonaceous chondrite material, and S, mostlyassociated with stony irons. These asteroid types occur at all distances within themain belt of asteroids between Mars and Jupiter but there is a distinct tendencyfor the C type asteroids to have larger orbital radii.There seems little doubt on the basis of the observational evidence that thestudy of meteorites is also tantamount to the study of asteroids. The importantquestion then is the way in which asteroids are related to planets because, if theyare intimately related, the information from laboratory meteorite studies could bedirectly applied to the problem of the origin and evolution of the planets. Thereis no consensus on the form of this relationship. It was early thought that asteroidswere the debris from a broken planet but this raised two difficulties. Thefirst concerned the source of energy which could break up a planet and the secondthat of disposing of the planetary material since the total mass of known asteroidsis much less than a lunar mass. A second theory, which assumes planetaryformation by an accumulation of asteroid-sized objects, claims that Jupiter exertsconsiderable influence on objects in the asteroid-belt region and so prevents theiraccumulation by a continual stirring process. However, if there had been enoughmaterial in the asteroid region to produce a planet then this again raises the questionof disposal. Certainly all observed solid bodies in the Solar System showsigns of damage by large projectiles so many former asteroids can be accounted