Asteroids, Comets, and Planet Formation

The solar system includes numerous small objects, generally classifiable as asteroids, comets, or interplanetary dust. Asteroids and comets are of considerable importance in the study of the terrestrial planets. The impacts of these bodies with the planets have repeatedly reshaped the planets' surfaces, redistributed the surface materials and changed their chemistry and mineralogy. In addition, impacts leave scars which provide a means of assessing ages. Asteroids, and possibly old comet nuclei, continually rain on the Earth, and many thousands are preserved and are displayed in our museums as meteorites. In this chapter the nature of comets and asteroids is briefly reviewed.The formation of planets is also examined here because most of the evidence as to how the planets formed comes from meteorites. The discussion is short and general. The chapter is included mainly to provide background for the subsequent more detailed treatment of the planets.