Diversity of comets: Formation zones and dynamical paths

The past dozen years have produced a new paradigm with regard to the source regions of comets in the early solar system. It is now widely recognized t hat the likely source of the Jupiter-family short-period comets (those with Tisserand parameters, T > 2 and periods: P, generally < 20 years) is the K uiper belt in the ecliptic plane beyond Neptune. In contrast, the source of the Halley-type and long-period comets (those with T < 2 and P > 20 years) appears to be the Oort cloud. However, the corners in the Oort cloud almos t certainly originated elsewhere, since accretion is very inefficient at su ch large heliocentric distances. New dynamical studies now suggest that the source of the Oort cloud comets is the entire giant planets region from Ju piter to Neptune, rather than primarily the Uranus-Neptune region, as previ ously thought. Some fraction of the Oort cloud population may even be aster oidal bodies formed inside the orbit of Jupiter. These comets and asteroids underwent a complex dynamical random walk among the giant planets before t hey were ejected to distant orbits in the Oort cloud, with possible interes ting consequences for their thermal and collisional histories. Observationa l evidence for diversity in cometary compositions is limited, at best.