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S.A. Stern (SwRI), P.R. Weissman (Jet Propulsion Laboratory), D.D. Durda (SwRI)
It is generally accepted that the Oort cloud was formed by the ejection of icy planetesimals (proto-comets) from the giant planets region of the solar nebula, during the clearing of small bodies from that region. Here we undertake the first investigation of the role of collisional evolution during this clearing. We use an established, time-dependent collisional evolution code developed for Kuiper belt studies (Stern & Colwell, AJ 114, 841, 1997) to carry out this work. We find that comets cannot escape without suffering intense collisional erosion. Further, for standard nebular disk models ranging upward from the minimum mass nebula, the collisional evolution is so severe that the erosion lifetimes for the proto-comets is one to two orders of magnitude less than their lifetimes for dynamical ejection. Indeed, it appears that collisions prevent proto-comets from escaping the giant planets region until the disk becomes optically thin (at least locally) to collisions. This, in turn, requires that either the disk mass must decrease (e.g., owing to radiation transport of collisionally-produced dust), or, sufficient mass must be trapped up in larger (lower area/mass ratio) bodies. Either case requires that the mass of solid bodies equal to or smaller than the size of typical comets, declines to a small fraction (~10%) of the minimum mass solar nebula. One consequence of this finding is that a significant fraction of the mass of the Oort cloud may have originated in the Kuiper belt region, having been ejected as it was gradually fed into the relatively cleared region between the giant planets. As we also describe, our results further imply that the mass of the Oort Cloud cannot far exceed 4--12 M\oplus. This work was supported by the NASA Origins and Planetary Geology and Geophysics programs.