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D. R. Davis (Planetary Science Institute), P. Farinella (Universit'a di Pisa)
Understanding the origin and evolution of Edgeworth-Kuiper objects (EKOs) is a challenging problem. Relative to the mass surface density based on "smearing" out planetary masses, there is a steep mass "shoulder" beyond Neptune. If this mass distributions is primordial, a major problem exists in forming QB1-sized bodies in 4.5 Byr. The usual solution to this conundrum is to speed up accretion by putting more mass in these regions initially. The excess mass was subsequently depleted, possibly by physical collisions.
Collisions have been shown to be an important process in the E-K belt. However, if collisions depleted E-K belt, they must have produced the present population. We have used our numerical collisional evolution model to study which initially massive E-K populations might have evolved to the observed belt. Basically, the only massive primordial E-K belts which collisionally erode to the present belt are those with a very steep size distribution. The initial accretion process had to be stopped at an early phase, when 105 bodies had grown to >100 km and most of the mass was in small planetesimals.
We next studied the size distribution in the region 24-50 AU, using a multizone accretion code. The largest bodies form at the inner part, near 25-30 AU and are nearly Earth-sized, far smaller than Neptune. In Pluto's region, bodies up to 4000 km in size form, while at 50 AU the largest bodies are around 500 km diameter. However, most of the mass is still at \leq 100-500 km, given a size distribution of the type required based on the collisional studies, i.e., most of the mass is at small sizes. What is missing, though, is a plausible scenario for the formation of Neptune at the right time to interrupt accretion and to instigate collisional erosion.