The Formation Of Large Kuiper Belt Objects: Many Chips Off Of Many Blocks
Michael E. Brown
The large Kuiper belt objects exhibit one of the largest compositional variations of any class of objects in the solar system. These bodies range in composition from nearly pure ice to nearly pure rock, with a loose trend of increasing rock fraction with increasing size. This compositional variability is unexpected: generic accretion and mixing models would suggest that these objects formed from essentially the same mixtures of primordial materials and thus their compositions should be nearly uniform and reflect a primordial ice-rock ratio. Many of the large KBOs have apparent signatures of giant impacts, and these giant impacts have been suggested to play a role in removing ice form these bodies. However, the expected velocities of impacts that were common in the early solar system are too low to allow more than modest mass removal in any single event. We propose, instead, that removal of ice from large KBOs occurred as the objects accreted from a series moderate-sized differentiated bodies. Each impact between ~like-sized objects removes ~10% of the total mass of the system. In a differentiated body, this mass could be exclusively water ice. We will present simulations showing how this process leads to a correlation between size and density and how the densities of many of the known large KBOs fit within this paradigm. A few of the most extreme densities still cannot be explained simply in this context, but we discuss possible solutions.
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42nd DPS Program published in BAAS volume 42 #4, 2010.
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