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D.D. Durda, S.A. Stern (SwRI), J.I. Lunine (UA/LPL), A. Morbidelli (Obs. Nice)
In the asteroid belt, the survival of Vesta's basaltic crust after 4.5 Gyr of collisional evolution provides a key constraint on the amount of impact processing that has occurred. By analogy, we extend this constraint to the Edgeworth-Kuiper Belt (EKB), employing Pluto's volatile N2/CO/CH4 crust in much the same way. (Smaller EKB objects are most likely not able to provide additional constraints, since they are unlikely to differentiate [Stern 1989] and thus obtain a crustal veneer of volatiles, and even if they did, volatiles are rapidly lost owing to intense hydrodynamic escape [Trafton et al.~1987]). Using an established EKB collision model (Durda & Stern 2000), we find that in the present-day collisional environment, some 1--2\times1030 ergs/Gyr of impact energy are deposited on Pluto, corresponding (for a pure N2 crust) to some 60--120 m/Gyr of impact devolatilization over 4 Gyr. This loss, perhaps 0.2--0.5 km, is small compared to the estimated 10--40 km maximum depth of Pluto's N2 crustal veneer. However, during the early, massive EKB epoch, which lasted perhaps ~0.3--1.5 Gyr (Stern & Colwell 1997), we estimate that up to ~18~km of crustal N2 could have been devolatilized. We will discuss the implications of impact devolitilization for constraints that may be placed upon the timing of Pluto's crustal differentiation, the amount of N2 in Pluto's crust, the mass of the early EKB, and the duration of the massive EKB era.