DPS 35th Meeting, 1-6 September 2003
Session 27. Planet and Satellite Origins II: Accretion, Terrestrial Planets
Oral, Chairs: F. Spahn and T. J. Ahrens, Thursday, September 4, 2003, 3:40-5:50pm, DeAnza I-II

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[27.08] Collisional Evolution of Terrestrial Planets

C. B. Agnor, E. I. Asphaug (University of California, Santa Cruz)

The currently accepted model for the formation of terrestrial planets describes their growth as the collisional accumulation of rocky or sometimes molten planetesimals. The characteristics of the planets produced by this process are, to a large degree, determined by their collisional evolution, and their associated differentiation and thermal evolution.

Studies of planet formation and planetary collisional evolution have typically been conducted separately. Most works of late-stage planet formation use perfectly inelastic mergers to model collisions (e.g. Agnor, Canup & Levison 1999, Chambers 2001, Levison & Agnor 2003), with certain recognized inadequacies, notably rotationally unstable spin rates acquired as a planet grows. Do planets really accrete in this manner? On the other hand, most of the work studying the collisional evolution of terrestrial planets has focused on determining the efficacy of single impacts to account for particular planetary characteristics and the formation of satellites (e.g. Benz et al. 1988, Canup & Asphaug 2001).

It has been recognized for some time (Wetherill 1985) that the final characteristics (e.g. spin state, bulk composition, isotopic age) of an accreting planet are determined not by the last or single largest collision (Agnor, Canup & Levison 1999) but by \emph{all} of the major collisional encounters in a planet's history. As demonstrated in our impact models, each major impact changes the silicate to metal ratio, the thermal state, and the spin state, and sets the stage for subsequent collisions.

We have commenced a detailed study of collision dynamics and outcomes common to the late stage of terrestrial planet accretion. We are modeling collisions using smooth particle hydrodynamics to examine, primarily, the regimes of impact that truly allow for accretion (\emph{i.e.\ } mass accumulation instead of mass loss). We are also studying the cumulative affect of giant impacts on major planetary characteristics (such as composition and spin) and the extent to which collisional processes may account for planetary heterogeneity. One initial outcome of this study, to be presented, is whether, and under which circumstances, the use of perfectly inelastic collisions in late stage accretion studies is appropriate.

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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.