Protostars and Planets VI, Heidelberg, July 15-20, 2013
SMALL PLANETESIMALS IN A MASSIVE DISK FORMED MARS
Kobayashi, Hiroshi (Nagoya University)
Dauphas, Nicolas (The University of Chicago)
Mars is likely to be a planetary embryo formed through collisions with planetesimals, which can explain its small mass and rapid formation timescale obtained from 182Hf-182W chronometry. In the classical theory of planet formation, the final embryo mass is determined only by the solid surface density. However, embryos can stir surrounding planetesimals, leading to destructive collisions and fragmen- tation. Radial drift of small fragments reduces the solid surface density. On the other hand, embryo growth is accelerated by fragment accretion. Since collisional fragmentation efficiency depends on the initial size of planetesimals, the final embryo mass and its growth time are determined by the initial planetesimal size and disk surface density. We have investigated the effect of these two parameters on the mass of Mars and the predicted radiogenic excess of 182W in the martian mantle. We conclude that Mars is likely to have formed in a massive disk of about 0.1 solar mass from planetesimals smaller than 10km in radius. Such small planetesimal size cannot explain core accretion of Jupiter, suggesting that there may have been a heliocentric gradient in planetesimal size in the solar nebula.
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