  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-182$W 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
fragmentation through erosive (cratering) collisions. We find that radial drift
of small fragments can drastically reduce 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. Two scenarios can explain the rapid
formation of small Mars: (i) it formed by accretion of small planetesimals in a
massive disk or (ii) it formed from large planetesimals but its growth was
arrested by the inward then outward migration of Jupiter. Taking into account
all constraints, we conclude that Mars is likely to have formed in a massive
disk of about ~ 0.1 solar mass from planetesimals smaller than ~ 10 km 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.
