The Effect of Semicollisional Accretion on Planetary Spins

Planetesimal accretion during planet formation is usually treated ascollisionless. Such accretion from a uniform and dynamically cold disk predictsprotoplanets with slow retrograde rotation. However, if the building blocks ofprotoplanets, planetesimals, are small, of order of a meter in size, then theyare likely to collide within the protoplanet's sphere of gravitationalinfluence, creating a prograde accretion disk around the protoplanet. Theaccretion of such a disk results in the formation of protoplanets spinning inthe prograde sense with the maximal spin rate allowed before centrifugal forcesbreak them apart. As a result of semi-collisional accretion, the final spin ofa planet after giant impacts is not completely random but is biased towardprograde rotation. The eventual accretion of the remaining planetesimals in thepost giant-impact phase might again be in the semi-collisional regime anddelivers a significant amount of additional prograde angular momentum to theterrestrial planets. We suggest that in our Solar System, semi-collisionalaccretion gave rise to the preference for prograde rotation observed in theterrestrial planets and perhaps the largest asteroids.Comment: 13 pages, 2 figure