SOURCES OF PLANETARY ROTATION - MAPPING PLANETESIMALS CONTRIBUTIONS TO ANGULAR-MOMENTUM

A systematic study of the motion backward and forward in time from a u niform distribution of bodies entering a planet's Hill sphere yields a mapping of the contribution to the planet's rotation from planetesima ls as a function of their source in heliocentric orbital element space . The mapping allows determination of the contribution to rotation fro m any assumed distribution of heliocentric planetesimal orbits. For ex ample, it can systematically reproduce earlier results from Monte-Carl o studies of contributions to rotational angular momentum. With our me thod of calculation, contributions to planetary rotation can be interp reted in terms of the arrival geometries at the planet's Hill sphere a nd the subsequent two-body motion inside the Hill sphere leading to im pact. Prograde rotation is strongly favored if a planet grows in a rel atively quiescent population of planetesimals with accretion nibbling at the edges of its feeding zone. However, if the impacting population were dominated by large bodies with high relative velocity, the direc tion and magnitude of rotation would be random. (C) 1997 Academic Pres s.