Physics
Scientific paper
Feb 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998icar..131..393o&link_type=abstract
Icarus, Volume 131, Issue 2, pp. 393-420.
Physics
15
Scientific paper
We study the rotation rate of a planet accreted from a disk of planetesimals with nonuniform spatial distribution. Recent N-body simulations of gravitational interactions between planetesimals and a protoplanet have shown that the spatial distribution of planetesimals in the vicinity of a protoplanet can become nonuniform because of strong gravitational perturbation by the protoplanet. On the basis of these results, we model the velocity and spatial distribution of planetesimals in the vicinity of a planet and evaluate the rotation rate of the planet produced by these planetesimals using three-body orbit calculations. Numerical results for a planet 1 AU from the Sun show that the rotation rate can be significantly larger than obtained by previous results where the spatial distribution of planetesimals was assumed to be uniform. The rotation rate could become still larger if the planet accretes a substantial amount of planetesimals in the form of small fragments with low orbital inclinations, which are considered to be a probable outcome of high velocity collisions between planetesimals in the late stage of accretion. It is found that the final rotation rate can be large enough to explain the present total angular momentum of the Earth-Moon system. These results suggest that the so-called giant impact of a planetary body of martian size would not be indispensable for the explanation of the present total angular momentum of the Earth-Moon system. However, the enhanced values of rotation rates obtained by the present study are still much smaller than the value required for the fission of the proto-Earth by rotational instability. Thus, formation of the Moon by this mechanism seems impossible. We also investigate the rotation rate of planets at the locations of Mars, Uranus, and Neptune and find that the mean value of angular momentum produced by a large number of planetesimals with nonuniform spatial distribution can be as large as the magnitudes of the present rotation rates of these planets.
Ida Shigeru
Ohtsuki Keiji
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