Other
Scientific paper
May 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999p%26ss...47..607p&link_type=abstract
Planetary and Space Science, Volume 47, Issue 5, p. 607-617.
Other
2
Scientific paper
The mean square momentum accumulated by a planet as a result of collisions and encounters with planetesimals during the accretionary epoch was computed. It is assumed that the present mean square eccentricity and inclination of the planetary orbits were determined by this processes. The encounters resulted the dominant effect, especially for Saturn, Uranus and Neptune, where using the current upper limit of the mass distribution of planetesimals m1/M inferred from the obliquities and spin periods of the planets (Safronov, 1969. Evolution of the Protoplanetary Cloud and Formation of the Earth and the Planets. NASA TTF-677, Nauka, Moscow; Lissauer and Safronov, 1991. Icarus 93, 288), the resulting orbital parameters would be higher than their present values. For this reason, a new upper limit to the power law mass distribution of planetesimals in the outer solar system, consistent with the present orbital parameters is obtained, which is one or two orders lower than the previous estimates cited above. In the case of the Earth and Venus we obtain a high upper limit of the mass distribution, since the present eccentricity and inclination of their orbits are probably determined mainly by the gravitational perturbations and not by the process of accumulation. Our results of m1/M in the inner and outer solar system are in a good agreement with those obtained by Harris and Ward (1982. Annu. Rev. Earth Planet. Sci. 10, 61). They carried out a somewhat different calculation of the random impulses than the one presented in this work, obtaining that the present eccentricity and inclination of the giant planets' orbits demand a very small mass ratio. On one hand, we have calculated m1/M considering that the total mass of the planets is due to the accretion of planetesimals. On the other hand, in the case of the giant planets, we considered the accretion of planetesimals to form a core of solid material which accreted prior to gas accumulation. The inclusion of the gas component leads to a higher value of the mass ratio for Jupiter and Saturn, while for Uranus and Neptune the results remain the same than neglecting the gas component. Even when the gas is taken into account, the present eccentricity and inclination of the planetary orbits in the outer solar system demand much smaller values of m1/M than most of the previous estimates. This is consistent with the runaway accretion scenario, where the largest planetesimals rapidly grow becoming detached from the distribution while the rest of the mass remains in smaller bodies. We also constrain the masses of the largest planetesimals which are probably out of the continuous mass distribution, studying the increase in the eccentricity of the planetary orbits caused by a single, close encounter and a single impact with these large bodies. Previous estimates of the largest planetesimal masses at the end of the accretionary epoch have been obtained assuming that the inclination of the spin axes of the planets were cause by off-center impacts (Safronov, 1969. Evolution of the Protoplanetary Cloud and Formation of the Earth and the Planets. NASA TTF-677, Nauka, Moscow; Lissauer and Safronov, 1991. Icarus 93, 288; Parisi and Brunini, 1996a. Muzzio, J. C., Ferraz-Mello, S., Henrard, J.(Eds.), Proceeding of the workshop: Chaos in Gravitational N-body Systems, p.291; Parisi and Brunini 1997. Planet. Space Sci. 45, 181). In the case of the giant planets our results of the maximum allowed masses of the largest bodies are, generally speaking, in good agreement with those previous estimates, although we obtain masses for the Earth and Venus which are much higher.
Brunini Adrián
Parisi Mirta Gabriela
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