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I. The Formation of Planetesimals. II. Tidal Friction and Generalized Cassini's Laws in the Solar System


Ward, William Roger (1973) I. The Formation of Planetesimals. II. Tidal Friction and Generalized Cassini's Laws in the Solar System. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/kqm5-4r97.


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In Part 1, four stages in the accretion of planetesimals are described. The initial stage is the condensation of dust particles from the gaseous solar nebula as it cools. These dust particles settle into a thin disk which is gravitationally unstable. A first generation of planetesimals, whose radii range up to ~10⁻¹ kilometer, form from the dust disk by direct gravitational collapse to solid densities on a time scale of order one year. The resulting disk, composed of first generation planetesimals, is still gravitationally unstable and the planetesimals are grouped into clusters containing approximately 10⁴ members. The contraction of these clusters is controlled by the rate at which gas drag damps their internal rotational and random kinetic energies. On a time scale of a few thousand years, the clusters contract to form a second generation of planetesimals having radii of the order of a few kilometers. Further coalescence of planetesimals proceeds by collisions which seem capable of producing objects with a growth rate of ~15 cm. yr⁻¹ at one A.U. The final stage of accretion during which planet-sized objects form is not considered here.

In Part 2 of this thesis, an investigation of a dynamical problem which has considerable application to the solar system is undertaken. The evolution of the obliquity of an object is determined when under the influence of three phenomena: (1) the precession of the object's orbit plane, (2) the precession of the object's spin axis, and (3) tidal friction. In the absence of tidal friction, it is concluded that if the period for precession of the spin axis is much shorter than the orbit precession period, the obliquity of the object will remain very nearly constant in spite of the movement of the orbit normal. It is further concluded, that since the obliquity is not changed by the motion of the orbit plane, the decay of the obliquity towards zero by tidal friction is not significantly altered by this motion. These results are applied to Mercury, Venus, Iapetus, Triton, as well as the equatorial satellites of Mars, Jupiter, Saturn, and Uranus. The final spin states of these objects satisfy a generalization of Cassini's laws for the moon.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:(Planetary Science)
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Planetary Sciences
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Goldreich, Peter Martin
Group:Astronomy Department
Thesis Committee:
  • Unknown, Unknown
Defense Date:21 November 1972
Record Number:CaltechETD:etd-03042009-085737
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:874
Deposited By: Imported from ETD-db
Deposited On:06 Mar 2009
Last Modified:16 Jul 2024 00:09

Thesis Files

PDF (Ward_wr_1973.pdf) - Final Version
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