Other
Scientific paper
Jan 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998lpico.957q..24m&link_type=abstract
Origin of the Earth and Moon, Proceedings of the Conference held 1-3 December, 1998 in Monterey, California. LPI Contribution N
Other
Earth (Planet), Earth-Moon System, Lunar Orbits, Lunar Rocks, Moon, Simulation, Orbital Mechanics, Capture Effect, Angular Momentum, Planetary Evolution, Trajectory Analysis
Scientific paper
Regardless of one's favorite model for the origin of the Earth Moon system, the early history of lunar orbital evolution would produce significant thermal and tidal effects on both interacting bodies. Three lunar origin models (fission, co-formation, and giant impact) feature a circular orbit that undergoes a progressive increase in orbital radius from the time of origin to the present. In contrast, a gravitational capture model places the Moon in an elliptical orbit undergoing progressive circularization from the time of capture (for model purposes about 3.9 Ma) for at least a few hundred million years following the capture event. Once the orbit is circularized, the tidal history for a gravitational capture scenario is similar to that for other models of lunar origin and features a progressive increase in orbital radius to the present. This elliptical orbit phase, if it occurred, should have left a distinctive signature in the terrestrial and lunar rock records. A typical numerical simulation of a coplanar, three-body stable prograde capture scenario features an initial close encounter at about 1.43 Earth radii and dissipation of sufficient energy from (1 to 2 X 1028 J, depending on the heliocentric orbital configuration at the time of the encounter) for capture of lunarlike (lunar mass and density) planetoid into an eliptical orbit of about 183 Earth radii and eccentricity of about 0.81. This orbit then undergoes a progressive circularization due to tidal energy dissipation within the two interacting bodies. Numerical simulation of the postcapture orbit evolution suggests a timescale for orbit circularization of about 1 b.y. for a range of reasonable postcapture body deformation and energy-dissipation parameters. If a lunarlike planetoid is captured into an orbit with the above dimensions, then the prograde angular momentum of the lunar orbit and prograde rotational angular momentum of a 10hr/day Earth equals the angular momentum of the Earth-Moon system. The predictable terrestrial deposition features are tidal and sUbtidal zone sediments recording abnormally high, but progressively decreasing, ocean tidal amplitudes and ranges between 3.9 and 3.0 Ga. Candidates for recording such information are the Onverwacht, Fig Tree, and Moodies groups of South Africa and the Warrawoona and Fortescue Groups of Western Australia. Since more the 90% of the energy for tidal capture must be dissipated within the body of the encountering planetoid, it is predictable that the Moon would undergo severe heating, crustal disruption, and major episodes of basaltic magmatism at about 3.9 Ga as it experiences repeated tidal oscillations of a few hundred meter orbit after orbit following the captture event. Although the thermal regime of the planet would not be greatly affected by the capture sequence of events, the repeated 10-20 km tidal amplitudes would tend to disrupt severly and precapture (primitive) crust, especially in the equatorial zone.
Malcuit Robert J.
Winters Ronald R.
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