SP-345 Evolution of the Solar System

 

FIGURE 24.5.1.- Noncatastrophic alternative; spin-orbit resonance prevents the Moon from reaching the Roche limit.

 

FIGURE 24.5.1.- Noncatastrophic alternative; spin-orbit resonance prevents the Moon from reaching the Roche limit. The retrograde lunar capture orbit contracts due to tidal dissipation until resonance between the lunar orbital period and the spin period of the Earth locks the Moon in a slowly expanding orbit. Since the Moon never comes very close, no breakup or autoejection of debris occurs and the tides do not reach catastrophic heights. When the orbital inclination has decreased below a critical angle (suggested in the diagram at about 25°), the resonance locking is broken and the Moon recedes to its present orbit at 60 Earth radii. The dotted curve represents the catastropic alternative (Moon reaching the Roche limit). (From Alfven and Arrhenius, 1969.)
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