Fission origin of the Moon: Cause and timing

A new scenario is offered for the origin of the Moon. It is assumed that the Earth formed initially with about the maximum amount of angular momentum consistent with dynamical stability. This state is approximated by the secularly unstable Maclaurin spheroids (highly flattened, hamburger-shaped bodies). It is shown that the Earth cannot depart from this state at a resonable rate as long as its viscosity is in the range of liquid rock. Since core formation supplies about 1600 kJ kg^?1 the Earth will not leave this state until core formation is complete. When cooling produces a rise in viscosity, the Earth will necessarily evolve along a path which is approximated by the Riemann ellipsoids (which have rapid internal motion). The evolution is toward a Jacobi ellipsoid, but it is intercepted by the development of a third-harmonic (pearshaped) instability, which is catastrophic and leads to fission. The process of fission itself may be fundamentally analogous to the breaking of a wave in water. We cannot exclude the possibility that some other planets evolved similarly.