Gravitational heating of the Moon

It is pointed out that if the mass of the Moon responds to a varying field of external force as a plastic body characterized by an effective viscosity μ of the order close to 10 18 gm/cm·sec, the viscous dissipation, into heat, of motion caused by monthly variation in the height of Earth-raised bodily tides would alone be sufficient for raising the temperature of the lunar interior to 1000° or more in the past 4 1 2 thousand million years. The efficiency of this purely gravitational heating process should be proportional to the product μe , where e denotes the eccentricity of the relative orbit of the Moon; and unless the rising temperature will reduce the effective value of μ with sufficient rapidity, any secular increase in e would thus be bound to accelerate this process in the course of time.