On the Rotation of Mercury

A planar, fixed-orbit model of the rotation of the planet Mercury is analyzed. The model includes only the solar torques on the planet's permanent asymmetry and its solar tidal bulge. For this model, it is shown that the zero of the averaged tidal torque corresponds to an asymptotically stable periodic solution of the second kind which, for two tidal torque representations, is close to the asymptotically stable equilibrium point corresponding to an exact 3∶2 spin-orbit resonance. A conjecture that the current rotation state of Mercury is due to transfer from capture by the zero of the averaged tidal torque to 3∶2 resonance capture with changes in the eccentricity of the planet's orbit is discussed briefly.