Tidal origin of the Laplace resonance and the resurfacing of Ganymede

I present a new scenario for the tidal origin of the Laplace resonance. The inner three Galilean satellites are supposed to have formed in orbits well away from the 2/1 pair-wise mean motion commensurabilities and the three body Laplace resonance. The dynamics is controlled by the evolution of the 2/1 critical frequencies, ω₁ = 2n₂ - n₁ and ω₂ = 2n₃ - n₂, where n₁, n₂, n₃ are the mean motions of Io, Europa, and Ganymede. I show that the Laplace relation, ω₁ = ω₂, may have been established from within a previous three body resonance defined by ω₁ ω₂ ≈ 1 2. The ω₁ ω₂ ≈ 1 2 resonance would have excited Ganymede's orbital eccentricity to a high value. The attendant enhanced tidal heating in Ganymede during that high eccentricity period provides a possible explanation for the geologically younger surface of Ganymede compared to that of its "twin satellite," Callisto. The extent of orbital evolution required for this scenario is easily accomodated by values of Q_J of several times 10⁵.