TIDAL EVOLUTION INTO THE LAPLACE RESONANCE AND THE RESURFACING OF GANYMEDE

We use the numerical model of R. Malhotra (1991, Icarus 94, 399-412) t o explore the orbital history of to, Europa, and Ganymede for a large range of parameters and initial conditions near the Laplace resonance. We identify two new Laplace-like resonances which pump Ganymede's ecc entricity and may help explain the resurfacing of Ganymede. Near the L aplace resonance, the Io-Europa conjunction drifts;at a mean angular v elocity omega(1) = 2n(2) - n(1), while the Europa-Ganymede conjunction drifts at a rate omega(2) = 2n(3) - n(2), where n(1), n(2), and n(3) are the mean motions of Io, Europa, and Ganymede. We find that Laplace -like resonances characterized by omega(1)/omega(2) approximate to 3/2 and omega(1)/omega(2) approximate to 2 can pump Ganymede's eccentrici ty to similar to 0.07, producing tidal heating several hundred times h igher than at the present epoch and 2 to 30 times greater than that oc curring in the omega(1)/omega(2) approximate to 1/2 resonance identifi ed previously by Malhotra. The evolution of omega(1) and omega(2) prio r to capture is strongly affected by Q(lo)'/Q(J)'. (Here, Q' = Q/k is the ratio of the tidal dissipation function to second-degree Love numb er; the subscript J is for Jupiter.) We find that capture into omega(1 )/omega(2) approximate to 3/2 or 2 occurs over a large range of possib le initial satellite orbits if Q(lo)'/Q(J)' less than or equal to 4 x 10(-4), but cannot occur for values greater than or equal to 8 x 10(-4 ). (The latter is approximately two-thirds the value required to maint ain Io's current eccentricity in steady state.) For constant Q/k, the system, once captured, remains trapped in these resonances. We show, h owever, that they can be disrupted by rapid changes in the tidal dissi pation rate in lo or Europa during the course of the evolution; the sa tellites subsequently evolve into the Laplace resonance (omega(1) = om ega(2)) with high probability. Because the higher dissipation in these resonances increases the likelihood of internal activity within Ganym ede, we favor the omega(1)/omega(2) approximate to 3/2 and 2 resonance s over omega(1)/omega(2) approximate to 1/2 for the evolutionary path taken by the Galilean satellites before their capture into the Laplace resonance. In addition to its surface appearance, Ganymede's large fr ee eccentricity (0.0015) has long been a puzzle. We find that the omeg a(1)/omega(2) approximate to 3/2 and omega(1)/omega(2) approximate to 2 resonances can pump Ganymede's free eccentricity up to similar to 10 (-3) independent of Q(Gany)'/Q(J)'. We also show that Ganymede's free eccentricity cannot have been produced by impact With a large asteroid or comet. (C) 1997 Academic Press.