Recent orbital evolution and the internal structures of Enceladus and Dione

We study the orbital behavior of Saturn’s satellites Enceladus and Dione during their passage through the 2:1 mean-motion resonances to constrain their interior structures, parameterized by the quantity k2=Q (assumed constant). Enceladus’ evolution after escape from the second-order e-Enceladus e-Dione resonance requires that ðk2=QÞEnceladus < 8 10 , for that QSaturn > 18;000. This result is in agreement with [Meyer, J., Wisdom, J., 2008b. Icarus 193, 213–223]. The present-day libration amplitude of Enceladus requires that ðk2=QÞEnceladus > 1:2 10 , assuming that QSaturn < 10 . Dione’s present-day eccentricity indicates that ðk2=QÞDione 6 3 10 4 for QSaturn > 18;000. Assuming Maxwellian viscoelastic behavior, we find that for Enceladus a convective ice shell overlying an ocean is too dissipative to match the orbital constraints. We conclude that a conductive shell overlying an ocean is more likely, and discuss the implications of this result. Dione’s ice shell is also likely to be conductive, but our results are less