Enceladus's and Dione's floating ice shells supported by minimum stress isostasy
Abstract
Enceladus's gravity and shape have been explained in terms of a thick isostatic ice shell floating on a global ocean, in contradiction of the thin shell implied by librations. Here we propose a new isostatic model minimizing crustal deviatoric stress, and demonstrate that gravity and shape data predict a 384\,km-thick ocean beneath a 234\,km-thick shell agreeing with -- but independent of -- libration data. Isostatic and tidal stresses are comparable in magnitude. South polar crust is only 74\,km thick, facilitating the opening of water conduits and enhancing tidal dissipation through stress concentration. Enceladus's resonant companion, Dione, is in a similar state of minimum stress isostasy. Its gravity and shape can be explained in terms of a 9923\,km-thick isostatic shell overlying a 6530\,km-thick global ocean, thus providing the first clear evidence for a present-day ocean within Dione.