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 $38\pm 4\mathrm{km}$-thick ocean beneath a $23\pm 4\mathrm{km}$-thick shell agreeing with -- but independent of -- libration data. Isostatic and tidal stresses are comparable in magnitude. South polar crust is only $7\pm 4\mathrm{km}$ 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 $99\pm 23\mathrm{km}$-thick isostatic shell overlying a $65\pm 30\mathrm{km}$-thick global ocean, thus providing the first clear evidence for a present-day ocean within Dione.

Main paper: 14 pages, 4 figures; Supplementary information: 15 pages, 5 figures, 7 tables. Paper accepted for publication in Geophysical Research Letters

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