Enceladus's and Dione's floating ice shells supported by minimum stress isostasy

TL;DR

This paper introduces a new isostatic model for Enceladus and Dione, showing that their gravity and shape data support the existence of subsurface oceans beneath relatively thin ice shells, consistent with observed libration and tidal stresses.

Contribution

It proposes a minimum stress isostatic model that explains gravity, shape, and libration data for Enceladus and Dione, providing evidence for present-day subsurface oceans.

Findings

Enceladus has a ~38 km ocean beneath a ~23 km ice shell.

South polar crust of Enceladus is only ~7 km thick, aiding water conduit formation.

Dione likely has a ~99 km shell over a ~65 km ocean, indicating a present-day subsurface ocean.

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 $\rm{38\pm4\,km}$-thick ocean beneath a $\rm{23\pm4\,km}$-thick shell agreeing with -- but independent of -- libration data. Isostatic and tidal stresses are comparable in magnitude. South polar crust is only $7\pm4\rm\,km$ 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 $\rm{99\pm23\,km}$-thick isostatic shell overlying a $\rm{65\pm30\,km}$-thick global ocean, thus providing the first clear evidence for a present-day ocean within Dione.