Need help understanding what an "isostatic model minimizing crustal deviatoric stress" means Despite the background information coming from another body besides the Earth, the terms I would like to understand are historically geophysical in nature.

Background:

I just read today's news "Saturn's moon Dione harbors a subsurface ocean" at Phys.org and this seems to bring the number of sub-surface oceans in the Solar System to about six (the number might vary depending on the level of certainty). 

This news about Saturn's moon Dione comes from a model which uses gravitational data from recent flyby's of the Cassini spacecraft. The model is simultaneously applied to another one of Saturn's moons Enceladus. 

Question:

My question is about the terms in the abstract of the recently published paper on the subject in Geophysical Research Letters and my question is limited to these terms which I believe come from Earth Science and apply to the understanding of the Earth's crust floating on it's magma "ocean" interaction with the hot and slightly viscous mantle below.

What is an isostatic model and what is deviatoric stress in the context of geophysics?


  Enceladus' 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 ± 4km-thick ocean beneath a 23 ± 4km-thick shell agreeing with – but independent from – libration data. Isostatic and tidal stresses are comparable in magnitude. South polar crust is only 7 ± 4km thick, facilitating the opening of water conduits and enhancing tidal dissipation through stress concentration. Enceladus' resonant companion, Dione, is in a similar state of minimum stress isostasy. Its gravity and shape can be explained in terms of a 99 ± 23km-thick isostatic shell overlying a 65 ± 30km-thick global ocean, thus providing the first clear evidence for a present-day ocean within Dione.
Isostatic means the equilibrium between the components of the crust in regards to their vertical movement owing to differences in density. 

Deviatoric stress is the kinda sideways stress that particles in a larger system under stress, impose on each other. It's what keeps objects from crumbling to dust when exert vertical stress on them.

So, there model simulates the vertical stress on the crust more than it does the horizontal stresses. Enceladus is a puzzle because it's squished down at the poles much more than any other known moon or planet.