ICY LITHOSPHERES...OR LACK THEREOF

The lithosphere of a planetary body is not an inherent, distinct layer like, for example, a planetary crust. Instead, the lithosphere represents a mechanical response to the thermal/stress state near the surface. Thus, the formation of a lithosphere depends on whether the material can support stresses elastically over long time scales or will flow to relax stresses via thermally activated viscous creep (or brittly fail for high enough stresses). Temperature primarily controls whether elastic or viscous behavior dominates, because materials at a significant fraction of their melting points can creep appreciably over geologic time. For materials in icy worlds, temperature changes as small as only a few tens of Kelvin can yield viscous behavior. While models of the mechanical lithosphere (which supports surface loads over geologic time) often prescribe the lithosphere a priori, numerically simulating the body with a composite elastic-viscous (and plastic, for brittle behavior) rheology has the advantage of allowing the lithosphere to develop "naturally" and can thus capture better some of the emergent behavior in the support of planetary loads. For instance, relatively high surface temperatures for the icy satellites of Jupiter and Saturn (and Ceres?) mean that even lithospheric ice can creep appreciably over geological time and thus thin progressively, rendering suspect models that assume static equilibrium. Moreover, even modest temperature anomalies associated with geologically active features can affect how the lithosphere can support these loads. Additionally, phenomena traditionally modeled as viscous actually have appreciably elastic effects. Growth rates for folding/necking lithospheres are much smaller when elasticity is included, and viscous relaxation of crater topography is actually a problem of flexural support, with the negative load of the crater causing (progressively) upwards flexure. Indeed, an icy lithosphere is more than just a simple layer.