Past exam of the physics of the Earth as a Planet course of the University of Cambridge / 2015 / Paper 1 / 2 / d / Solution

Use the supplied surface temperature, $420^{\circ }\mathrm{C}\simeq 693\mathrm{K}$, as the premise. With otherwise comparable material and geothermal gradient, this hotter boundary increases thermally activated material creep at shallow depth and places a chosen mechanical cutoff temperature closer to the surface. That effect alone would favour a thinner elastic layer than in a cool terrestrial oceanic plate.

Water weakening of rocks makes dryness act in the opposite direction. Water can enhance material creep; sufficiently dry material can remain strong at temperatures at which its wet equivalent relaxes. Experiments on dry diabase provide a concrete basis for a strong dry crust and possible mechanical coupling between crust and mantle, rather than requiring a weak crustal layer solely because the surface is hot.^[ref]

For an illustrative steady conductive geotherm with thermal conductivity $k_T$ and heat flux $q$, a fixed strength cutoff givesTherefore the comparison would scale aswhere all temperatures are compared consistently and a dry rheology may have a higher effective cutoff $T_{\ast ,V}$. This is a conditional strength-layer estimate, not a universal definition of $T_e$.

Venus can retain appreciable elastic thickness despite its hot surface; temperature favours thinning, while dryness favours strengthening. The supplied facts alone determine neither a numerical $T_e$ nor a definite ordering relative to Earth's oceanic plates. The comparison requires heat flow, subsurface thermal history, material rheology, interior water content and loading timescale. Little atmospheric water is evidence relevant to the hypothesis of dry rocks, not a direct measurement of deep water activity.