Flow strength of highly hydrated Mg- and Na-sulfate hydrate salts, pure and in mixtures with water ice, with application to Europa

摘要

We selected two Europan-ice-shell candidate highly hydrated sulfate salts for a laboratory survey of ductile flow properties: MgSO_4·7H_2O (epsomite) and Na2SO4·10H2O (mirabilite), called MS7 and NS10, respectively. Polycrystalline samples in pure form and in mixtures with water ice I were tested using our cryogenic high-pressure creep apparatus at temperatures 232 ≤ T ≤ 294 K, confining pressures P = 50 and 100 MPa, and strain rates 4 × 10~-8 ≤ ≤ 7 × 10~-5 s~-1. Grain size of NS10 samples was >100 μm. The flow strength σ of pure MS7 was over 100 times that of polycrystalline ice I at comparable conditions; that of pure NS10 over 20 times that of ice. In terms of the creep law = Aσ n eQ/RT , where R is the gas constant, we determine parameter values of A = 10~12.1 MPan s~-1, n = 5.4, and Q = 128 kJ/mol for pure NS10. Composites of ice I and NS10 of volume fraction NS10 have flow strength σ c = [ NS10σ NS10 J + (1 - NS10)σ iceI J ]1/J where J ≈ -0.5, making the effect on the flow of ice with low volume fractions of NS10 much like that of virtually undeformable hard rock inclusions. Being much stronger and denser than ice, massive sulfate inclusions in the warmer, ductile layer of the Europan ice shell are less likely to be entrained in convective ice flow and more likely to be drawn to the base of the ice shell by gravitational forces and eventually expelled. With only smaller, dispersed sulfate inclusions, at probable sulfate < 0.2, the shell may be treated rheologically as pure, polycrystalline ice, with boundary conditions perhaps influenced by the high density and low thermal conductivity of the hydrated salts.