Experimental investigation of the mechanical properties of synthetic magnesium sulfate hydrates: implications for the strength of hydrated deposits on Mars

[1] We have carried out uniaxial compression experiments to determine the mechanical properties of three crystalline magnesium sulfate hydrates that may be present in the near-surface environment of Mars. Our synthetic samples of kieserite (MgSO4 · H2O), epsomite (MgSO4 · 7H2O), and meridianiite (MgSO4 · 11H2O) have mean values of unconfined compressive strength of 6.3 ± 0.7, 12.9 ± 1.8, and 30.1 ± 4.5 MPa, respectively, Young's modulus of 0.8 ± 0.1, 2.9 ± 0.4, and 5.9 ± 0.8 GPa, respectively, and mean porosity values of 47.8% ± 0.5%, 11.1% ± 0.6%, and 2.9% ± 0.2%, respectively. Although our tests cannot quantify a systematic relationship between hydration state and mechanical properties, the different porosities produced by consistent sample preparation methods suggest that the addition of non-cation-coordinated water molecules likely reduces the strength of individual sulfate hydrate phases. However, the bulk mechanical properties of our synthetic specimens are instead controlled predominantly by the sample porosity; generally, the strength increases as the porosity decreases. We expect the mechanical properties of sulfate hydrate deposits on Mars to be governed by the bulk porosity rather than the strength of the pure solid phase. We have performed cyclic stressing tests, replicating possible periodic depositional and erosional periods on Mars resulting from obliquity changes. A gradual compaction and reduction in sample porosity, rather than an increase in crack damage, is observed with each loading cycle, suggesting that the evolution of mechanical properties will depend on local factors such as bulk density, in addition to the overall stress history