Simulating the evolution of brine-saturated regolith on Mars

Abstract

We investigate the sublimation behaviour of brine-saturated Martian analogue materials in laboratory experiments and how compositional variations affect the development of the sample surfaces and subsurfaces. As a regolith analogue for our experiments, we used granular Saddleback basalt with the three grain size distributions 212–500 $\mu$m, 212–2000 $\mu$m, and 1000–2000 $\mu$m. Our brine was composed of ferric sulphate, magnesium sulphate, and magnesium chloride. The samples were placed in a environmental chamber and exposed to a temperature of 243 K and a CO₂ atmosphere of 5 mbar for several days. The samples were then insolated at 590 W m⁻² with a solar simulator, representing the insolation rate at Mars whilst changes in surface morphology were monitored in vertical and lateral directions by two cameras. No explicit salt efflorescence could be produced on the surface. However, we found a stratification characterised by four layers developed within the samples. Beneath (i) a solidified crust at the surface, (ii) a dry, unconsolidated layer formed, partially characterised by white magnesium salt precipitates. A (iii) third layer was characterised by yellowish hydrated ferric sulphate precipitates, which filled considerable parts of the pore space between the regolith analogue grains. The (iv) fourth layer still contained water in the form of ice crystals and moisture. The spectral and mechanical properties of the sample surfaces were analysed after the sublimation experiment using reflectance spectroscopy in the visual and near/mid-infrared spectral range and cone penetration testing. The spectral comparisons of the uppermost crust with the non-saline regolith analogue material displayed a distinct spectral blue slope, indicating the presence of hydrated ferric sulfates. The strength of the upper crusts varied from marginally consolidated to strongly cemented depending on grain size and salt content. Our observations indicate that extensive salt crusts on the Martian surface are less likely to have been formed by brines present in the subsurface. Due to high evaporation rates, significant amounts of dissolved salts already precipitated below the surface of Mars, potentially leading to the formation of underground duricrusts.