Thermal properties and temperature variations in Martian soil analogues

The present work reviews the information on mineralogy and chemistry of Martian soils from the data of Landers and remote sensing. Compositional information from Mars Pathfinder Landers indicates a mixture of weathered local rocks of mafic composition. If the Martian soil is composed by sediments only, porosity can be 50% at the surface. In other areas of interest a surface porosity of 20% can be present for lavas. Martian soil analogues, in dry and frozen conditions, are investigated in the present work as far as thermal conductivity and temperature variation, along its depth, are concerned. The thermal conductivity is predicted theoretically with the cubic cell model, which considers the dry soil as a cubic cell, with a solid cubic particle at the center, and gas around it. The thermal conductivity of the porous medium is evaluated from the solution of the one-dimensional Fourier conduction equation with the assumption of isothermal lines. The model needs to know the thermal conductivities of the solid particle and of the materials present, i.e. atmospheric gas and/or frozen ice, and the porosity of the soil analogue. Martian soil analogue is simulated with a kind of olivine tested in laboratory. The soil mineral composition allows to evaluate experimentally the thermal conductivity of the olivine particle, with the help of the theoretical method, which results equal to Ks = 2.94 W/m K. The thermal conductivity of dry Martian soil analogue, evaluated with the theoretical model in the porosity range p = 0 – 1 and at several temperatures, increases with the temperature and decreases with the porosity. The Martian atmospheric pressure is about 6 mbar and the Martian atmospheric gas is composed of CO2 (95%), N2 (3%), Ar (1.5%) and traces of water vapour. Heat capacity of soil analogue is evaluated with the knowledge of its physical properties, the porosity and the specific heats of the materials present. Thermal diffusivity, calculated as the ratio of thermal conductivity and heat capacity, is a function of porosity and ice mass content of the soil analogue. The temperature of Mars surface is assumed variable during the day in the range: 148 K – 298 K. Temperature variations in dry and partially frozen soil analogues are predicted during a Martian day. The temperature variation at different depth is attenuated, as compared to the surface variation, and a phase delay is present depending on the soil thermal properties. The temperature variation, as well as the derivative of the temperature variation with the depth, is dependent on the thermal diffusivity of the soil analogue. In conclusion, the temperature measurement along the depth of a Martian soil analogue can be used to verify its physical status, i.e. dry or partially frozen.