Thermal Conductivity of Dry Soil at Low Pressure.

The thermal conductivity of dry soil at atmospheric pressure depends on bulk density and mineral composition, but is typically around 0.2 W m^-1 C^-1.  The same soil on Mars has a much lower conductivity, around 0.07 W m^-1 C^-1.  The reduction is due to the low atmospheric pressure on Mars, which affects the thermal conductivity of the gas phase in the soil.  These changes in thermal conductivity of porous media with pressure were investigated in the laboratory and with mixing models.  Conductivities approached an apparently limiting value around 0.03 W m^-1 C^-1  at very low pressure, which varied little with mineralogy, even though the conductivity of these same samples at atmospheric pressure varied by a factor of almost 2.  A modified mixing model was able to give a reasonable fit to the data, and provided some information about pore size distribution.