Non-equilibrium between dust and gas temperatures in the Mars atmosphere

A radiative-conductive balance calculation is presented to predict dust temperatures in the Mars atmosphere and to evaluate the assumption that dust and gas in the atmosphere are in thermal equilibrium. Mie theory is used to calculate the absorption and scattering properties of dust, important for the heating of dust by sunlight; scattering is neglected in the treatment of infrared absorption and emission by grains. Besides radiating in the infrared, dust grains cool via collisions with gas molecules, at a rate proportional to the gas density. We find that dust grains are in thermal equilibrium with the atmospheric gas near the surface. At a predictable pressure, which depends on grain size, the dust and gas temperatures decouple, as radiative absorption and emission increasingly dominate over exchange of thermal energy due to collisions with gas molecules. This non-equilibrium between dust and gas will affect calculations of heating in the atmosphere.