ATMOSPHERIC EFFECTS ON THE REMOTE DETERMINATION OF THERMAL INERTIA ON MARS

The thermal inertia of the uppermost 1-10 cm of the Martian surface is determined by comparing measurements of the infrared brightness temperature of the surface at many different times of day with temperatures predicted with a thermal model. These models allow sunlight to reach the surface unattenuated, and assume that emission to the surface by the atmosphere is constant throughout the day and equal to 2% of the noontime insolation. We have assessed the consequences of these assumptions with a thermal model that allows for the transfer of radiation through a dusty CO2 atmosphere, and sensible heat exchange with the surface. The radiative properties of the dust are "tuned" to produce a near-neutral greenhouse effect that is similar to that observed. Thus, at the surface averaged over a day, absorption by CO2 and dust decreases the solar flux by an amount roughly equal to the increase in IR. However, the increased downward IR, which can be two to five times greater than the 2% assumption depending on dust load, significantly reduces derived thermal inertias. When using the model to fit IRTM data at the Viking lander 1 site, for example, the derived thermal inertia is 25% smaller than that based on the 2% assumption. Smaller thermal inertias imply smaller particle sizes, and our results suggest that low-thermal-inertia regions consist of particles closer to 5 μm in diameter rather than 50 μm as was previously believed. This is more consistent with suggestions that these regions form by atmospheric sedimentation. Finally, atmospheric emission also appears capable of explaining much, but not all, of the discrepancy between measured afternoon brightness temperatures and temperatures predicted by thermal models that use the 2% assumptions, i.e., the so-called "afternoon cooling" effect. © 1991.