A comparison of the thermal and radar characteristics of Mars

There is a correlation between Martian thermal inertia and radar cross section data centered on +22° latitude. The correlation is strongest with 70-cm radar, except between longitudes 10 and 90° where there is a slight anticorrelation, and gets progressively weaker at 12.5- and 3.8-cm wavelengths, respectively. A correlation is expected because of the dependence of both properties on density, but an increase in the average particle size of the surface with increasing dielectric constant is also required in order to explain the data. This may take the form of an increased number of small rocks. The anticorrelation may result from either the effects of atmospheric dust on the surface temperature or from the effects on radar of local variations in large-scale roughness or scattering by rocks. The relative behavior between the wavelengths can be understood in terms of appropriately sized rocks which act as radar scatterers. The trend of the correlation agrees with the dichotomy of the planet into two types of terrain, as noted in other remote-sensing data, and is consistent with an erosional versus depositional surface nature. Variations in the surface dielectric constant, inferred from the 3.8-cm radar data, can explain discrepancies between 2.8-cm radio emission observations and a simple model based on the global distribution of thermal inertia and albedo.