LOW BRIGHTNESS TEMPERATURES OF MARTIAN POLAR CAPS - CO2 CLOUDS OR LOW SURFACE EMISSIVITY

One of the major surprises from the spacecraft missions to Mars of the 1970s was the finding of anomalously low brightness temperatures in the winter polar regions, far below the expected CO2 frost point, Since then, many explanations have been advanced for this puzzling behavior, including the low emissivities of carbon dioxide frost and the presence of carbon dioxide clouds, but no conclusion has been reached, We have carefully analyzed the measurements obtained by the Mariner 9 infrared interferometer spectrometer (IRIS) and the Viking infrared thermal mapper (IRTM). Besides their low brightness temperatures, the anomalous areas are characterized by their high variability and their complex spectral signature, Also, there is evidence suggesting that their occurrence is related to the condensation of CO2 in the atmosphere, We have used a combination of data analysis and modeling to compare these observations with simulated spectra obtained with radiative models of CO2 ice clouds and CO2 ice deposits, We show that precipitating CO2 cloud with particle radius larger than 10 mu m and CO2 snow deposits with millimeter-sized grains are able to produce the observed features, In both cases matching the IRIS spectra requires the CO2 ice particles to be mixed with small amounts of water or dust, as expected for the northern winter cap observed by the Mariner 9 mission, Nonprecipitating CO2 clouds, if they exist, should be transparent in the infrared, On the other hand, CO2 ice deposits composed of large grains or monolithic ice which have directly condensed on the ground could have an emissivity close to unity and in any case much higher than that of small CO2 ice particles originating from atmospheric condensation. We conclude that the low brightness temperatures are likely to be created by CO2 snow falls and that both falling snow particles and fresh snow deposits could contribute to create the observed features.