F. Forget, G. B. Hansen, and J. B. Pollack. Low brightness temperatures of Martian polar caps: CO2 clouds or low surface emissivity? Journal of Geophysical Research, 100:21219-21234, 1995. [ bib | DOI | ADS link ]
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 puzzing 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 ocurrence 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 μm 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 particles and fresh snow deposits could contribute to create the observed features. .
F. Hourdin, F. Forget, and O. Talagrand. The sensitivity of the Martian surface pressure and atmospheric mass budget to various parameters: A comparison between numerical simulations and Viking observations. Journal of Geophysical Research, 100:5501-5523, 1995. [ bib | DOI | ADS link ]
The sensitvity of the Martian atmospheric circulation to a number of poorly known or strongly varying parameters (surface roughness length, atmospheric optical depth, CO2 ice albedo, and thermal emissivity) is investigated through experiments performed with the Martian version of the atmospheric general circulation model of Laboratoire de Meteorologie Dynamique, with a rather coarse horizontal resolution (a grid with 32 points in longitude and 24 points in latitude). The results are evaluated primarily on the basis of comparisons with the surface pressure records of the Viking mission. To that end, the records are decomposed into long-period seasonal variations due to mass exchange with the polar caps and latitudinal redistribution of mass, and short-period variations due to transient longitudinally propagating waves. The sensitivty experiments include a 5-year control simulation and shorter simulations (a little longer than 1 year) performed with 'perturbed' parameter values. The main conclusions are that (1) a change of horizontal resolution (twice as many points in each direction) mostly affects the transient waves, (2) surface roughness lengths have a significant impact on the near-suface wind and, as a matter of consequence, on the latitudinal redistribution of mass, (3) atmospheric dust optical depth has a significant impact on radiative balance and dynamics, and (4) CO2 ice albedo and thermal emissivity strongly influence mass exchange between the atmosphere and the polar caps. In view of this last conclusion, an automatic procedure is implemented through which the albedo and emissivity of each of the two polar caps are determined, together with the total (i.e., including the caps) atmospheric CO2 content, in such a way as to get the closest fit of the model to the Viking pressure measurements.