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. The measurements obtained by the Mariner 9 infrared interferometer spectrometer (IRIS) and the Viking infrared thermal mapper (IRTM) have been analysed. It is shown 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. It is concluded 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. -from Authors