Cloud reflection observations: Results from the Mars orbiter laser altimeter

Formation of the perennial ice caps is one of the most important climatic processes on Mars. Up to 25% of the atmosphere is recycled each year through the ice caps. CO(2) ice may form on cold surfaces when temperatures are low enough (similar to146 K) or condense in the atmosphere and fall down on the surface as snow. Condensation of CO(2) is easily predicted by the models but has not been observed directly in any significant detail. However, formation of thick CO(2) clouds may have important implications for the martian climate. We describe observations of cloud echoes during the polar night by the Mars Orbiter Laser Altimeter. We argue that most of these clouds are mostly composed of CO(2) ice. We present observations of cloud heights, and their locations and properties over the course of 1.5 martian years. We were able to distinguish at least two distinctive types of clouds on the basis of their location and spatial structure. We compare cloud echo locations with the Mars Global Surveyor Thermal Emission Spectrometer (TES) brightness temperature data. The results show some correlation with the anomalously low brightness temperatures (cold spots) in TES data. We suggest that these zones are due to snow or frost freshly formed on the ground and only in some rare cases to CO(2) snowfall. (C) 2001 Elsevier Science.