Martian polar clouds and cold spots

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Scientific paper

The Mars Orbiting Laser Altimeter aboard the Mars Global Surveyor (MGS) spacecraft, has observed echoes from cloud tops at 1.064μm on ˜ 60% of its orbital passes over the winter poles. These clouds are unlikely to be composed of water ice since the vapor pressure of H_2O is very low at the temperature of the polar night, as measured by TES, the MGS Thermal Emission Spectrometer, and by an analysis of MGS radio occultations. Dust clouds can also be ruled out since no correlation is seen between clouds and global dust storms. The most likely candidate is CO_2 ice. We have constructed a database of MOLA clouds, identified by clusters of closely grouped echoes. Each set of cloud echoes was plotted by altitude and along-track distance, inspected visually, and assigned to a category by its morphology and relation to surface features. When cloud locations are correlated with one another, some classes show a high correlation during the polar night, consistent with their motion in a polar vortex. The IRIS instrument on Mariner 8, and later TES on MGS, have observed anomalous ``cold spots'' in the polar night. Three models have been put forward to explain them: (1) CO_2 ice clouds, (2) a deep layer of CO_2 frost, or (3) surface layers of CO_2 slab ice. In each case, the emissivity is lowered by volume scattering, but in very different conditions. To choose between these models, we have made maps of the spectral slope, partial E / partial λ, for 20μm < λ < 25μm at intervals of 5^o in L_s. The cold spots show up clearly as isolated minima. Having identified a particular cold spot, we note the first time at which TES detected it and the last time, if any, at which TES observed that region without detecting any anomaly. From this, an estimate of the average lifetime of a cold spot can be determined. We go on to correlate MOLA clouds with TES cold spots to see whether the two are related, and whether particular types of cloud are responsible.

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