Viking era water-ice clouds

We have spatially and temporally mapped water ice clouds on Mars for 1.25 Martian years using the Viking infrared thermal mapper (IRTM) data. Our technique compares brightness temperatures in the 11 and 20 mu m IRTM channels, utilizing the 11 mu m water ice absorption feature. A complication arises because of the surface nonunit and wavelength-dependent emissivities. We developed a technique for removal of this effect. Using a surface thermal model, we calculated brightness temperatures and their differences for the IRTM channels resulting from the surface emissivity effect alone. These were then subtracted from the measured brightness temperatures, yielding brightness temperature differences dominated by atmospheric effects. The ability to identify water ice clouds in the infrared provides potential new information about particle size and physical processes by comparing these clouds to those seen in visible wavelengths. We found that water ice clouds were more widespread and frequent during the Viking period than had been previously recognized, with the northern spring and summer being the cloudiest periods on Mars. We interpret some of the identified cloudy zones as the southern and northern solstice season upwelling branches of the Hadley cell, although these were shifted 15 degrees-20 degrees southward from model predictions. Additionally, the transition between the two branches occurred later in time than in the model predictions. We observed the extension of the north polar hood below 60 degrees N in longitudes 120 degrees-200 degrees. We did not find evidence for a south polar hood north of 60 degrees S nor any evidence for interannual variability within our limited data set.