Measurements of the atmospheric water vapor on Mars by the Imager for Mars Pathfinder

The Imager for Mars Pathfinder (IMP) was the first instrument to measure the atmospheric water on Mars from its surface. It took the images of the Sun through the Martian atmosphere in five narrowband filters, two in the 0.94 μm H₂O band and three in the continuum around it. The observations were carried out in the mornings and in the evenings when the Sun was between 3° and 8° above the horizon. The absorption due to the atmospheric water vapor did not exceed 2%. An average column density of 6±4 precipitated microns (pr μm) was derived from the IMP data. The dependence of the observed H₂O transmittance on Sun elevation tentatively implies that the water vapor is not uniformly mixed in the atmosphere but is rather confined to a layer 1-3 km thick near the surface. IMP observations also indicate a horizontal inhomogeneity of the layer but show no significant morning-to-evening variations of the water vapor amount.