Viking: Mars Atmospheric Water Vapor Mapping Experiment—Preliminary Report of Results

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Abstract
Observations made from the Viking I orbiter show very little water vapor in the Mars atmosphere in the southern hemisphere (0 to 3 precipitable micrometers) with a gradual increase across the equator to northern latitudes. Maximum amounts between 20 and 30 micrometers have been observed in the short period covered by the observations to date. The season, northern midsummer, corresponds to the beginning of the water vapor cycle in that hemisphere. A strong repetitive diurnal cycling between the solid and vapor phases is observed at a site to the east of the Tharsis Ridge at 10° north latitude; the vapor lies close to the martian surface and is most probably in saturation equilibrium with a surface haze or fog throughout much of the day.