Venus: New Microwave Measurements Show No Atmospheric Water Vapor

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Abstract
Two sets of passive radio observations of Venus—measurements of the spectrum of the disk temperature near the 1-centimeter wavelength, and interferometric measurements of the planetary limb darkening at the 1.35-centimeter water vapor resonance—show no evidence of water vapor in the lower atmosphere of Venus. The upper limit of 2 x 10-3 for the mixing ratio of water vapor is substantially less than the amounts derived from the Venera space probes (0.5 x 10-2 to 2.5 x 10-2). This amount of water vapor cannot produce dense clouds, and it is doubtful that it may contribute significantly to a greenhouse effect.