The Evolution of Water Vapor in the Atmosphere of Venus

Abstract

The atmosphere of Venus appears to be deficient in water vapor by a factor of about 104 compared with the total amount of water on Earth. The feasibility of loss of water vapor from the Venus atmosphere is examined, assuming H20 as the sole initial constituent. A steady-state model is constructed, and the photochemistry establishes the distribution of important products in the upper atmosphere. Calculations of exospheric temperatures yield values as high as 100,000K. Such large temperatures result from the large abundance of atomic hydrogen in the exosphere, and imply a dynamic outflow of all constituents from the upper region of the atmosphere. Such an outflow would cause the escape of all hydrogen and some of the oxygen resulting from dissociation of H20. Little loss of CO2 would result due to its low abundance in the upper region permitting its accumulation to the present observed value. It is concluded that if Venus formed from the same mix of materials as Earth, much tectonic activity and fairly rapid outgassing must have occurred during the early phase of its history to account for the loss of water vapor.