Estimate of radiogenic He⁴and Ar⁴⁰concentration in the Cytherean atmosphere

Abstract

Mixing ratios of 2 × 10−4 and 1 × 10−4 for He4 and Ar40, respectively, in a 100% Co2 Mariner 5 model of the Cytherean lower atmosphere are derived by assuming that the He4 and Ar40 are secondary in origin and result entirely from radioactive disintegration with subsequent outgassing from the planet. Production rates on Venus are assumed equal to those for the earth. Thermal escape of He4 is shown to be negligible, and other loss mechanisms are assumed unimportant. Concentration of these gases in the upper atmosphere of Venus are calculated from the mixing ratios and a turbopause at 6180 km radius. The He4 concentration in the upper atmosphere is a factor of 10 larger than the minimum value estimated by McElroy and Strobel [1969] to account for the observed nighttime ionosphere and is about a factor of 4 larger than that suggested by Whitten [1969] to explain the observed daytime ionosphere. This degree of agreement among three separate approaches strengthens the possibility that He4 is a dominant constituent in the upper atmosphere of Venus and also implies that loss of He4 on Venus by plasma expansion is unimportant. He4 will be the dominant constituent out to a radius of about 7600 km if H2 is present at the upper limit suggested by McElroy and Hunten [1969] and out to a radius of about 8700 km if H2 is negligible.