Venus: eddy coefficients in the thermosphere and the inferred helium content of the lower atmosphere

Abstract

The Pioneer Venus bus neutral mass spectrometer (BNMS) has measured number densities of CO2, N2, and He in the dayside Venus thermosphere. Valid data were received down to 128 km, which is below the Venus homopause. From the BNMS data, and other measurements of the N2/CO2 ratio in the lower atmosphere, we derive eddy coefficients K by using one‐dimensional models. A good description of the data is obtained with K = 2 × 1013n− 1/2[cm²s−1], where n is the total number density [cm−3]. The homopause falls close to 132 km altitude for He and 139 km for N2 in our preferred model. Extrapolation of the measured thermospheric He/CO2 ratio to the low atmosphere yields He/CO2 ≃ 2 × 10−5. This He abundance can have accumulated in less than the lifetime of the planet by outgassing from the planetary body as well as by accretion from the solar wind.