Venus Thermosphere: In situ Composition Measurements, the Temperature Profile, and the Homopause Altitude

The neutral mass spectrometer on board the Pioneer Venus multiprobe bus measured composition and structural parameters of the dayside Venus upper atmosphere on 9 December 1978. Carbon dioxide and helium number densities were 6 × 10$^{9}$ and 5 × 10$^{6}$ per cubic centimeter, respectively, at an altitude of 150 kilometers. The mixing ratios of both argon-36 and argon-40 were approximately 80 parts per million at an altitude of 135 kilometers. The exospheric temperature from 160 to 170 kilometers was 285 ± 10 K. The helium homopause was found at an altitude of about 137 kilometers.