Analysis of data from the Pioneer Venus Sounder Probe mass spectrometer

The composition of the lower atmosphere of the planet Venus from 62 km to the surface was measured by a neutral gas mass spectrometer onboard of the Pioneer Venus Sounder Probe. Fifty-one mass spectra were obtained with an average altitude resolution of approximately 1 km. The instrument measured the composition of the gases relative to CO2, the dominant gas, that is sampled from the Venus atmosphere through a special leak. The mass range extended from 1 to 208 amu with a sensitivity of the order of 1 ppm relative to CO2, but for the noble gases it was nearly 100 times better. A description of the instrument and the initial results are reported.