The Pioneer Venus Large Probe Neutral Mass Spectrometer collected data that apparently, but almost surely misleadingly, indicate the presence of a large amount of methane (1000--6000 ppm) in the atmosphere of Venus from 60 km to the surface. The measured ratio of CH3D to CH4 was about 5¿10-3. The ratio for Venus methane equilibrated with water would have been 9¿10-2 and for terrestrial methane 6¿10-4. Transfer of deuterium from atmospheric HDO to poorly deuterated methane can account for the strange gradient in the water vapor mixing ratio below 10 km previously reported. Similar deuterium transfer within the mass spectrometer causes reduction in the apparent ratio of HDO to H2O. A full accounting for the deuterium atoms increases the ratio from 100 to 157 times terrestrial. This, in turn, leads to a revised value of 28 ppm, constant with altitude below 25 km, for the mixing ratio of water vapor.

Terrestrial methane mixed with 136Xe was introduced into the instrument for technical reasons and is the logical candidate for the source. But the methane detected, unlike 136Xe, closely mimics the behavior of an atmospheric gas. Despite this and the strange D/H ratio, the arguments against this methane being purely atmospheric are overwhelming. A preferable, but not provable, explanation is that it was generated by a reaction between an unidentified highly deuterated atmospheric consituent and a poorly deuterated instrumental contaminant. ¿ Amperigan Geophysical Union 1993