Deuterium in the upper atmospheres of Venus and Earth

The anomalous Lyman-α glow observed by Mariner 5 between 6500 km and 9000 km from the center of Venus would imply a 10 to 1 deuterium-to-hydrogen ratio at 6500 km if it were caused by deuterium. It is pointed out here that because at 650°K the escape of deuterium is only 10−4 times as fast as hydrogen, this ratio may only require a planetary abundance ratio of order 10−3. The argument depends on the escape flux of deuterium being determined primarily by the photodissociation rate of deuterium compounds. It is shown that if downward mixing and recombination of atoms is not to compete too seriously with escape, the eddy mixing coefficient on Venus must be much smaller than that on earth.