Release of radiogenic gases from the moon

The rate of escape of Ar-40 from the moon is calculated from mass-spectrometer data obtained at the Apollo-17 landing site. It is shown that the rate of loss of Ar from the moon varies significantly over periods the order of one lunation and that the average loss rate is about 3 t/a, corresponding to about 6% of the present rate of Ar production by K decay within the moon. These features of the Ar loss-rate data are interpreted as evidence that this gas originates in the partially molten asthenosphere, which in turn requires that early differentiation only affected the outer 600 to 1,000 km of the moon, trapping significant amounts of radioactive materials in the present asthenosphere. The relationship between the venting of Ar and other radiogenic gases in the lunar atmosphere is discussed.