Preamble

Although we believe that the terrestrial planets all have essentially the same major chemical constituents, the atmospheres, where they exist, are very different (Tables 2.8 and 2.9). This diversity means that atmospheric composition is a sensitive indicator of the evolutionary history of the planets. The radiogenic isotope 40Ar, being a decay product of 40K, is of particular interest to the Earth as a whole because it would have been very rare indeed when the Earth first formed, and has leaked to the atmosphere progressively over the life of the Earth. We suppose that the potassium abundances of Venus, Earth and Mars are similar, an assumption justified for Venus by data plotted in Fig. 21.1. Then the three-fold greater value of the ratio mass of atmospheric 40Ar to mass of planet for the Earth than for Venus shows that the leakage of 40Ar to the atmosphere has been much more effective for the Earth. This requires the convective stirring of the Earth to have been more vigorous.

We also need to explain the fact that the ratio 36Ar/40Ar for Venus is 270 times the terrestrial value. We can be confident that in all cases the 40Ar was produced in the planets after they formed, but that 36Ar was either inherited from the solar nebula or was acquired subsequently from the solar wind. Solar Ar is dominated by 36Ar.