5 - Evidence of the Earth's evolutionary history

Summary

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.