COMPOSITION OF TERRESTRIAL PLANETS AND CHONDRITIC METEORITES

The close similarity in the abundances of elements heavier than oxygen in chondritic meteorites and the solar atmosphere (Figure 1.5) suggests a chondritic composition as a reasonable assumption for the composition of the inner rocky planets—Mercury, Venus, Earth, and Mars. However, even simple considerations show that there may be important differences in chemical composition between meteorites and the inner planets. For example, Table 10.1 gives the densities of the four terrestrial planets and chondritic meteorites (Brown and Mussett, 1993). The table shows that the terrestrial planets have densities, which when corrected for their different internal pressure, vary significantly from that of chondritic meteorites. In fact, Mars with an uncompressed density of ∼ 3,700 kg m–3 is the only planet which lies in the chondritic range of 3,400–3,900 kg m–3. Earth, Venus, and Mercury are denser than chondrites, and the Earth's moon is less. The only sufficiently abundant element to influence such strong density difference is iron, because its atomic mass 56 is much higher than that of the next three most abundant elements, O(16), Mg(24), and Si(28).

More subtle is the evidence that volatile elements have been systematically depleted in terrestrial planets. Figure 10.1 shows a plot of the ratio of volatile elements, K and Rb to refractory (less volatile) elements U and Sr, respectively, in meteorites, inner planets, and the moon (Halliday and Porcelli, 2001). This shows the volatile depleted nature of the moon relative to the Earth, and the Earth and Mars relative to meteorites. The most depleted ratio in angrites is understandable as they are differentiated meteorites (Chapter 9).

The systematic decrease in iron metal/silicate ratios and systematic variations in ratios of volatile elements to refractory trace elements were probably caused by decreasing condensation pressures and temperatures, respectively, in the solar nebula away from the sun (Brown and Mussett, 1993).