The composition of the terrestrial planets

Abstract

The mean atomic weight is a useful parameter with which to discuss the composition of the terrestrial planets. Using the density-pressure relationships of the earth and assigning a mean atomic weight to the mantle and core with the help of shock wave data, it is possible to construct a mass-mean density curve for planets of a given mean atomic weight. The Mariner IV occultation experiment and recent telescopic measurements have provided new data for the radius of Mars which are in substantial agreement and yield a density for Mars between 3.96 and 4.10 g/cm^3. Planetary radar data have been processed to yield improved estimates of the radii and masses of Venus and Mercury. The corresponding densities are ϱ (Venus) = 5.27 and ϱ (Mercury) = 5.50 g/cm^3. These new determinations of density are used to estimate the mean atomic weight of the terrestrial planets. The results are Mars, 25.3 ± 0.4; Venus. 26.4; Mercury, 36; and the moon 22.0. The mean atomic weight of the earth is 27.0. These results can be compared with the following representative values for meteorites and terrestrial rocks: igneous rocks, 21–22; carbonaceous chondrites, 23.4–24.0; ordinary chondrites 24.4; "high iron" chondrites, 25.1; enstatite chondrites, 25.6; and iron meteorites, 55.

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