Geochemistry

As judged from comet Halley, elemental abundances in comets are almost chondritic. C, N, and O are even closer to solar than to chondritic abundances, as can be seen in Fig. 2.5 (Delsemme, 1991a). In particular, the siderophile elements are in solar (or chondritic) proportions in the cometary silicates, and their contribution to the outer layers of the Earth (crust and upper mantle) is of the order of 2 x 1025 g (see Table 2.2). This is quite enough to solve the apparent paradox of the "siderophile excess," which is a puzzle that has not been easy to explain in geochemistry.

The term siderophile dates back to V.M. Goldschmidt, who suggested in 1922 a geochemical classification of the elements, on the basis of their tendency to concentrate in one of the three principal solid phases that appear in a cooling magma. The elements that follow metallic iron are called siderophiles, as opposed to those that follow sulfur (chalcophile) and those that follow silicates (lithophile). Because of the formation of an iron core at the center of the Earth, liquid iron should have scavenged the siderophile elements like Ni, Co, Ir, Au, Os, and Pd. However, they are still found in chondritic proportions in some samples of the crust and the mantle (Morgan et al., 1981).

Fig. 2.5. The abundance of known element in comet Halley is compared with the same elements in the Sun. Except hydrogen (and presumably the noble gases) all elements seem to be in solar proportions in comet Halley. A curious exception is the Fe/Si ratio which is only 25% of that in the Sun. The silicon seems to be overabundant by a factor of 2, and the iron underabundant by the same factor, in comet Halley. This diagram is shown to establish that most siderophile elements are in solar proportions in comet Halley and, assumedly in all comets, explaining the surprising "siderophile excess" of the Earth's crust, as well as their depletion in the upper mantle.

Fig. 2.5. The abundance of known element in comet Halley is compared with the same elements in the Sun. Except hydrogen (and presumably the noble gases) all elements seem to be in solar proportions in comet Halley. A curious exception is the Fe/Si ratio which is only 25% of that in the Sun. The silicon seems to be overabundant by a factor of 2, and the iron underabundant by the same factor, in comet Halley. This diagram is shown to establish that most siderophile elements are in solar proportions in comet Halley and, assumedly in all comets, explaining the surprising "siderophile excess" of the Earth's crust, as well as their depletion in the upper mantle.

Murthy (1991) suggests a possible explanation, on the basis of a purely geophysical model, for this "excess" of siderophiles. In his explanation, the "excess" of volatiles remains unexplained. The "excess" volatiles are those elements that are found in the biosphere and cannot be derived by weathering of igneous rocks (Rubey, 1955). The exogenous origin of both the "excess volatiles" and the "excess siderophi1es" on the terrestrial planets is a natural explanation that does not require any "ad hoc" hypothesis, because it derives, of necessity, from the formation mechanism of the planets by planetesimals, and of the growth of the giant planets. This growth has deflected chondritic and cometary bodies whose bombardment of the Earth brought the needed amounts of "excess" volatiles and of "excess" siderophiles.

Chyba (1991) has also shown that the mass flux of the impacts corresponding to the visible craters on the Moon, when properly extrapolated by the gravitation focusing action of the Earth, explains the right order of magnitude for the "excess" siderophiles.

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