The Earth and Mars are similar in many respects. Nevertheless the two neighbouring planets have quite different histories. This is confirmed by a new study published recently in the scientific journal Nature by a group of researchers from ETH Zurich, the University of Oxford and the University of California in Los Angeles*. The results show that the metallic cores in the centres of the planets Earth and Mars formed under quite different conditions.

The scientists based their work on a study of silicon isotopes in various rock samples. They analysed not only rocks from the Earth and the Moon, but also meteorites from Mars and from the large asteroid Vesta as well as primitive meteorites with what is known as a chondritic composition. The fact that these studies were carried out at ETH Zurich is no accident. Ben Reynolds, assistant professor at the Institute for Isotope Geology and Mineral Resources of ETH Zurich, explains that “The composition of the silicon isotopes varies only slightly in the different samples. Only a few laboratories in the world are able to measure such small differences at all."

Lighter isotopes into the core

The measurements have now revealed that the silicon isotopes in the rocks from Mars and the asteroid Vesta have a composition comparable to that in the chondritic meteorites. According to the current theory, the latter reflect the original composition of the solar system, because they were not modified by subsequent geological processes. Thus the composition of the silicon isotopes in the Mars mantle was not affected by the metallic core separating out.

Compared to this, the samples from the Earth are distinctly different. The rocks from the Earth’s mantle have a higher proportion of heavier silicon isotopes than the rocks from Mars and Vesta. The researchers’ explanation for this is that a differentiation took place during the formation of the Earth’s metallic core. In this process the lighter silicon isotopes obviously went preferentially into the Earth’s core, and so the heavier silicon accumulated in the Earth’s mantle. On the other hand this kind of differentiation did not take place in the case of Mars. Although our planetary neighbour also has a metallic core, this seems to have absorbed the heavy and light silicon isotopes equally well.

In the researchers’ opinion, the reason why differentiation occurred in the case of the Earth and not with Mars lies in the size of the planets. Because the Earth is much bigger than Mars, the pressure and temperature prevailing at the transition between the mantle and the core of our planet are much higher. When the phase equilibria under these conditions are studied, it turns out that the light silicon preferentially enters the Earth’s core.

Early differentiation

Another remarkable fact is that the Moon’s rocks have a silicon isotope composition comparable to that of the samples from the Earth. Since the Moon is much smaller than the Earth, the processes that caused differentiation cannot be the same in this case. Therefore the researchers offer a different explanation for these measured values: Nowadays it is assumed that the Moon came into existence when the Earth collided with the protoplanet Theia about 40 million years after the formation of the solar system. The new data indicate that the material from which the Moon was later formed mixed with that from the Earth’s mantle. This in turn means that the differentiation of the silicon isotopes must already have taken place at that time.

* Bastian Georg et. al.: Silicon in the Earth's core. Nature Volume 447 Number 7148 pp. 1031-1142 (2007).