Cracks in the foundation
Chondritic meteorites have a similar chemical composition to the sun and are therefore reliable witnesses as to what the solar nebula, from which the planets formed, was composed of. This can be used to deduce what the Earth consists of chemically. However, ETH Zurich researchers have now discovered that strictly speaking this fundamental geological assumption is not true.
Many moons ago, geochemists discovered that the Earth must be identical to the so-called chondritic meteorites in terms of its chemical composition. The latter consist of exactly the same mixture of elements as the sun, which suggests that they mirror the composition of the solar nebula, from which the planets once emerged. This reasoning enables geologists to draw many significant conclusions. For example, we can work out which elements make up the Earth’s core as a result.
Or so we thought: whilst it may not exactly shatter this geochemical fundament, a new publication does expose various cracks in the theory. Based on the results of their experiments, a team of researchers, which includes Bernard Bourdon, Professor of Isotope Geochemistry at ETH Zurich, recently concluded in the journal Nature that the Earth may have a slightly different composition to chondritic meteorites after all.
Theory not all that plausible
The basis for the study was the discovery by another team that the element neodymium in the rock found in the Earth’s surface has a somewhat different isotopic make-up compared to the meteorites. As neodymium is a lithophilic element and therefore not present in the Earth’s core, the team suggests, there must be a hidden reservoir in the Earth’s mantle that exhibits a different composition to the rest of it. However, on account of the strong convection in the Earth’s mantle, which gives rise to a continual mixture of the rock, Bourdon’s team did not find this theory all that plausible, and so they started looking for another explanation.
The scientists scrutinized the samarium and neodymium isotopes in rocks from the Earth, meteorites from Mars and the asteroid Vesta more closely and supplemented the values with data from the literature on moonstones. The two elements samarium and neodymium are closely related: the isotopes samarium 147 and samarium 146 namely decompose in the daughter isotopes neodymium 143 and neodymium 142. If you measure the isotopic composition of the two elements, you can reconstruct the processes that occurred in the early stages of the solar system on account of the degradation’s different half-lives.
The new data reveals that rocks on the moon and Mars also exhibit an isotopic composition that differs significantly from that of the chondritic meteorites. However, the values match those of the terrestrial rocks: according to this, the Earth, Moon and Mars have a samarium-neodymium ratio that is five to eight percent above that of the chondritic meteorites. “The variance may not seem all that much”, explains Bourdon. “But it is significant enough to be inconsistent with the classic model”.
How homogenous was the solar nebula?
According to Bourdon, the fact that the three celestial bodies the Earth, the Moon and Mars could have the same isotopic composition proves that the theory of a hidden reservoir in Earth’s mantle is far from holeproof. “Our analyses indicate that a process must have occurred in the first 30 million years of our solar system which resulted in the uneven distribution of matter in the solar system.” As far as the scientists are concerned, there are two possibilities: the first is that the matter in the solar nebula ceased to be homogenous even before the planets formed, a theory which astrophysicists consider perfectly plausible. This explanation is supported by that fact that the meteorites from the asteroid Vesta, which is considerably further from the sun than Mars, has a different isotopic composition compared to rocks from the Earth, the Moon or Mars. The data indicates that Vesta could have a similar composition to the chondritic meteorites, which also come from the asteroid belt.
The second explanation assumes that a crust formed on the first planetary bodies, the so-called planetesimals. In the course of this, the crusts and mantles of these bodies each exhibited a different composition. According to this theory, when the planets collided with one another their crust was blown away. This left bodies that had another isotopic composition to the original solar nebula, from which today’s planets later emerged.
G. Caro et.al.: Super-chondritic Sm/Nd ratios in Mars, the Earth and the Moon. Nature 452, p.336-339 (2008).