Solar Nebula Homogeneity Contested

According to solar system formation theory, the Sun and the planets all emerged from a homogeneous cloud of dust and gas, called the solar nebula, which collapsed on itself about 5 billion years ago. Chondritic meteorites are believed to be among the first objects that were formed in the solar nebula, some of which being in the composition of the planets while others are still wandering through the solar system.

Because the premise of the theory says that the solar system was formed from a single cloud of gas and dust, it is only natural to believe that the chemical composition of the Sun is fairly similar to that of chondritic meteorites and therefore to the composition of the Earth. Yet, ETH Zurich researchers believe otherwise. Geochemical studies showed that, in fact, the chemical composition of the Earth is slightly different than the composition of the chondritic meteorites.

For example, neodymium concentration in Earth's crust is different from that of chondritic meteorites. Because neodymium cannot exist in Earth's core, some scientists believe that it may be found in high concentrations inside the mantle. However, this theory is not plausible because the heat convection inside the mantle would push neodymium to the surface, which is clearly not the case.

While looking for an alternative explanation, Bernard Bourdon, Professor of Isotope Geochemistry at ETH Zurich and leader of the study, started comparing the concentrations of samarium and neodymium isotopes in rocks from Earth, Mars and the Vesta asteroid. Samarium 147 and samarium 146 both decompose into neodymium 143 and neodymium 142, thus they can reveal what kind of processes could have accounted for the different decomposing rates in the early life of the solar system.

The results of the study show that the chemical composition of chondritic meteorites and that of Mars and the Moon are different, whereas the Earth, Moon and Mars have the same samarium-neodymium isotope ratio, up to eight percent higher than that of chondritic meteorites. "The variance may not seem all that much. But it is significant enough to be inconsistent with the classic model", explains Bourdon.

"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", he continues.

There are two possible explanations for how this might have happened. The first implies that the solar nebula ceased being homogeneous at some point in time, causing the Earth, Moon and Mars to have identical isotopic compositions but different from that of Vesta and chondritic meteorites, which is extremely plausible if considering the involved distances.

Alternatively, the first planetary bodies in the solar system, the planetesimals, may have had different chemical compositions but the planetary collisions that formed the planets we observe today may have ejected into space the original crust of the planetesimals exposing the material coming from the solar nebula.


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