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New theory to explain why the planets of our solar system have different compositions

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A team of researchers from the University of Copenhagen and the Museum für Naturkunde, Leibniz-Institut für Evolutions has presented a new explanation about the difference in the composition of the planets in our solar system. In their article published in the journal Nature they describe their study of the calcium isotope composition of certain meteorites, Earth itself and Mars, and use what they learned to explain how planets could be so different. Alessandro Morbidelli with Observatoire de la Côte d'Azur in France offers a News and Views article about the work done by the team in the same issue of the magazine.

As Morbidelli points out, most planetary scientists agree that the planets in our solar system had origins similar to the small rocks orbiting the sun, which comprise the protoplanetary disk, which collided and merged, creating rocks each time larger ones that eventually became protoplanets. But as of that moment, it is not clear why the planets were so different. In this new effort, researchers have devised a new theory to explain how this happened.

All the protoplanets grew at the same speed, the group suggests, but they stopped growing at different times. Those that were smaller, continue, stopped growing before those that were larger. During this time, they also suggest, the material is constantly added to the disk. At first, it seems that the composition of the material was different from the material that came later, which explains why the rocky planets we see today have such differences in composition.

The researchers developed their theory after studying the calcium isotope composition of several meteorites called angrites and ureilites, as well as those of Mars and Earth, and also of the asteroid Vesta. The calcium isotopes, they point out, are involved in the formation of rock and, because of that, offer clues to their origins. The researchers found that the isotopic ratios in the samples correlate with the masses of their parent planets and asteroids, which they claim provide a proxy for their accretion schedule. And that, they say, also provides evidence of the different compositions of the planets, since the smaller ones stopped accumulating material while the larger ones continued to add material that was different from what had come before.

Explore more:
Study sheds new light on how Earth and Mars were created

More information:
Martin Schiller et al. Isotopic evolution of the protoplanetary disk and the building blocks of the Earth and the Moon, Nature (2018). DOI: 10.1038 / nature25990


The variability of nucleosynthetic isotopes among objects in the Solar System is often used to investigate the genetic relationship between meteorite groups and rocky planets (Mercury, Venus, Earth and Mars), which in turn, can provide information about the basic components of the Earth-Moon system. Using this approach, it has been inferred that no primitive meteorite coincides with the terrestrial composition and the material of the protoplanetary disk from which the Earth and the Moon have increased is not greatly restricted6. This conclusion, however, is based on the assumption that the observed nucleosynthetic variability of the internal objects of the solar system predominantly reflects spatial heterogeneity. Here we use the isotopic composition of the calcium refractory element to show that the nucleosynthetic variability in the internal solar system reflects mainly a rapid change in the isotope composition of calcium independent of mass of protoplanetary disk solids associated with the early accumulation of mass at proto -Sun. We measured the independent mass proportions 48Ca / 44Ca of the samples originating in the parent bodies of ureilite and anthrasite meteorites, as well as of Vesta, Mars and Earth, and found that they are positively correlated with the masses of their asteroids and parent planets, which are a proxy of their accretion time scales. This correlation implies a secular evolution of the bulk calcium isotope composition of the protoplanetary disk in the terrestrial region of planetary formation. Individual chondrites of common chondrites formed within a million years of the collapse of proto-Sun7 reveal the complete range of 48Ca / 44Ca independent ratios of mass from the inner Solar System, indicating a rapid change in the composition of protoplanetary disk material. We infer that this secular evolution reflects the mixture of pristine material from the external Solar System in the thermally processed internal protoplanetary disk associated with mass accumulation in the proto-Sun. The identical calcium isotope composition of Earth and Moon reported here is a prediction of our model if the formative impact of Luna involved protoplanets or precursors that completed their accretion near the end of the life of the protoplanetary disk.

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