International research team studied isotopic composition of rocky planets in the inner solar system – sciencedaily

Earth and Mars were formed from material largely derived from the inner solar system; only a few percent of the building blocks of these two planets originate from beyond Jupiter’s orbit. A group of researchers led by the University of Münster (Germany) reports these findings today in the journal Scientists progress. They present the most comprehensive comparison to date of the isotopic makeup of Earth, Mars, and pristine building materials from the inner and outer solar system. Part of this material is still found largely unchanged in meteorites today. The results of the study have far-reaching implications for our understanding of the process that formed the planets Mercury, Venus, Earth, and Mars. The theory that the four rocky planets reached their current size by accumulating millimeter-sized pebbles of dust from the outer solar system is not tenable.

About 4.6 billion years ago, in the early days of our solar system, a disk of dust and gas circled around the young Sun. Two theories describe how, over millions of years, the rocky inner planets formed from this original building material. According to the old theory, the dust in the inner solar system has agglomerated into larger and larger pieces gradually reaching approximately the size of our Moon. The collisions of these planetary embryos eventually produced the inner planets Mercury, Venus, Earth, and Mars. A more recent theory, however, prefers a different growth process: Dust “pebbles” the size of a millimeter migrated from the outer solar system to the Sun. Along the way, they accreted themselves to the planetary embryos of the inner solar system and gradually enlarged them to their current size.

Both theories are based on theoretical models and computer simulations aimed at reconstructing the conditions and dynamics of the early solar system; both describe a possible path for the formation of planets. But which one is right? What process actually took place? To answer these questions, in their current study, researchers from the University of Münster (Germany), the Côte d’Azur Observatory (France), the California Institute of Technology (USA), the Natural History Museum Berlin (Germany) and the Free University of Berlin (Germany) determined the exact composition of the rocky planets Earth and Mars. “We wanted to know whether the building blocks of Earth and Mars came from the outer or inner solar system,” says Dr Christoph Burkhardt of the University of Münster, the study’s first author. To this end, isotopes of the rare metals titanium, zirconium and molybdenum found in minute traces in the silicate-rich outer layers of both planets provide crucial clues. Isotopes are different varieties of the same element, which differ only in the weight of their atomic nucleus.

Meteorites as a reference

Scientists assume that at the start of the solar system, these isotopes and other metals were not distributed evenly. On the contrary, their abundance depended on the distance from the Sun. They therefore hold precious information on the origin, in the primitive solar system, of the constituent elements of a certain body.

As a benchmark for the original isotopic inventory of the outer and inner solar system, the researchers used two types of meteorites. These pieces of rock generally found their way to Earth from the asteroid belt, the region between the orbits of Mars and Jupiter. They have been considered largely virgin material since the early days of the solar system. While the so-called carbonaceous chondrites, which can contain up to a few percent carbon, appeared beyond Jupiter’s orbit and were only later moved into the asteroid belt due to the influence of growing gas giants, their lower carbon cousins, the non-carbon chondrites, are true children of the inner solar system.

The precise isotopic composition of the outer rock layers accessible to Earth and that of the two types of meteorites have been studied for some time; however, there have not been such comprehensive analyzes of Martian rocks. In their current study, the researchers have now looked at samples from a total of 17 Martian meteorites, which can be attributed to six typical Martian rock types. In addition, scientists have studied for the first time the abundance of three different metal isotopes.

The Martian meteorite samples were first powdered and subjected to a complex chemical pretreatment. Using a multi-collector plasma mass spectrometer from the University of Münster’s Institute of Planetology, the researchers were then able to detect minute amounts of titanium, zirconium and molybdenum isotopes. They then performed computer simulations to calculate the ratio in which building materials found today in carbonaceous and non-carbonaceous chondrites must have been incorporated into Earth and Mars in order to reproduce their measured compositions. In doing so, they considered two different accretion phases to account for the different history of titanium and zirconium isotopes as well as molybdenum isotopes, respectively. Unlike titanium and zirconium, molybdenum mainly accumulates in the metallic planetary core. The tiny amounts that are still found today in the silicate-rich outer layers can therefore only have been added during the very last phase of the planet’s growth.

The researchers’ results show that the outer rock layers of Earth and Mars have little in common with the carbonaceous chondrites of the outer solar system. They represent only about four percent of the original building blocks of the two planets. “If the early Earth and Mars had mainly accreted dust grains from the outer solar system, this value would have been almost ten times higher,” says Professor Thorsten Kleine of the University of Münster, who is also director of the Institute. Max Planck for Solar System Research in Göttingen. “We therefore cannot confirm this theory of the formation of the inner planets,” he adds.

Lost building material

But the makeup of Earth and Mars also doesn’t exactly match the material of non-carbonaceous chondrites. Computer simulations suggest that another different type of building material must also have been involved. “The isotopic composition of this third type of building material, as inferred by our computer simulations, implies that it must come from the innermost region of the solar system, ”explains Christoph Burkhardt. Since the bodies so close to the Sun have hardly ever been dispersed in the asteroid belt, this material has been almost completely absorbed by the inner planets and therefore is not found in meteorites. “It is, so to speak, a ‘lost building material’ to which we no longer have direct access today”, explains Thorsten Kleine.

The surprising finding does not change the consequences of the study for the theory of planetary formation. “The fact that Earth and Mars apparently mainly contain material from the inner solar system corresponds well to the formation of planets from collisions of large bodies in the inner solar system,” concludes Christoph Burkhardt.

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