Building materials for Earth and Mars come from the inner solar system

The research team examined the isotopic composition of the rocky planets

Earth and Mars were made of materials that came mostly from the inner solar system. Only a small percentage of the “building materials” of these two planets have an origin outside the orbit of Jupiter. This is the result of a group of researchers led by Westphalian Wilhelms University (WWU) in Münster. In today’s Science Advances they present the most comprehensive comparison of the isotopic compositions of Earth and Mars and the original building materials of the inner and outer solar system. Some of this material can still be found today largely unadulterated in meteorites. The results of the study have far-reaching consequences for our ideas about the formation process of the planets closest to the Sun, Mercury, Venus, Earth and Mars. It disproves the theory that the four rocky planets grew to their current size through the accretion of millimeter-sized clumps of dust from the outer solar system. Thorsten Kleine, the study’s lead, recently became director of the Max Planck Institute for Solar System Research in Göttingen.

When our solar system was born about 4.6 billion years ago, a disk of dust and gas was orbiting the still young sun. Two theories describe how the rocky inner planets formed from this original building material over millions of years. According to the ancient theory, the dust clumps together in larger and larger clumps, which gradually reached the size of our moon in the inner solar system. Collisions between these former planets finally led to Mercury, Venus, Earth, and Mars. On the other hand, a newer theory favors a different growth process: according to this, millimeter-sized clumps of dust migrate from the outer solar system towards the sun. On their way, they encountered the planetary ancestors of the inner solar system, settled there and gradually helped them reach their current size.

Both theories are based on model calculations and computer simulations that simulate relationships and motions in the early solar system; Both describe a possible method of planet formation. But which one is right? What process actually happened? To clarify this question, researchers from the WWU, the Observatoire de la Côte d’Azur (Nice, France), the California Institute of Technology (Pasadena, USA), the Musée für Naturkunde (Berlin) and the Free University of Berlin in their current study considered the microstructure The rocky planets Earth and Mars. The first author, Dr. Christoph Burckhardt of the WWU. Isotopes of rare metals such as titanium, zirconium and molybdenum, which are found in small traces in the silicate-rich outer layers of both planets, provide important clues. Isotopes refer to different types of the same element, which differ only in the weight of their atomic nuclei.

Scientists hypothesize that these and other metallic isotopes were not evenly distributed in the early solar system. Rather, its frequency depends on the distance from the sun. Thus, the isotope frequencies provide information about where the body building materials originated in the early solar system.
The researchers used two types of meteorites as references for the original inventory of isotopes in the outer and inner solar system. These rocks usually found their way to Earth from the asteroid belt, the area between the orbits of Mars and Jupiter. They are considered largely unchanged materials since the beginnings of the solar system. Whereas the so-called carbonaceous chondrites, which can contain as little as carbon, originated on the far side of Jupiter’s orbit and later moved into the asteroid belt due to the influence of the growing giant gas, their low-carbon cousins, the non-carbonaceous chondrites, are real babies. In the inner world of the solar system.

The exact isotopic composition of Earth’s accessible outer rock layers and those of the two meteorite types have been researched for some time; To date, no comprehensive comparable analyzes of Martian rocks have been performed. In their current study, the researchers examined samples from a total of 17 Martian meteorites, which can be assigned to six typical types of Martian rocks. In addition, scientists are tracking for the first time the effects of three different mineral isotopes.

Samples from Martian meteorites were first crushed and chemically treated in a complex process. With the help of a multi-complexer plasma mass spectrometer at the Institute of Planetary Science at the University of Münster, the researchers were then able to track down trace amounts of the isotopes of titanium, zirconium and molybdenum. On a computer, the scientists also simulated the ratio of building materials found today in carbonate and non-carbonate chondrites to be entered in order to reproduce the measured values. They also considered two different phases of material input. Because unlike titanium and zirconium, molybdenum accumulates mainly in the core of the metal planets. The small amounts that are still present today in the silicate-rich outer layers can only be added at the last stage of the planet’s growth.

The researchers’ findings show that the outer rocky layers of Earth and Mars have little in common with the carbonaceous chondrites of the outer solar system. Their share in the original building materials of both planets is only about four percent. Says Professor Dr. WWU’s Thorsten Kleine, who is also director of the Max Planck Institute for Solar System Research in Göttingen. “We cannot confirm this theory of inner planet formation,” he adds.

But the composition of Earth and Mars doesn’t quite match that of non-carbonaceous chondrites either. The model’s calculations indicate that various other types of building materials must be included. The origin of this third type of building material must be in the deepest part of the solar system. This can be inferred from the isotopic composition that our calculations must have,” explains Christoph Burckhardt. Since masses of rocks from such close proximity to the Sun have never been scattered in the asteroid belt, they are almost completely absorbed into the inner planets. They do not occur in meteorites. He says Thorsten Kleine: “The ‘lost building materials’, so to speak, we no longer have direct access to today.”

The surprising discovery does not change the study’s consequences for theories of planet formation. Christoph Burckhardt concludes that “the fact that Earth and Mars seem to contain primarily material from the inner solar system is well in line with the formation of planets from collisions of large bodies in the inner solar system.”