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Earth and Mars formation

Jan 10, 2022

Science

Earth and Mars formation scenario in the Solar System

 

By looking at the range of isotopic variations in terrestrial and meteoritic samples, a Lawrence Livermore National Laboratory (LLNL) scientist and collaborators have figured out that Earth and Mars formed by collisions of planetary embryos originating from the inner solar system. 1)

Figure 1: An artist's conception of the developing solar system, with the young sun at its center and (proto-)planets accreting dust and debris from the disk (image courtesy of NASA)
Figure 1: An artist's conception of the developing solar system, with the young sun at its center and (proto-)planets accreting dust and debris from the disk (image courtesy of NASA)

Rocky planets may have formed by two fundamentally different processes, but it is unclear which one built the terrestrial planets of our solar system. The planets formed either by collisions among planetary embryos from the inner solar system or by accreting sunward-drifting millimeter-sized "pebbles" from the outer solar system.

In the new research, the team showed that the isotopic compositions of Earth and Mars predominantly result from the accretion of planetary bodies from the inner solar system, including material from the innermost disk unsampled by meteorites, with only a few percentages of a planet's mass coming from outer solar system bodies. The research appears in the Dec. 22 issue of Science Advances. 2)

"Our data refute a pebble accretion origin of the terrestrial planets but are consistent with collisional growth from inner solar system embryos," said LLNL scientist and co-author Jan Render, who performed part of the measurements while working as a postdoc at his previous position at the University of Münster. "This low fraction of outer solar system material in Earth and Mars suggests the presence of a persistent dust-drift barrier in the disk and highlights the specific pathway of rocky planet formation in the solar system."

Determining which of the two processes governed the formation of the terrestrial planets of our solar system is crucial for understanding the solar system's architecture and dynamical evolution, and for placing planet formation in the solar system into the context of general planet formation processes, such as those observed in exoplanetary systems.

The amount of outer solar system material accreted by the terrestrial planets may be determined using nucleosynthetic isotope anomalies. These arise from the heterogeneous distribution of presolar matter within the solar protoplanetary disk and provide a record of the heritage of a planet's building material. These isotope anomalies permit distinguishing between non-carbonaceous (NC) and carbonaceous (CC) meteorites, which are commonly assumed to represent planetary bodies that accreted in the inner and outer solar system, respectively.

The team used the recent observation of correlated isotope variations among NC meteorites to show that both Earth and Mars incorporated material unsampled among meteorites, determined the provenance and isotopic composition of this lost planetary building material and used this information to assess the amount of CC material accreted by Earth and Mars.

Other contributors include researchers from the University of Münster, the Université de Nice Sophia-Antipolis, the California Institute of Technology and the Museum für Naturkunde and the Freie Universitat in Berlin. The work is funded by the German Research Foundation.

Figure 2: Possible scenarios of terrestrial planet formation. In the classic "Wetherill-type" model of oligarchic growth, the terrestrial planets formed by mutual collisions among Moon- to Mars-sized planetary embryos after the gas disk dissipated and accreted only a small fraction of CC planetesimals, which were scattered inward during Jupiter's growth and/or putative migration. Alternatively, the terrestrial planets may have formed within the lifetime of the gas disk by efficiently accreting "pebbles" from the outer solar system, which drift sunward through the disk due to gas drag. The two models differ in the amount of outer solar system (CC) material accreted by the terrestrial planets, which may be quantified using nucleosynthetic isotope anomalies (image credit: LLNL)
Figure 2: Possible scenarios of terrestrial planet formation. In the classic "Wetherill-type" model of oligarchic growth, the terrestrial planets formed by mutual collisions among Moon- to Mars-sized planetary embryos after the gas disk dissipated and accreted only a small fraction of CC planetesimals, which were scattered inward during Jupiter's growth and/or putative migration. Alternatively, the terrestrial planets may have formed within the lifetime of the gas disk by efficiently accreting "pebbles" from the outer solar system, which drift sunward through the disk due to gas drag. The two models differ in the amount of outer solar system (CC) material accreted by the terrestrial planets, which may be quantified using nucleosynthetic isotope anomalies (image credit: LLNL)

 

Some background

• December 22, 2021: International research team investigated the isotopic composition of rocky planets in the inner Solar System. 3)

- Earth and Mars were formed from material that largely originated in the inner Solar System; only a few percent of the building blocks of these two planets originated beyond Jupiter's orbit. A group of researchers led by the University of Münster (Germany) report these findings today in the journal "Science Advances". They present the most comprehensive comparison to date of the isotopic composition of Earth, Mars and pristine building material from the inner and outer Solar System. Some of this material is today still found largely unaltered in meteorites. The results of the study have far-reaching consequences for our understanding of the process that formed the planets Mercury, Venus, Earth, and Mars. The theory postulating that the four rocky planets grew to their present size by accumulating millimeter-sized dust pebbles from the outer Solar System is not tenable.

- Approximately 4.6 billion years ago in the early days of our Solar System, a disk of dust and gases orbited the young Sun. Two theories describe how in the course of millions of years the inner rocky planets formed from this original building material. According to the older theory, the dust in the inner Solar System agglomerated to ever larger chunks gradually reaching approximately the size of our Moon. Collisions of these planetary embryos finally produced the inner planets Mercury, Venus, Earth, and Mars. A newer theory, however, prefers a different growth process: millimeter-sized dust "pebbles" migrated from the outer Solar System towards the Sun. On their way, they were accreted onto the planetary embryos of the inner Solar System, and step by step enlarged them to their present size.

- Both theories are based on theoretical models and computer simulations aimed at reconstructing the conditions and dynamics in the early Solar System; both describe a possible path of planet formation. But which one is right? Which process actually took place? To answer these questions, in their current study researchers from the University of Münster (Germany), the Observatoire de la Cote d'Azur (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 find out whether the building blocks of Earth and Mars originated in the outer or inner Solar System", says Dr. Christoph Burkhardt of the University of Münster, the study's first author. To this end, the isotopes of the rare metals titanium, zirconium and molybdenum found in minute traces in the outer, silicate-rich 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.

Figure 3: The four terrestrial planets: Mercury, Venus, Earth and Mars (image credit: NASA/Lunar and Planetary Institute)
Figure 3: The four terrestrial planets: Mercury, Venus, Earth and Mars (image credit: NASA/Lunar and Planetary Institute)

Meteorites as a reference

- Scientists assume that in the early Solar System these and other metal isotopes were not evenly distributed. Rather, their abundance depended on the distance from the Sun. They therefore hold valuable information about where in the early Solar System a certain body's building blocks originated.

- As a reference for the original isotopic inventory of the outer and inner Solar System, the researchers used two types of meteorites. These chunks of rock generally found their way to Earth from the asteroid belt, the region between the orbits of Mars and Jupiter. They are considered to be largely pristine material from the beginnings of the Solar System. While so-called carbonaceous chondrites, which can contain up to a few percent carbon, originated beyond Jupiter's orbit and only later relocated to the asteroid belt due to influence of the growing gas giants, their more carbon-depleted cousins, the non-carbonaceous chondrites, are true children of the inner Solar System.

- The precise isotopic composition of Earth's accessible outer rock layers and that of both types of meteorites have been studied for some time; however, there have been no comparably comprehensive analyses of Martian rocks. In their current study, the researchers now examined samples from a total of 17 Martian meteorites, which can be assigned to six typical types of Martian rock. In addition, the scientists for the first time investigated the abundances of three different metal isotopes.

- The samples of Martian meteorites were first powdered and subjected to complex chemical pretreatment. Using a multicollector plasma mass spectrometer at the Institute of Planetology at the University of Münster, the researchers were then able to detect tiny amounts of titanium, zirconium, and molybdenum isotopes. They then performed computer simulations to calculate the ratio in which building material 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 phases of accretion to account for the different history of the titanium and zirconium isotopes as well as of the molybdenum isotopes, respectively. Unlike titanium and zirconium, molybdenum accumulates mainly in the metallic planetary core. The tiny amounts 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.

Figure 4: The Martian Meteorite Elephant Moraine (EETA) 79001. The scientists examined these and other Martian meteorites in the study. (image credit: NASA/JSC)
Figure 4: The Martian Meteorite Elephant Moraine (EETA) 79001. The scientists examined these and other Martian meteorites in the study. (image credit: NASA/JSC)

- 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 account for only about four percent of both planets' original building blocks. "If early Earth and Mars had mainly accreted dust grains from the outer Solar System, this value should be almost ten times higher," says Prof. Dr. Thorsten Kleine of the University of Münster, who is also director at the Max Planck Institute for Solar System Research in Göttingen. "We thus cannot confirm this theory of the formation of the inner planets," he adds.

Lost building material

- But the composition of Earth and Mars does not exactly match the material of the non-carbonaceous chondrites either. The computer simulations suggest that another, different kind of building material must also have been in play. "The isotopic composition of this third type of building material as inferred by our computer simulations implies it must have originated in the innermost region of the Solar System", explains Christoph Burkhardt. Since bodies from such close proximity to the Sun were almost never scattered into the asteroid belt, this material was almost completely absorbed into the inner planets and thus does not occur in meteorites. "It is, so to speak, 'lost building material' to which we no longer have direct access today," says Thorsten Kleine.

- The surprising find does not change the consequences of the study for theory of planet formation. "The fact that Earth and Mars apparently contain mainly material from the inner Solar System fits well with planet formation from the collisions of large bodies in the inner Solar System," concludes Christoph Burkhardt.



1) Anne M Stark, "Lost in space: Rocky planets formed from missing solar system material," LLNL News, 22 December 2021, URL: https://www.llnl.gov/news/lost-space-rocky-planets-formed-missing-solar-system-material

2) Christoph Burkhardt, Alessandro Morbidelli, Gerrit Budde, Jan H. Render , Thomas S. Kruijer and Thorsten Kleine , "Terrestrial planet formation from lost inner solar system material," Science Advances, Volume 7, Issue 52, Published: 22 December 2021, URL: https://www.science.org/doi/pdf/10.1126/sciadv.abj7601, URL:

3) "Earth and Mars were formed from inner Solar System material," University of Muenster News, 22 December 2021, URL: https://www.uni-muenster.de/news/view.php?cmdid=12266

The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (eoportal@symbios.space).

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