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Researchers at Lawrence Livermore National Laboratory believe isotopic variations in terrestrial and meteoritic samples prove that Earth and Mars were formed by collisions of planetary embryos originating from the inner solar system.

Scientists have long believed that rocky planets throughout the universe were formed by the collisions of planetary embryos from inner star systems or by the coalescence of small starward-drifting “pebbles” from the outer star systems, two fundamentally different processes. However, there has been no consensus on how the two outermost terrestrial planets in the Earth’s solar system – Earth and Mars -- were formed.

In new research, which appears in the Dec. 22 issue of Science Advances, the scientists reported that the isotopic compositions of Earth and Mars predominantly result from the accretion of planetary bodies from the inner solar system, with only a small percentage of their mass coming from outer solar system bodies.

“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 the University of Münster in Germany. “This low fraction of outer solar system material in Earth and Mars suggests the presence of a persistent dust-drift barrier in the planetary disk and highlights the specific pathway of rocky planet formation in the solar system.”

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

The research team used data on isotope variations among non-carbonaceous meteorites to show that Earth and Mars contain material unsampled among meteorites. After determining the origin and isotopic composition of this “lost” planetary building material, they were able to assess the amount of carbonaceous material accreted by Earth and Mars.

The team included researchers from the University of Münster, the Université de Nice Sophia-Antipolis in France, 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.