Ancient meteorite discovery may explain Earth’s missing elements

Scientists have long debated why Earth and Mars lack certain essential elements. A new study reveals that these missing pieces may not have been absent from the start but were instead lost during violent cosmic collisions that shaped the planets.

Moderately volatile elements (MVEs), including copper, zinc, and potassium, are vital for planetary chemistry. These elements frequently accompany life-essential components like carbon, nitrogen, and water. Understanding their distribution in the early solar system sheds light on why Earth developed into a habitable planet while Mars remained barren.

Compared to chondrites, the most primitive meteorites, Earth and Mars have far fewer MVEs. Scientists have proposed two primary explanations: either these elements never fully condensed in the early solar nebula, or they were lost through differentiation and collisions as planetesimals—the early building blocks of planets—formed.

To investigate, a research team led by Assistant Professor Damanveer Grewal from Arizona State University analyzed iron meteorites, which are remnants of the metallic cores of some of the earliest planetesimals. Their study, published in Science Advances, challenges existing theories and suggests that early planetesimals in the inner solar system were not as depleted in MVEs as previously thought.

Meteorites provide a unique window into the formation of planets. Since they are fragments of planetesimals from the early solar system, they preserve chemical signatures that can reveal the history of planetary building blocks.

Previous research suggested that inner solar system planetesimals were depleted in MVEs from the beginning. Scientists had largely based this assumption on two types of differentiated meteorites—angrites and howardite-eucrite-diogenite (HED) meteorites.

These are highly depleted in MVEs, leading researchers to believe that planetary building blocks formed under conditions that favored volatile loss. However, angrites and HED meteorites primarily sample the outer crust and upper mantle of their parent bodies, leaving uncertainty about whether this depletion extended to entire planetesimals.

By analyzing magmatic iron meteorites, the Arizona State University team found evidence that the first-generation planetesimals of the inner solar system actually retained significant amounts of MVEs.

This finding upends the idea that volatile depletion occurred solely during the condensation of solid materials in the early solar nebula. Instead, it suggests that planets like Earth and Mars initially formed from planetesimals that were rich in MVEs but lost these elements during later stages of planetary growth.

The loss of MVEs in Earth and Mars likely resulted from a period of intense collisional evolution rather than from conditions in the early solar nebula. When planetesimals collided and merged, some of their volatile-rich crust and mantle material was stripped away, leading to a gradual depletion of MVEs over time.

“Our work redefines how we understand the chemical evolution of planets,” Grewal explained. “It shows that the building blocks of Earth and Mars were originally rich in these life-essential elements, but intense collisions during planetary growth caused their depletion.”

This study also highlights a key difference between the inner and outer solar system. While planetesimals in the outer solar system accreted under cooler conditions and retained more of their original volatiles, those in the inner solar system formed at higher temperatures and experienced more energetic impacts. These events gradually removed volatile elements, leading to the compositions observed in Earth and Mars today.

One of the major advantages of using magmatic iron meteorites in this study is that they represent the metallic cores of ancient planetesimals. Unlike basaltic meteorites such as angrites and HEDs, which mainly sample crustal material, iron meteorites provide a record of planetesimal differentiation without the effects of surface-level processes like volcanic activity and impact melting.

The researchers reconstructed the bulk chemical compositions of these early planetesimals using elemental abundances in their metallic cores. They found that while some planetesimals were depleted in MVEs, many retained chondrite-like compositions, indicating that depletion was not a universal characteristic of early planetary building blocks.

Another advantage of using iron meteorites is that they sample a wider range of parent bodies than angrites and HEDs, which come from only two distinct sources. By analyzing multiple iron meteorite groups, the researchers were able to compare planetesimals from different regions of the early solar system, providing a more comprehensive picture of planetary formation.

This study suggests that instead of starting out MVE-poor, the building blocks of Earth and Mars initially contained significant amounts of these elements. Their loss was a later consequence of the violent processes that shaped terrestrial planets.

By rethinking how MVEs were distributed in the early solar system, scientists can refine models of planetary formation and better understand why Earth evolved into a habitable world.

Future research will likely focus on identifying the specific types of collisions and impacts that contributed to the volatile depletion of terrestrial planets, further unraveling the complex history of our solar system.

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