Earth and Mars may have formed through different mixtures of planetary building processes, despite developing side by side.
Earth and Mars formed near each other about 4.5 billion years ago, yet new research suggests they grew through very different processes.
“The most surprising result was that Earth and Mars appear to have formed in different ways. You might have expected that two planets formed side by side in the same solar system would share a more similar formation history,” says Professor Anders Johansen, who studies planet formation at the Globe Institute, University of Copenhagen. He co-led the new study with Assistant Professor Haiyang Wang.
Two Different Paths to Planet Formation
Rocky planets can grow through collisions between large bodies called planetesimals, through pebble accretion, in which smaller particles are swept up, or through a combination of both.
The new study supports that hybrid picture, but also indicates Earth and Mars relied on those processes in very different proportions.
“At least 75 percent of Earth’s mass appears to originate from two young planets, known as protoplanets, that grew large by accreting pebbles, while planetesimals contributed up to 25 percent. In contrast, roughly three-quarters of Mars’ mass appears to come from planetesimals, with the remaining quarter originating from pebble accretion,” says Haiyang Wang.
Chemical Clues From the Early Solar System
To reconstruct the planets’ histories, the researchers analyzed volatile elements such as sodium, zinc, and potassium in their crusts and mantles. Because these elements can evaporate at high temperatures, their present-day abundances preserve clues about the conditions each planet experienced while forming.
The team combined those chemical fingerprints with computer models to test which formation scenarios best matched the planets seen today.
“It is a major detective job to figure out what happened back then when most of the evidence disappeared long ago. But even after 4.5 billion years, the compositions of Earth’s and Mars’ mantles remain the same. You can think of them as an imprint of the formation process,” says Anders Johansen.
The models still depend on assumptions because the exact composition of the Solar System’s original building materials is unknown. Even so, the researchers say the contrast between Earth and Mars remained consistent when those assumptions were changed.
“The exact percentages may vary somewhat, but our analyses consistently indicate that Earth and Mars formed in two different ways. Our method provides a more precise and direct way of understanding planet formation than the more widely used isotope-based approach, which can often be interpreted in multiple ways,” says Haiyang Wang.
Implications for Habitable Worlds
The same approach could eventually help researchers study rocky planets beyond the Solar System. Formation conditions influence how much volatile material a planet retains, including water and other substances linked to habitability.
“If we understand how planets lose volatile elements during their formation, we can also become better at predicting how much water and other life-supporting substances they ultimately retain,” says Anders Johansen.
Reference: “Volatile depletion in rocky planets as a chemical fingerprint of hybrid accretion” by Haiyang S. Wang, Anders Johansen, Ziyan Xu, Marie-Luise Steinmeyer, Michiel Lambrechts, Elishevah van Kooten, Chao-Chin Yang, Zhaohuan Zhu, Dante S. Lauretta and Martin Bizzarro, 25 September 2026, Nature Astronomy.
DOI: 10.1038/s41550-026-02984-6
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