Plate Nº 66 · recorded September 30, 2026
Space & AstronomyReported finding
Earth and Mars Were Built From Different Cosmic Recipes
A new University of Copenhagen study finds Earth formed mostly by pebble accretion while Mars formed mostly from colliding planetesimals, based on chemical fingerprints in their mantles.
By Nathan Brooks4 min read782 words
In brief
- At least 75% of Earth's mass comes from two protoplanets that grew by pebble accretion, while roughly three-quarters of Mars' mass comes from planetesimals.
- Researchers used volatile elements — sodium, zinc, and potassium — in the crust and mantle as chemical fingerprints of planetary formation.
- The study appears in Nature Astronomy (2026), DOI: 10.1038/s41550-026-02984-6, co-led by Anders Johansen and Haiyang Wang of the University of Copenhagen.
Four and a half billion years ago, Earth and Mars took shape in the same rotating cloud of gas and dust that became our solar system. You might expect two neighboring planets to share a similar origin story. According to a new study from the University of Copenhagen, they do not.
By analyzing the chemical composition of both planets, researchers reconstructed the earliest stages of their formation. The result caught the team off guard.
"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 Anders Johansen, professor of planet formation at the Globe Institute, University of Copenhagen. He co-led the study with assistant professor Haiyang Wang. The paper appears in the journal Nature Astronomy.
Two ways to build a planet
Planetary scientists broadly agree that planets form in one of two ways, or through a combination of both. In the first scenario, giant space rocks called planetesimals — bodies roughly tens to hundreds of kilometers across — collide and merge. In the second, called pebble accretion, these large rocks sweep up much smaller particles as they orbit the young star.
The scientific community has not settled on which process, or what mix of the two, best explains rocky planets like Earth and Mars. The Copenhagen team believes their findings support the hybrid model, in which both processes play a role. But their study goes further, offering a more explicit breakdown of how each planet came together 4.5 billion years ago.
"At least 75% 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%. In contrast, roughly three-quarters of Mars' mass appears to come from planetesimals, with the remaining quarter originating from pebble accretion," says Wang.
In plain terms: Earth is mostly a product of pebble accumulation, while Mars is mostly a product of colliding rocky bodies.
Reading chemistry as a record of the past
How do you study events that happened 4.5 billion years ago? Scientists must work with traces that survive today. The researchers examined specific elements in the outer layers of both planets — the crust and the mantle, the rocky layer beneath it.
They focused on volatile elements such as sodium, zinc, and potassium, which evaporate relatively easily at high temperatures. Whether these elements are present or absent acts as a chemical fingerprint, revealing the processes each planet underwent as it formed. The team then used advanced computer models to test which formation scenarios best explain the observed compositions.
"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 Johansen.
Caveats behind the numbers
The researchers are upfront about the limits of their work. Their computer models contain uncertainties. For one, the exact chemical composition of the original building blocks in our solar system — the material that eventually became Earth and Mars — remains unknown.
The models therefore rely on several assumptions. Still, the team reports that when they adjusted for those assumptions, the central conclusion held.
"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 Wang.
The isotope-based approach he refers to studies variants of chemical elements that differ slightly in mass, a common but sometimes ambiguous tool for reconstructing planetary history.
Why it matters beyond our solar system
The findings have implications for the search for life elsewhere. Several future space missions aim to discover and study Earth-like planets in other galaxies, and understanding their chemical compositions matters for judging whether they could support life.
"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 Johansen.
Water, after all, is the key volatile. A planet that keeps enough of it, along with other easily evaporated substances, has a better chance of being habitable.
The study, "Volatile depletion in rocky planets as a chemical fingerprint of hybrid accretion," by Haiyang S. Wang et al., was published in Nature Astronomy (2026), DOI: 10.1038/s41550-026-02984-6.
via Phys.org Space & Astronomy (Source)
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