The research team, led by Haiyang S. Wang of the University of Copenhagen together with planet-formation theorist Anders Johansen and colleagues across Europe and the United States, combined models of pebble accretion with collisional growth from volatile-depleted planetesimals. Pebble accretion is the process by which young planets sweep up millimetre-to-centimetre-sized particles of dust and ice drifting through the protoplanetary disk, a mechanism known to be extraordinarily fast. Planetesimal accretion, by contrast, is the slower, more chaotic route in which larger rocky bodies collide and merge over millions of years. Rather than pitting these two theories against each other, the team asked whether both could have contributed, and whether the resulting mixture could be read directly from the planets’ chemistry.

The key insight is that each growth channel leaves a distinct thermal signature. Pebbles falling onto a growing protoplanet must pass through a hot hydrogen and helium gas envelope that surrounds the embryo. Using a one-dimensional model of this envelope, the researchers calculated what happens to elements of different volatility during the descent. Elements whose sublimation temperatures fall below the temperature at the bottom of the envelope are vaporized, and once vaporized they are efficiently carried away. The team validated this assumption with three-dimensional hydrodynamical simulations of the convective envelope, tracking nearly a million tracer particles. Within a single year only about thirty percent of the released volatiles remained in the envelope, and after five years less than three percent survived, because large-scale recycling flows continuously exchange envelope gas with the surrounding disk.

Crucially, this thermal processing is selective. Refractory elements such as aluminium, calcium and titanium, along with the moderately refractory magnesium and silicon, are protected. When silicates sublimate deep in the envelope, they form a dense vapour atmosphere in equilibrium with the underlying magma ocean, and an inner radiative zone where silicate clouds form strongly suppresses convection, preventing these vapours from diffusing upward and escaping. The moderately volatile lithophile elements, by contrast, are lost almost completely once the growing protoplanet becomes massive enough for its envelope base to exceed their sublimation temperatures. The result is a characteristic volatility pattern: a sharp drop in abundance for elements more volatile than roughly the lithium and manganese threshold, with refractory elements essentially untouched.

The second ingredient in the hybrid recipe comes from meteorites. The asteroid Vesta, often regarded as a surviving planetary embryo, shows strong depletion of elements with sublimation temperatures below about 1,200 kelvin, a pattern likely produced when short-lived radioactive isotopes such as aluminium-26 melted early-formed planetesimals and boiled away their volatiles. The angrite meteorite group shows an even more extreme depletion, losing even silicon and magnesium, which suggests that thermal processing of the first generation of planetesimals was widespread. The team adopted Vesta’s composition as representative of this population of baked planetesimals, which then contributed their volatile-poor material to the growing planets through collisions.

With these building blocks characterized, the researchers turned to Bayesian inference, a statistical framework that weighs competing models by how well they explain the data while penalizing unnecessary complexity. For Earth, the best-fitting model describes a three-component mixture: a proto-Earth and a giant impactor, both grown by pebble accretion and therefore stripped of their moderately volatile elements, plus a contribution from Vesta-like planetesimals. The analysis indicates that at least about seventy-five percent of Earth’s mass came from the two pebble-grown bodies, with up to roughly twenty-five percent from the baked planetesimals. If the planetesimal population is allowed to have a hypothetical depletion pattern not observed among known meteorite parent bodies, its contribution could rise to around forty percent. Models lacking the pebble-grown components were disfavoured decisively, and adding volatile-rich, carbonaceous-chondrite-like material improved nothing, contributing no more than ten percent of Earth’s mass.

The impactor component is particularly intriguing because it connects naturally to the Moon-forming giant impact. The inferred mass fraction of the impactor, between eight and forty-three percent of Earth’s mass, comfortably brackets both the canonical Theia mass of about ten to fifteen percent and the larger masses required in scenarios where a giant impact disrupts an early resonant chain of planets. The team also tested scenarios with two or three impactors. A second, small impactor remains statistically plausible, but a third is disfavoured, and its mass would be negligible. Notably, the preferred number of giant impacts is lower than the average of about three seen in classical N-body simulations, which the authors attribute to pebble accretion needing fewer planetary seeds to build a terrestrial-mass world, and therefore producing fewer embryos to collide.

Mars tells a complementary story. Because isotopic evidence indicates that Mars formed and differentiated its core rapidly, within the lifetime of the protoplanetary disk, the researchers modelled it as a combination of pebbles and planetesimals accreted early. The Bayesian solution assigns twenty-seven plus or minus five percent of Mars’s mass to pebble accretion and seventy-three plus or minus five percent to Vesta-like planetesimals, a result consistent with independent silicon isotope constraints. Allowing the planetesimal composition to vary freely reveals that an even more volatile-depleted population, more extreme than Vesta, could also fit the data. In every scenario tested, planetesimals dominate Mars’s budget while pebble accretion never falls below about ten percent, reinforcing that even the smaller terrestrial planet grew by hybrid means. The contrast with Earth, where pebble accretion was dominant, suggests Mars’s pebble supply was comparatively sluggish, perhaps because gravitational stirring by larger neighbouring protoplanets perturbed its orbit.

The implications extend well beyond the two planets we know best. The hybrid-accretion framework makes testable predictions about core chemistry: because moderately volatile siderophile elements such as sulfur can hide in the iron cores of planetesimals, the model yields sulfur abundances for bulk Earth and Mars that align with recent geophysical estimates of core composition rather than with older, lower values inferred from volatility trends alone. The model also dovetails with hafnium-tungsten chronometry, since a three-component Earth with substantial planetesimal input naturally explains our planet’s low tungsten-182 excess in the mantle. And because the same thermal physics operates around other stars, the framework offers a general tool for interpreting the volatile depletion observed in polluted white dwarf atmospheres and in Sun-like stars that have ingested planetary material, evidence that devolatilization may be a universal feature of rocky planet formation.

Perhaps most profoundly, the study reframes the origin of the ingredients for life. Carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur would have been largely stripped away during the pebble accretion phase, even for planets growing just outside the water ice line. Their delivery to rocky worlds must therefore have come through other channels: early accretion of volatile-rich pebbles before the protoplanet grew hot enough to expel them, impacts with smaller differentiated bodies, and late arrival of primitive planetesimals that escaped thermal processing. Reading the volatility patterns of rocky planets as accretion histories in this way transforms planetary chemistry from a static inventory into a dynamic record, one that astronomers can now begin to apply to the growing catalogue of rocky exoplanets as they assess which distant worlds might have retained the volatile inventory that habitability demands.

Subject of Research: Volatile element depletion in Earth and Mars as a record of hybrid pebble and planetesimal accretion

Article Title: Volatile depletion in rocky planets as a chemical fingerprint of hybrid accretion

Article References: Volatile depletion in rocky planets as a chemical fingerprint of hybrid accretion. (n.d.). https://doi.org/10.1038/s41550-026-02984-6

Image Credits: AI Generated

DOI: 10.1038/s41550-026-02984-6

Keywords: pebble accretion, planetesimals, volatile elements, Earth formation, Mars, protoplanetary disk, Bayesian inference, Vesta, giant impact, cosmochemistry, rocky exoplanets, planetary differentiation

The study, led by Professor Anders Johansen of the Globe Institute and Assistant Professor Haiyang Wang, was published in the journal Nature Astronomy. Rather than relying on the isotopic measurements that have dominated this field for decades, the team developed a novel approach based on planetary chemistry itself. By analyzing the elemental composition of the crust and mantle of Earth and Mars, and feeding those compositions into advanced statistical computer models, they reconstructed the earliest stages of each planet’s growth. The result, Johansen admits, surprised even the researchers who produced it.

“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 Johansen, who studies planet formation at the Globe Institute. To appreciate why that finding matters, it helps to understand the two competing theories of planet formation that have divided the community for years.

Researchers broadly agree that rocky planets are built primarily through one of two processes, or through a hybrid of both. In the first scenario, planets emerge when giant space rocks known as planetesimals collide and merge, accumulating mass through a series of enormous impacts. In the second, known as pebble accretion, these large bodies grow by sweeping up much smaller particles, millimeter to centimeter sized pebbles that drift inward through the gas of the young solar disk. Which of these mechanisms, or what combination of them, best explains the origin of Earth and Mars has never been definitively settled.

The Copenhagen team believes their findings support the so-called hybrid model, in which both mechanisms contributed, but their analysis goes a step further by assigning explicit proportions to each process for each planet. “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 Wang. In other words, Earth’s growth was dominated by the steady harvesting of pebbles onto two large precursor bodies, while Mars was built mainly through collisions between massive planetesimals.

The key to this detective work lies in volatile elements, substances such as sodium, zinc, and potassium that evaporate relatively easily at high temperatures. The researchers examined the abundance of these elements in the outer layers of the two planets, the crust and mantle, where their presence or absence acts as a chemical fingerprint of the formation process. Because pebbles lose volatile elements as they travel toward a planet’s surface, a planet assembled largely from pebbles should end up more depleted in these elements than one built primarily from planetesimal collisions. The team used computer simulations to demonstrate exactly this effect, showing that volatile elements are stripped from pebbles during their journey to the growing planet.

“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. This durability is what makes the method powerful. While the surfaces of both planets have been reshaped by volcanism, impacts, and erosion over billions of years, the bulk chemical composition of their mantles preserves a record of the material from which they were assembled, and therefore of the mechanism that assembled them.

The researchers are careful to acknowledge the uncertainties inherent in their models. The exact chemical composition of the original building blocks in the young Solar System that eventually became Earth and Mars is unknown, so the models must rely on several assumptions. These include the assumption that the planetesimal building blocks of both planets were chemically similar to the asteroid Vesta, and that volatile elements are lost more efficiently during pebble accretion than during giant impacts. Yet even when the team adjusted for those assumptions, the central conclusion held firm. “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.

That point about interpretability is significant. Isotope-based methods, which compare the ratios of variant atoms preserved in planetary materials, have long been the standard tool for probing planet formation, but their results can be ambiguous, admitting multiple plausible formation scenarios. The volatile-element fingerprinting approach developed by the Copenhagen team offers, in their view, a sharper and more direct diagnostic. By constraining which accretion process dominated for each planet, it narrows the space of acceptable models and brings the field closer to a consensus on how rocky planets actually grow in the inner regions of a forming solar system.

The implications may extend far beyond our own cosmic neighborhood. Several future space missions aim to discover and characterize Earth-like planets orbiting other stars, and understanding the chemical composition of such worlds is essential for assessing whether they could be habitable. Volatile elements are not merely tracers of formation history; they include the ingredients of habitability itself, and the amount of water and other life-supporting substances a planet retains depends directly on how those volatiles were processed during assembly. “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. In that sense, the chemical imprint preserved in Earth’s and Mars’ mantles for four and a half billion years may ultimately help scientists judge which of the thousands of known exoplanets are worth searching for signs of life, turning a question about our own origins into a tool for exploring everyone else’s.

Subject of Research: Chemical fingerprinting of volatile elements to reconstruct the differing formation histories of Earth and Mars

Article Title: New study rekindles contested question: Earth and Mars had different origins

Article References: New study rekindles contested question: Earth and Mars had different origins. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Earth, Mars, planet formation, pebble accretion, planetesimals, volatile elements, protoplanets, mantle composition, Nature Astronomy, hybrid accretion model, habitability, University of Copenhagen