Earth and Mars Formed in Different Ways, Volatile Chemistry Study Finds

Four and a half billion years ago, the planets of our Solar System emerged from a vast rotating cloud of gas and dust. Yet the precise mechanics of how rocky worlds like Earth and Mars assembled from that primordial material remains one of the most fiercely contested questions in planetary science. Now a team of researchers at the University of Copenhagen has added a striking new piece of evidence to the debate, and their conclusion is unexpected: the two neighboring planets, born side by side in the same disk of material, appear to have formed in fundamentally different ways.

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