Volatile element fingerprints suggest Earth and Mars were built by different processes

Four and a half billion years ago, Earth and Mars were born in the rotating cloud of gas and dust that would become the Solar System. Exactly how the planets formed remains one of the most disputed questions in planetary science.

Researchers at the University of Copenhagen are now adding new evidence to the debate. Using a new approach, they have reconstructed the earliest stages of the planets' formation by analysing their chemical composition, and the results surprised them.

"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," said 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.

Researchers broadly agree that planets form primarily in one of two ways, or through a hybrid of both. Planets either emerge when giant space rocks known as planetesimals collide, or when these large rocks accumulate smaller particles, a process known as pebble accretion.

Planetary scientists have not settled on which of the two processes, or what combination of them, best explains how rocky planets such as Earth and Mars formed.

The researchers behind the new study believe their findings support the hybrid model. The study goes a step further and offers a more explicit account of how Earth and Mars formed 4.5 billion years ago.

"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," said Wang.

To understand how planets formed billions of years ago, scientists must rely on traces that still exist today.

In the new study, the researchers examined specific elements in the outer layers of the planets, the crust and mantle. They focused on volatile elements, which evaporate relatively easily at high temperatures, such as sodium, zinc and potassium.

According to the researchers, the presence or absence of these elements acts as a chemical fingerprint that reveals the processes planets went through during their formation. Using computer models, they can then determine which formation scenarios most likely explain the origins of Earth and Mars.

"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," said Johansen.

The researchers emphasize that their computer models contain uncertainties. For example, the exact chemical composition of the original building blocks in the Solar System that eventually became Earth and Mars remains unknown.

As a result, the models rely on several assumptions. These include the assumption that the planetesimal building blocks of Earth and Mars were chemically similar to the asteroid Vesta, and that volatile elements are lost more efficiently during pebble accretion than during giant impacts. The researchers used computer simulations to show that volatile elements are lost from pebbles as they travel toward a planet's surface. Even when adjusting for those assumptions, the main conclusion remains unchanged.

"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," said Wang.

The researchers also see applications beyond the Solar System. Several future space missions aim to discover and study Earth-like planets around other stars, and understanding the chemical compositions of such planets is important for assessing whether they are potentially habitable, the researchers point out.

"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," said Johansen.

The study was published in the journal Nature Astronomy.

CONTACT: William Brons Petersen, University of Copenhagen, [email protected]