Earth and Mars: Built from different cosmic recipes

Four and a half billion years ago, Earth and Mars emerged from the same nebula of gas and dust. However, the exact mechanisms by which these planets formed continue to be a matter of debate among planetary scientists. A team of researchers from the University of Copenhagen has now provided fresh insights into this longstanding mystery.

By analyzing the chemical composition of Earth and Mars, the researchers have reconstructed the earliest stages of their formation using a novel approach. Their findings suggest that the two planets' origins followed different paths. Professor Anders Johansen, who co-led the study, explained that while it might seem intuitive for planets born in the same solar system to share a similar formation history, Earth and Mars appear to have formed from distinct cosmic ingredients.

The study indicates that over 75% of Earth's mass likely originated from two young protoplanets that grew by accreting smaller particles, known as pebbles, while the remaining mass came from larger planetesimals. In contrast, approximately 75% of Mars' mass is thought to have come from planetesimals, with the rest accruing from pebble accretion.

This contrasts with the prevailing theories that planets form primarily through either a pebble accretion process or a collision between massive rocks. The researchers believe their results lend support to the hybrid model, where both processes likely played a role in forming Earth and Mars. However, to understand the planets' formation billions of years ago, scientists must rely on remnants still present today.

For Earth and Mars, the key clues lie in the elements in their crust and mantle, particularly volatile elements like sodium, zinc, and potassium. These elements are easily lost at high temperatures, so their presence or absence provides a chemical fingerprint revealing the conditions under which the planets formed. Using advanced computer models, the researchers traced these volatile elements back to the initial stages of the planets' formation.

While acknowledging uncertainties in their models, such as the unknown exact chemical composition of the solar system's original building blocks, the researchers' findings remain robust even when accounting for various assumptions. The percentage breakdown of how Earth and Mars formed, though not exact, consistently points to distinct formation pathways for each planet.

This approach offers a more precise way of studying planet formation compared to traditional isotope-based methods, which can sometimes lead to ambiguous interpretations.

Beyond its implications for Earth and Mars, the study has broader implications for our search for habitable exoplanets. As future space missions explore Earth-like planets in distant galaxies, understanding how volatile elements are depleted during planetary formation will help predict the water and other life-supporting substances these worlds might retain. This research marks a significant step forward in unraveling the cosmic recipe that gave birth to our neighboring worlds.