New research published in Nature Astronomy offers fresh insights into the distinct formation histories of Earth and Mars, suggesting that the two planets—despite their similar ages and starting materials—emerged through notably different processes. By combining chemical analyses of planetary rocks with advanced computer simulations, scientists have developed a model indicating that Mars primarily formed through the collision and accumulation of larger, asteroid-sized bodies, while Earth’s growth was dominated by the gradual aggregation of smaller pebble-like particles.

The study, led by planetary scientist Anders Johansen of the University of Copenhagen, involved creating virtual planet-building scenarios to replicate the current compositions and masses of Earth and Mars. These models relied on patterns observed in the volatile elements—such as sodium, potassium, lithium, and zinc—found in the planets’ crusts and mantles. Both planets show depletion of these moderately volatile elements compared to primordial solar system materials preserved in meteorites, but the nature of these depletions differs, suggesting divergent assembly histories.

According to the researchers, Mars likely formed with about 73% of its mass derived from large, asteroid-like impacts, supplemented by the accumulation of smaller particles. In contrast, the team estimates that approximately 75% of Earth’s mass originated from the gentle clumping of pebbles into two primary protoplanets, which later collided. This scenario may help explain Earth’s enduring supply of water, as previous theories posit that icy pebbles from the outer solar system contributed significantly to delivering life-sustaining volatiles.

The study further supports the idea that both planets formed from a combination of material sizes rather than solely one type, bridging a gap between competing theories. Jupiter’s intense gravity, long thought to have influenced Mars’s smaller size by scattering planetary building blocks, remains a key factor in the models.

Not all experts fully agree on the role pebbles play in planet formation. Paolo Sossi, a planetary scientist at ETH Zurich who was not involved in the study, described the model’s proportions as uncertain due to simplifications inherent in the simulations. However, he acknowledged that Earth’s volatile element patterns suggest multiple types of building blocks were involved, making the new model plausible.

Conel Alexander, a cosmochemist at Carnegie Science, noted that the study contributes valuable ideas but emphasized that volatile element ratios represent only one aspect of the complex process that shaped Earth and Mars. He described the hybrid approach as “intriguing” but cautioned it is not definitive, highlighting the ongoing debate among researchers.

This work advances understanding of the early solar system’s dynamics and the processes that led to the distinct characteristics of neighboring terrestrial planets, underscoring the complexity and variety inherent in planetary formation.