Scientists have developed a new model to explore the contrasting formation processes of Earth and Mars, revealing that the two neighboring planets may have assembled through markedly different pathways despite originating in the same early solar system environment. The research, published in Nature Astronomy, combines chemical analyses of planetary materials with computer simulations to propose a hybrid planetary formation scenario.
The study suggests that Mars formed predominantly through the accretion of large, asteroid-sized bodies, while Earth’s mass was mainly built up from smaller, pebble-sized particles that gradually clumped together before undergoing later collisions between larger protoplanets. According to Anders Johansen, a planetary scientist at the University of Copenhagen and one of the study’s authors, the findings were unexpected given the proximity and shared starting materials of both planets.
By analyzing the volatile element content—such as sodium, potassium, lithium, and zinc—in the crust and mantle of Earth and Mars, the researchers inferred distinct assembly histories. These moderately volatile elements tend to be lost at high temperatures generated during giant impacts. Mars, which has about one-tenth of Earth’s mass, appears to have acquired about 73% of its mass from bigger, high-energy collisions with asteroid-like objects, preserving fewer volatiles. Earth, on the other hand, likely accumulated around 75% of its mass from pebble-sized particles, which would have retained a greater portion of volatiles during the initial stages.
The results provide support for the hypothesis that Earth’s supply of life-sustaining water could have been delivered via icy pebbles originating from the outer solar system, complementing prior theories involving comet and damp asteroid contributions. This mixed origin could help explain differences in planetary compositions and their ultimate environmental conditions—Earth as a habitable, water-rich world versus Mars as a colder, volcanic desert.
The study also touches on the influence of Jupiter’s massive gravity, which is believed to have shaped the distribution of materials available to the inner planets by shepherding and scattering rocky bodies, thereby affecting Mars’s growth.
Some experts caution that while the hybrid model offers a compelling framework, it simplifies various complex processes and that precise quantification of the proportions of pebbles and larger objects is still uncertain. Paolo Sossi of ETH Zurich noted that Earth’s chemical composition strongly indicates a combination of different building block types, and the presence of pebbles in its formation is plausible. Conel Alexander, a cosmochemist at Carnegie Science, emphasized that volatile element ratios are only one factor among many and that alternative explanations remain viable.
The new findings contribute to the ongoing debate over planetary formation mechanisms, highlighting divergent paths that nearby planets might take in their early assembly and raising fresh questions about how water and other volatiles were delivered to terrestrial worlds.