Earth and Mars formed within the same stellar system, yet their evolutionary paths and growth mechanisms turned out to be cardinally opposed. In a recent study published in the authoritative scientific journal Nature Astronomy, scientists presented a detailed analysis of how the rocky worlds of our system were born. For a long time, the scientific community has debated fiercely about what prevailed in the early cosmos: the peaceful absorption of small cosmic grains or destructive collisions of large planetesimals.
Two Hypotheses of Planet Formation
Traditionally, astrophysicists considered two scenarios of planetary accretion. The first assumed the gradual and peaceful accumulation of small "pebbles" drifting in the protoplanetary disk. The second scenario was based on dynamic and aggressive collisions of massive fragments. Recent computer modeling has shown that both processes actually occurred in parallel, but the proportions of their participation in the formation of Earth and Mars differ radically.
The Role of Volatile Elements and Thermal Evaporation
The key to the mystery lay in the chemical composition features of the mantle of both planets. Researchers recorded a noticeable deficit of moderately volatile elements compared to the solar composition. It turned out that during the absorption of small pebbles, the growing protoplanet retained a temporary gas envelope. The generated heat led to the evaporation of volatile fractions, which were carried back into the disk by gas flows, while heavy minerals remained, forming a solid core.
Controversial Data
It is worth noting that early theoretical calculations were based predominantly on the hypothesis of exclusively impact-driven planet formation due to chaotic collisions of large bodies. At the same time, the latest Bayesian models prove the critical importance of pebble accretion, generating discussions about the exact chronology and rates of accretion processes in the young Solar System, leaving room for a revision of classical models.
Different Scenarios for Earth and Mars
Using statistical modeling, scientists found that about 75% of Earth's mass was formed from objects that grew through pebble accretion (proto-Earth and a large impactor that created the Moon), and only 25% from planetesimals. For Mars, the situation looks different: about 73% of its mass was assembled from planetesimals similar to the asteroid Vesta, and only 27% from pebbles. Probably, the early orbital migration of the Red Planet prevented it from efficiently absorbing small substance.
