Recent research from Yale University indicates that the early stages of Solar System formation were more selective than previously understood. As planets, moons, and protoplanets began to form, two primary types of materials were available: chondrules, small rock pieces formed at high temperatures, and matrix, a cold dust rich in water ice and organic content.
Read More
The study, published on September 18 in Nature Astronomy, shows that chondrules were preferentially incorporated into solid bodies during the Solar System's first million years. This finding contrasts with earlier beliefs, which suggested that sorting occurred between 2 to 4 million years after formation.
Damanveer Grewal, an assistant professor of Earth and planetary sciences at Yale and the study's first author, stated, "Our work shows that this assembly process was remarkably selective from the very beginning," noting that early bodies in the outer Solar System comprised 83% to 92% chondrules, with minimal icy dust that would later dominate other formations.
Chondrules are found within chondrites, some of the most primitive meteorites. They provide a direct connection to the Solar System's earliest history. Grewal emphasized the significance of chondrules, which started forming billions of years ago.
Previous studies have indicated that carbonaceous chondrites from the outer Solar System exhibit a pattern where older samples have a higher proportion of chondrules compared to younger ones. This suggested that the areas where the first solid objects, known as planetesimals, formed favored heat-formed materials.
Confirming the events of the Solar System's first million years has been challenging due to the absence of preserved undifferentiated bodies from that time. To tackle this, Grewal and his team examined iron meteorites from the outer regions, which contained radioactive aluminum-26, leading to their complete melting and the loss of earlier physical structures.
The researchers identified two chemical indicators linked to matrix: sulfur, which is prevalent in matrix, and the oxidation state of iron, which reflects the amount of water ice and oxidized dust originally present. By analyzing these tracers, the team concluded that matrix constituted only 8% to 17% of the original material in these ancient parent bodies, indicating a substantial dominance of chondrules.
Grewal pointed out that both tracers supported the finding that early planetesimals were remarkably poor in matrix. These results also help clarify why ancient chondrules are rarely found today; many were likely incorporated into early planetesimals that later melted, erasing physical evidence.
The study concludes that the sorting and selection of materials began almost immediately as solid bodies formed, favoring chondrules significantly. Grewal remarked, "These ubiquitous little beads of rock are the basic building blocks from which the planets themselves were eventually assembled."
Co-authors of the study include Zhongtian Zhang from Princeton University and Joanna Drążkowska from the Max Planck Institute for Solar System Research in Germany. The research was funded by Yale University.