When the solar system first took up the task of building solid bodies — such as planets, moons, and protoplanets — it basically had a choice between two ingredients. There were heat-forged chondrules, which were millimeter-sized bits of rock; and there was matrix, a fine-grained, cold dust loaded with water ice and organic molecules.
And from the get-go, the solar system chose fire.
In a new, Yale-led study, researchers provide the first geochemical evidence that within the first million years after the solar system began to form, it was already preferentially sorting for chondrules over matrix. Prior research had only been able to document this sorting process in objects that formed 2 to 4 million years after the solar system's origin.
The study was published Sept. 18 in the journal Nature Astronomy .
"Our work shows that this assembly process was remarkably selective from the very beginning," said Damanveer Grewal, an assistant professor of Earth and planetary sciences in Yale's Faculty of Arts and Sciences, and first author of the study. "The earliest bodies in the outer solar system were built from 83% to 92% chondrules, with very little of the icy, volatile-rich dust that dominates later-forming objects."
Chondrules are found inside chondrites — the most primitive meteorites in geological collections. "You can hold them in your hand and know that they began as part of a process that started billions of years ago," Grewal said. "It's a timescale that's hard to wrap your head around."
It has been known for some time that among carbonaceous chondrites (primitive, stony meteorites that contain organic compounds and water among their silicate minerals) from the outer solar system, chondrites that formed earlier contained a higher percentage of chondrules and a lower percent of matrix. This suggested that in areas where the first solid bodies — called planetesimals — were forming, icy dust was already being muscled out in favor of heat-forged chondrules.
But no preserved undifferentiated bodies survive from that early epoch — the first million years of the solar system — to confirm the original chondrule-to-matrix ratio.
Grewal's solution was to look for chemical tracers within iron meteorites from the outer solar system that would point to an earlier era. The parent bodies sampled by these meteorites had accumulated so much radioactive aluminum-26 that they melted completely, destroying all physical traces of what they were originally made of. Yet a pair of independent chemical tracers enabled the researchers to reconstruct the original composition.
Both tracers are tied to matrix: sulfur, which exists in concentrated form in matrix, and the oxidation state of iron, which reveals how much water ice and oxidized dust the original body incorporated.
Using the tracers, the researchers calculated matrix levels of only 8% to 17% in the original bodies sampled by these iron meteorites — lower than what had been found in any known chondrites. "Both tracers independently tell the same story: these early planetesimals were remarkably matrix-poor," Grewal said. "That convergence is what makes the result robust."
The findings, he said, also help explain why older chondrules are scarce in the meteorite record; they were incorporated into bodies that later melted, erasing the physical evidence.
"These ubiquitous little beads of rock are the basic building blocks from which the planets themselves were eventually assembled," Grewal said. "And now we know they were already being sorted and incorporated into the first generation of solid bodies from the very start."
Co-authors of the study are Zhongtian Zhang of Princeton University and Joanna Drążkowska of the Max Planck Institute for Solar System Research in Germany.
Funding for the research came from Yale University.
