The earliest solid bodies in the outer solar system formed from 83% to 92% heat-forged chondrules rather than icy dust, according to a Yale-led study published in Nature Astronomy on September 18. Researchers used iron meteorite chemical tracers to reveal that aerodynamic sorting immediately separated rocky grains from volatile-rich material.
Aerodynamic Sorting and Early Planetesimal Composition
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. One choice consisted of heat-forged chondrules, which were millimeter-sized bits of rock. The other ingredient was a fine-grained, cold dust matrix loaded with water ice and organic molecules. From the get-go, the solar system chose fire. According to a study published in Nature Astronomy titled ‘Planetesimal compositions governed by aerodynamic sorting from the onset of Solar System formation’, the early solar system preferentially sorted for chondrules over matrix within its first million years.
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 new findings show that aerodynamic sorting was active from the very beginning. This process preferentially sorted the larger, heat-processed chondrules from the fine-grained matrix. As a result, the earliest planetesimals incorporated far less of the volatile-rich, ice-bearing material than later bodies did.
Chemical Tracers in Iron Meteorites Reveal Matrix Poverty
Studying these earliest planetesimals is difficult because the original bodies did not survive intact, and some of their parent bodies accumulated enough radioactive aluminum-26 to melt completely. To overcome this limitation, researchers turned to iron meteorites for chemical clues. Sulfur and iron oxidation measurements independently indicated matrix levels of just 8% to 17%, supporting evidence that this sorting began almost immediately.
The study’s authors relied on independent chemical tracers tied to the matrix material to evaluate these ancient bodies. Both indicators independently pointed to low matrix levels in the original material sampled by these iron meteorites.
Damanveer Grewal, assistant professor of Earth and planetary sciences in Yale’s Faculty of Arts and Sciences
Grewal served as the first author of the study. He stated, Our work shows that this assembly process was remarkably selective from the very beginning.
He further noted, 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.
He also noted of primitive meteorites: You can hold them in your hand and know that they began as part of a process that started billions of years ago. It's a timescale that's hard to wrap your head around.
Broader Implications for Asteroid History and Ryugu Samples
The convergence of the sulfur and iron oxidation indicators confirmed that these early planetesimals were assembled from a strongly filtered mixture of material rather than from an undifferentiated reservoir of dust.
Scientists have laid out the history of a near-Earth asteroid in unprecedented detail. Samples returned from the asteroid Ryugu by the Hayabusa2 probe in 2020 have revealed hints about some of the events that took place as the Sun and its planets formed, after being analysed by an international team of scientists, including researchers from the Museum. Dr Ashley King is a Museum researcher and UKRI Future Leaders Fellow who co-authored the analysis of Ryugu. He says, ‘We found that Ryugu’s parent asteroid formed in the cold, outer regions of the solar system where there was both carbon dioxide and water ice.’ He added, ‘Those ices accreted into the asteroid, where they melted and then reacted with rocky materials to create abundant hydrated minerals. The original asteroid was then smashed apart, with some of the fragments accumulating into what we now know as Ryugu.’ The findings of that study were published in the journal Science.
Продолжение темы
