Analysis of iron meteorites suggests that the first rocky bodies in the outer solar system were formed primarily from heated grains, with their separation from ice-rich dust beginning as early as the first million years.
If the first solid bodies in the outer solar system came with a recipe, it would have included a generous helping of chondrules, grains of rock about a millimeter in diameter that formed from heat, and very little cold dust. This fine dust, called matrix, carried water ice and organic molecules, but only a small fraction of it made it to the early bodies studied by a team led by Yale University.
In a study published on September 18, 2026 in the journal Nature Astronomy The team found geochemical evidence that this chondrule preference existed as early as the first million years of the solar system. Previous evidence of material sorting came from bodies that formed about two to four million years after its inception.
To reconstruct a mixture that no longer exists in its original form, the researchers examined iron meteorites. Their parent bodies had been completely melted, so the chondrules they contained had disappeared. Chemical traces that were preserved even after melting allowed the team to calculate back the composition of the materials from which the bodies were formed.
"These tiny, ubiquitous rock grains are the basic building blocks from which the planets themselves were ultimately assembled," said the study's first author, Damonvir Grewal. "And now we know that they were already sorted and incorporated into the first generation of solid bodies right from the beginning."
Two markers restore the original mixture
These early bodies, called planetesimals, accumulated enough radioactive aluminum-26 to heat up and melt them. No body preserved from the first million years retained the original, unmelted mixture of chondrules and matrix. So Grewal and his colleagues used two independent chemical markers associated with the matrix.
The first marker is sulfur, which is concentrated in the matrix and helps estimate how much dust was in the parent body. The second is the oxidation state of the iron, which provides a separate measure of the amount of water ice and oxidized dust it contained. Both measurements indicated an original matrix content of only 8%–17%, less than in any known chondrites, ancient meteorites in which the chondrules are still preserved.
“Both markers tell the same story independently: these early planetesimals were very matrix-poor,” Grewal said. “This convergence reinforces the result.” According to the reconstruction, chondrules made up 83%–92% of the material in the bodies examined.
Where did the ancient grains go?
Carbonaceous chondrites from the outer solar system contain organic materials and water alongside silicate minerals. It was already known that chondrites that formed relatively early have a higher proportion of chondrules and less matrix than those that formed later. The new study extends the picture to the first generation of solid bodies and also offers an explanation for the paucity of ancient chondrules in the meteorite record: they were included in bodies that melted and lost their shape.
Questions and Answers
What are chondrules? Tiny grains of rock formed by heat and preserved in some ancient meteorites. They are one of the building blocks of planetary bodies.
How do we know what was in the body that melted? Chemical markers that survived melting are measured in iron meteorites, including sulfur and the oxidation state of iron, and from these they infer the proportion of the original matrix.
Does the study mean there was no ice in the early solar system? No. He refers to the parent bodies examined in the outer solar system and found that they contained little volatile-rich matrix relative to chondrules.
The scientific article
Damanveer S. Grewal, Zhongtian Zhang and Joanna Drążkowska, Planetesimal compositions governed by aerodynamic sorting from the onset of Solar System formation,Nature Astronomy, 18.9.2026. Yale University announcementTranslation: Nachum Shereshevsky.
