Iron meteorite chemistry puts the earliest planetesimals at 8 to 17 percent icy dust

The two tracers rest on different chemistry. Sulfur is highly concentrated in matrix, so a planetesimal that swept up a lot of cold dust should have inherited a lot of sulfur. Iron's oxidation state records how much water ice and oxidized dust went into the body ^[8]. "Both tracers independently tell the same story: these early planetesimals were remarkably matrix-poor," said Damanveer Grewal, the Yale assistant professor of Earth and planetary sciences who is first author of the study. "That convergence is what makes the result robust" ^[12][13].

The indirect route is necessary because the rocks lost the direct evidence. Chondrules are small rocky spheres visible inside chondrites, among the most primitive meteorites in collections, and counting those spheres is how the sorting has been documented before ^[15]. The iron meteorite parent bodies held enough radioactive aluminum-26 to melt all the way through. Melting erased the structures ^[7].

Grewal described the result from the chondrule side: the earliest bodies in the outer Solar System, he said, were "built from 83% to 92% chondrules, with very little of the icy, volatile-rich dust that dominates later-forming objects" ^[11]. That range is the complement of the measured matrix fraction, 100 minus 17 and 100 minus 8 ^[16]. The tracers constrain matrix; the chondrule figure follows by subtraction, so the reconstruction counts everything that is not fine dust as a chondrule.

Matrix is where the water ice and the organic material sit ^[5]. A body that accreted that little of it started out drier than any chondrite yet measured ^[9]. The direction of the trend was already known from carbonaceous chondrites out of the outer Solar System, where older examples hold more chondrules and less matrix and younger ones hold more of the cold volatile-rich material ^[14]. "Our work shows that this assembly process was remarkably selective from the very beginning," Grewal said ^[10].

What the tracers constrain is the matrix content of outer Solar System planetesimals in the first million years ^[19]. Getting from there to the water on any planet requires knowing how much of the later, matrix-rich generation was moved inward, which is a transport question these melted bodies do not answer. The Yale announcement does not name the physical process that separated millimeter beads from micron dust ^[1].