Constraints on planetesimal accretion inferred from particle-size distribution in CO chondrites

TL;DR

This study analyzes particle-size distributions in CO chondrites to infer that planetesimals likely formed through gravitational collapse, with aerodynamic sorting during accretion influencing chondrule sizes.

Contribution

It provides evidence supporting gravitational collapse as the formation mechanism for planetesimals, based on particle-size distribution analysis and modeling of chondrule sorting.

Findings

Chondrule sizes increase with petrographic grade, indicating thermal metamorphism effects.

Aerodynamic sorting during accretion influenced chondrule distribution within parent bodies.

Modeling shows larger chondrules are more centrally concentrated, supporting gravitational collapse formation.

Abstract

The formation of planetesimals was a key step in the assemblage of planetary bodies, yet many aspects of their formation remain poorly constrained. Notably, the mechanism by which chondrules -- sub-millimetric spheroids that dominate primitive meteorites -- were incorporated into planetesimals remains poorly understood. Here we classify and analyze particle-size distributions in various CO carbonaceous chondrites found in the Atacama Desert. Our results show that the average circle-equivalent diameters of chondrules define a positive trend with the petrographic grade, which reflects the progressive role of thermal metamorphism within the CO parent body. We show that this relationship could not have been established by thermal metamorphism alone but rather by aerodynamic sorting during accretion. By modeling the self-gravitational contraction of clumps of chondrules, we show that (i) the accretion of the CO parent body(ies) would have generated a gradual change of chondrule size with depth in the parent body, with larger chondrules being more centrally concentrated than smaller ones, and (ii) any subsequent growth by pebble accretion would have been insignificant. These findings give substantial support to the view that planetesimals formed via gravitational collapse.