Protracted core formation and impact disruptions shaped the earliest outer Solar System planetesimals

Area of Science:

• Planetary Science
• Cosmochemistry
• Early Solar System Evolution

Background:

• Iron meteorites provide insights into planetesimal formation.
• Distinct compositions of carbonaceous (CC) and noncarbonaceous (NC) iron meteorite parent bodies (IMPBs) suggest different accretion and differentiation histories.
• CC IMPBs exhibit smaller, sulfur (S)-poor, highly siderophile element (HSE)-enriched cores with younger formation ages compared to NC IMPBs.

Purpose of the Study:

• To investigate the origins of differences in core compositions between CC and NC IMPBs.
• To explain the HSE enrichment in specific meteorite groups (IID, IIF, IIIF, IVB).

Main Methods:

• Utilized equilibrium partitioning models between solid and liquid metal.
• Modeled the segregation of S-rich and S-poor metal phases.
• Incorporated multistage evolutionary sequences including collisional disruption and reaccretion.

Main Results:

• Proposed a four-stage model for HSE enrichment in CC IMPB cores.
• Model involves initial S-rich protocore segregation, disruption, reaccretion, and ²⁶Al-driven heating.
• Suggests CC iron meteorites originate from second-generation, S-poor cores.

Conclusions:

• The proposed evolutionary sequence reconciles differences between NC and CC IMPBs.
• Early collisional processing played a crucial role in shaping planetesimal chemical evolution.
• Accounting for "missing" S-rich protocores is key to understanding these differences.