Reconstructing the formation of planetesimals in the early solar system by Graham Edwards

Graham Edwards

UC Santa Cruz

Chondrite meteorites are samples of some of the first planetary bodies to form at the dawn of our solar system. This study uses thermochronologic information recorded in the phosphate minerals of several “LL” chondrites to reconstruct the size and formation history of the LL parent planetesimal.

ABSTRACT

Chondritic meteorites derive from asteroidal parent bodies, are composed of millimeter-sized chondrules, and record the early stages of planetary assembly. Yet, both the original size of the parent planetesimals and the relationship between parent planetesimal accretion and chondrule formation, if any, remain disputed. We use Pb-phosphate thermochronology with planetesimal-scale thermal models to constrain the minimum size of the LL chondrite parent body and the timing of its accretion from protoplanetary material. Cooling ages of LL chondrites calculated from phosphate mineral Pb-isotope compositions record a total duration of cooling ?75 Ma, with a high-temperature, “type 6” interior that cools over ?30 Ma. Since the duration of conductive cooling scales with parent body size, these data require a ?150-km radius parent body and a plausible timeframe of accretion that overlaps the timeframe of formation for constituent LL chondrules. The concordance suggests that rapid accretion of a large LL parent asteroid occurred shortly after a major chondrule-forming episode and supports a growing body of evidence that accretion of planetesimals from millimeter-scale material “leaped” past the so-called meter-size barrier that plagues models of planetary accretion.

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