Early Solar System Turbulence Constrained by High Oxidation States in the Oldest Noncarbonaceous Planetesimals

Abstract

Early solar system (SS) planetesimals constitute the parent bodies of most meteorites investigated today. Nucleosynthetic isotope anomalies of bulk meteorites have revealed a dichotomy between noncarbonaceous (NC) and carbonaceous (CC) groups. Planetesimals sampling NC and CC isotopic signatures are conventionally thought to originate from the "dry" inner disk and volatile-rich outer disk, respectively, with their segregation enforced by a pressure bump close to the water–ice sublimation line, possible tied to Jupiter's formation. This framework is challenged by emerging evidence that the oldest NC planetesimals (i.e., the iron meteorites parent bodies (IMPBs)) were characterized by far higher oxidation states than previously imagined, suggesting abundant (≳ few weight percent) liquid water in their interiors prior to core differentiation. In this paper, we employ a model for a degassing icy planetesimal (heated by ²⁶Al decay) to map the conditions for liquid water production therein. Our work culminates in threshold characteristic sizes for pebbles composing the said planetesimal, under which water–ice melting occurs. Adopting a model for a disk evolving under both turbulence and magnetohydrodynamic disk winds, and assuming pebble growth is fragmentation limited, we self-consistently translate the threshold pebble size to lower limits on early SS turbulence. We find that if NC IMPBs were "wet," their constituent pebbles must have been smaller than a few centimeters, corresponding to typical values of the Shakura–Sunyaev α_ν turbulence parameter in excess of 10⁻³. These findings argue against a quiescent SS disk (for <10 au), are concordant with astronomical constraints on protoplanetary disk turbulence, and suggest pebble accretion played a secondary role in building our rocky planets.

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