Early Solar System Turbulence Constrained by High Oxidation States of the Oldest Non-Carbonaceous Planetesimals

TL;DR

This study links the high oxidation states of ancient non-carbonaceous planetesimals to early Solar System turbulence levels, suggesting a more dynamic, turbulent disk environment than previously thought, based on models of water melting in icy bodies.

Contribution

It introduces a model connecting pebble size thresholds for water melting to turbulence levels in the early Solar System, challenging the idea of a quiescent protoplanetary disk.

Findings

Implication of high turbulence levels (>10^{-3}) in the early Solar System.

Constraints on pebble sizes (<a few centimeters) for water melting in planetesimals.

Evidence against a quiescent disk within 10 AU during planetesimal formation.

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 non-carbonaceous (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, possibly 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 ($\gtrsim$ few wt.%) liquid water in their interiors prior to core differentiation. In this paper, we employ a model for a degassing icy planetesimal (heated by $^{26}$Al 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 $\alpha_{\nu}$ 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.