Evidence of a primordial isotopic gradient in the inner region of the solar protoplanetary disc

TL;DR

This study provides evidence for a primordial isotopic gradient in the inner solar protoplanetary disc, suggesting that Earth, Mars, and Vesta formed in different regions with limited mixing, preserving their distinct isotopic signatures.

Contribution

The paper introduces a model-based analysis supporting the existence of an isotopic gradient in the early solar disc and its role in planetary formation.

Findings

Vesta likely formed locally in the asteroid belt with limited material mixing.

Isotopic differences between Earth and Mars support a gradient-preserving formation process.

The isotopic gradient remained intact despite giant planet migration.

Abstract

Not only do the sampled terrestrial worlds (Earth, Mars, and asteroid 4 Vesta) differ in their mass-independent (nucleosynthetic) isotopic compositions of many elements (e.g. $\varepsilon^{48}$Ca, $\varepsilon^{50}$Ti, $\varepsilon^{54}$Cr, $\varepsilon^{92}$Mo), the magnitudes of some of these isotopic anomalies also appear to correlate with heliocentric distance. While the isotopic differences between the Earth and Mars may be readily accounted for by the accretion of mostly local materials in distinct regions of the protoplanetary disc, it is unclear whether this also applies to asteroid Vesta. Here we analysed the available data from our numerical simulation database to determine the formation location of Vesta in the framework of three planet-formation models: classical, Grand Tack, and Depleted Disc. We find that Vesta has a high probability of forming locally in the asteroid belt in models where material mixing in the inner disc is limited; this limited mixing is implied by the isotopic differences between the Earth and Mars. Based on our results, we propose several criteria to explain the apparent correlation between the different nucleosynthetic isotopic compositions of the Earth, Mars, and Vesta: (1) these planetary bodies accreted their building blocks in different regions of the disc, (2) the inner disc is characterised by an isotopic gradient, and (3) the isotopic gradient was preserved during the formation of these planetary bodies and was not diluted by material mixing in the disc (e.g. via giant planet migration).