Spatial distribution of isotopes and compositional mixing in the inner protoplanetary disk

TL;DR

This study uses N-body simulations to investigate the spatial distribution of isotopes in the inner protoplanetary disk, revealing a gradient in nucleosynthetic Cr isotopes but not in oxygen isotopes, suggesting different formation and alteration processes.

Contribution

It demonstrates that nucleosynthetic Cr isotopic anomalies follow a spatial gradient in the inner disk, while oxygen isotopic variations do not, indicating different origins or processing histories.

Findings

Cr isotopic anomalies show a spatial gradient consistent with simulations.

Oxygen isotopic variation does not follow a simple gradient, implying complex alteration.

Asteroid transport in the disk explains Cr isotope distribution among meteorites.

Abstract

The mass-independent isotopic signatures of planetary bodies have been widely used to trace the mixing and transport processes in planet formation. The observed isotopic variations among meteorites have been further linked to the modeled mass-weighted mean initial semimajor axes, assuming a spatial isotopic gradient in the inner protoplanetary disk. However, nucleosynthetic isotopic anomalies of nonvolatile elements and mass-independent oxygen isotopic variation ($\Delta ^{17}$O) show different relationships with distance from the Sun. Therefore, it is crucial to know whether isotopes were distributed systematically with heliocentric distance. In this study, we performed N-body simulations on compositional mixing during the collisional accretion and migration of planetary bodies to investigate the spatial distributions of Cr and O isotopes in the inner protoplanetary disk. The modeled mass-weighted mean initial semimajor axes of the parent bodies of noncarbonaceous (NC) meteorites and terrestrial planets were used to calculate the isotopic compositions of these bodies. Our simulations successfully reproduced the observed nucleosynthetic Cr isotopic anomaly among Earth, Mars, and the NC meteorite parent bodies, consistent with a spatial gradient of isotopic anomalies in the inner disk. Asteroids originating from different regions in the inner disk were transported to the main belt in our simulations, resulting in the Cr isotopic anomaly variation of the NC meteorite parent bodies. However, the $\Delta ^{17}$O distribution among the terrestrial planets and the NC meteorite parent bodies could not be reproduced assuming a $\Delta ^{17}$O gradient. The absence of a $\Delta ^{17}$O gradient reflects that the oxygen isotopic mass-independent fractionation might have altered the spatial distribution of the nucleosynthetic $\Delta ^{17}$O variation before protoplanets formed.