A very early origin of isotopically distinct nitrogen in inner Solar System protoplanets

TL;DR

This study reveals that the earliest inner Solar System protoplanets acquired isotopically distinct nitrogen from organic sources or dust, indicating early heterogeneity in nitrogen reservoirs that influenced planetary composition.

Contribution

It provides evidence for early, isotopically distinct nitrogen sources in inner and outer protoplanets, highlighting non-nebular nitrogen's role in planetary formation.

Findings

Inner and outer protoplanets accreted nitrogen from different sources.

Early protoplanets had isotopically distinct nitrogen signatures.

Earth's nitrogen isotopic ratio reflects a mixture of inner and outer Solar System materials.

Abstract

Understanding the origin of life-essential volatiles like N in the Solar System and beyond is critical to evaluate the potential habitability of rocky planets. Whether the inner Solar System planets accreted these volatiles from their inception or had an exogenous delivery from the outer Solar System is, however, not well understood. Using previously published data of nucleosynthetic anomalies of Ni, Mo, W and Ru in iron meteorites along with their 15N-14N ratios, here we show that the earliest formed protoplanets in the inner and outer protoplanetary disk accreted isotopically distinct N. While the Sun and Jupiter captured N from nebular gas, concomitantly growing protoplanets in the inner and outer disk possibly sourced their N from organics and/or dust - with each reservoir having a different N isotopic composition. A distinct N isotopic signature of the inner Solar System protoplanets coupled with their rapid accretion suggests that non-nebular, isotopically processed N was ubiquitous in their growth zone at 0-0.3 Myr after the formation of CAIs. Because 15N-14N ratio of the bulk silicate Earth falls between that of inner and outer Solar System reservoirs, we infer that N in the present-day rocky planets represents a mixture of both inner and outer Solar System material.