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

Abstract

Understanding the origin of life-essential volatiles such as nitrogen (N) in the Solar System and beyond is critical to evaluate the potential habitability of rocky planets1,2,3,4,5. 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 nickel, molybdenum, tungsten and ruthenium 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 gas6, 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 accretion7,8 suggests that non-nebular, isotopically processed N was ubiquitous in their growth zone between 0 and ~0.3 Myr after Solar System formation. Because the 15N/14N ratio of the bulk silicate Earth falls between that of the 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.

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Fig. 1: Variations in 15N/14N ratios of various Solar System objects and reservoirs.
Fig. 2: CC–NC dichotomy of iron meteorites plotted in δ15N–ε64Ni, δ15N–ε183W, δ15N–ε94Mo and δ15N–ε100Ru space.
Fig. 3: N content in iron meteorites can be explained by the segregation of N into protoplanetary cores rather than via nebular ingassing into Fe–Ni alloys.
Fig. 4: Contribution of CC and NC material to the N budget of the BSE.

Data availability

The authors declare that the data supporting the findings of this study are available within the article and its Supplementary Information files.

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Acknowledgements

We acknowledge C.-T. Lee for fruitful discussions during the early stage of this research. A. Vyas and J. D. Seales helped with the CC–NC reservoir mixing calculations for the BSE. A. P. Vyas helped improve the clarity of our communication. D.S.G. received support from NASA FINESST grant 80NSSC19K1538 and Lodieska Stockbridge Vaughn Fellowship by Rice University. R.D. was supported by NASA grants 80NSSC18K0828 and 80NSSC18K1314. B.M. was supported by the European Research Council grant PHOTONIS 695618.

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D.S.G. conceived the project, compiled the data and developed thermodynamic models. D.S.G., R.D. and B.M. interpreted the data. D.S.G. wrote the manuscript with input from R.D. and B.M.

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Correspondence to Damanveer S. Grewal.

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Extended data

Extended Data Fig. 1 Statistical variation of N abundances and isotope ratios in different groups of iron meteorites.

Probability distributions function using kernel density distribution function (using Matlab®) are used to ascertain the statistical variation of N abundances and isotope ratios for each group of iron meteorites. For each group, δ15N varies within a small range and shows sharp peaks for mean values, while N abundances show large variation.

Extended Data Fig. 2 δ15N of iron meteorites plotted against their N abundances.

For each iron meteorite group, δ15N falls in a narrow range, while N contents may vary over two orders of magnitude. Lack of any relationship between δ15N values and N abundances within any given group of iron meteorites argues against notable mass-dependent fractionation of N isotopes via volatility-related losses during planetary processing19,20,44.

Extended Data Fig. 3 Partitioning of N into Fe, Ni-alloy melts can explain N abundances for a variety of core-mantle differentiation scenarios.

a) For alloy-silicate equilibration at 1,000 bar-1,600 °C and varying alloy/silicate mass ratio between 0.01 and 1 (within the range of all differentiated rocky bodies in the inner Solar System except Mercury), N content in the core forming Fe, Ni-alloy melts varies between ~10 and 100 ppm. Fixed values of N content in the alloy at ~100 ppm are owing to low N solubility in the alloy at 1000 bar. b) For alloy-silicate equilibration at 10,000 bar-1,600 °C and varying alloy/silicate mass ratio, N content in the core forming Fe, Ni-alloy melts varies between ~10 and 10,000 ppm. The pink shaded regions represent the range of N contents in iron meteorites.

Supplementary information

Supplementary Table 1

A compilation, in machine-readable format, of N abundances and isotopic ratios in iron meteorites.

Supplementary Table 2

A compilation, in machine-readable format, of N isotopic ratios and isotopic anomalies in iron meteorites and rocky bodies in the inner Solar System.

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Grewal, D.S., Dasgupta, R. & Marty, B. A very early origin of isotopically distinct nitrogen in inner Solar System protoplanets. Nat Astron 5, 356–364 (2021). https://doi.org/10.1038/s41550-020-01283-y

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