Abstract
How and where the first generation of inner Solar System planetesimals formed remains poorly understood. Potential formation regions are the silicate condensation line and water snowline of the solar protoplanetary disk. Whether the chemical compositions of these planetesimals align with accretion at the silicate condensation line (water-free and reduced) or water snowline (water-bearing and oxidized) is, however, unknown. Here we use the Fe/Ni and Fe/Co ratios of magmatic iron meteorites to quantify the oxidation states of the earliest planetesimals associated with non-carbonaceous (NC) and carbonaceous (CC) reservoirs, representing the inner and outer Solar System, respectively. Our results show that the earliest NC planetesimals contained substantial amounts of oxidized Fe in their mantles (3–19 wt% FeO). In turn, we argue that this required the accretion of water-bearing materials into these NC planetesimals. The presence of substantial quantities of moderately and highly volatile elements in their parent cores is also inconsistent with their accretion at the silicate condensation line and favours, instead, their formation at or beyond the water snowline. Similar oxidation states in the early formed parent bodies of NC iron meteorites and those of NC achondrites and chondrites with diverse accretion ages suggest that the formation of oxidized planetesimals from water-bearing materials was widespread in the early history of the inner Solar System.
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Acknowledgements
We thank A. P. Vyas for helping to improve the clarity of our communication and we thank C. M. O’D. Alexander, G. Blake, K. Batygin and Y. Miyazaki for fruitful discussions during early stages of the research presented in this study. This study was funded by a Barr Foundation postdoctoral fellowship by Caltech awarded to D.S.G. B.Z. was supported by NASA grants 80NSSC19K1238 and 80NSSC23K0035.
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D.S.G. conceived the project. D.S.G. compiled the data and performed the numerical calculations along with N.X.N. B.Z. performed the fractional crystallization calculations. A.I. helped with the astrophysical implications. All authors interpreted the data. D.S.G. wrote the manuscript with inputs from N.X.N., B.Z., A.I. and P.D.A.
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Extended data
Extended Data Fig. 1 Fractional crystallization modeling of P, Ni, Co, and Ir for group IIIE.
The Ir-Co (a), Ir-Ni (b), and P-Ni (c) models use bulk 8 wt.% S and 0.6 wt.% P. The red lines, blue lines, and green dashed lines denote solid from simple fractional crystallization (SFC solid), solid from trapped melt (TM solid), and liquid (Liquid), respectively. Ir, Co, and Ni data are from ref. 73. Phosphorus data are from ref. 74.
Extended Data Fig. 2 Comparison between the FeO contents and fO2 of IMPBs based on the Fe/Co ratios of their parent cores.
In agreement with the calculations based on Fe/Ni ratios (Fig. 2), the estimated FeO contents (a) and fO2 (b) of CC IMPBs (blue) based on Fe/Co ratios are either similar to or only modestly higher than those of NC IMPBs (red). Error bars for FeO content and fO2 represent 1σ deviation obtained by the propagation of standard deviation of individual terms in Eqs. 1 and 2, respectively (for details refer to the caption of Fig. 2).
Extended Data Fig. 3 Comparison between the FeO contents and fO2 of IMPBs based on the Fe/Ni and Fe/Co ratios of their parent cores.
The FeO contents (a) and fO2 (b) of each NC and CC IMPB, except for groups IC and IID, estimated via Fe/Ni and Fe/Co ratios of their parent cores broadly agree with each other.
Extended Data Fig. 4 Comparison between the mean FeO contents of the IMPBs based on the Fe/Ni ratios of CI and ordinary/CM chondrites.
The FeO contents of IMPBs estimated using Fe/Ni ratios of ordinary and CM chondrites (for NC and CC IMPBs, respectively) and CI chondrites are approximately similar.
Extended Data Fig. 5 Comparison between the FeO contents and fO2 of the IMPBs based on the Fe/Ni ratios determined by different fractional crystallization models.
The FeO content (a) and fO2 (b) of each NC and CC IMPB, as determined by several fractional crystallization models, broadly agree with each other. The X-axis represents FeO contents and fO2 values determined using the results from ref. 19,20,21,22, while the Y-axis represents values from ref. 13. Note that the data plotted on the X-axis are used for the discussion in this study. Data for group IC and group IIAB were not plotted since the compositions of their parent cores have only been determined in ref. 13. Additionally, data for group IIIE was not plotted as the composition of its parent core was determined only in this study.
Extended Data Fig. 6 Minimum water required to explain the FeO contents of the parent bodies of iron meteorites based on the Fe/Ni and Fe/Co ratios of their parent cores.
Although CC IMPBs generally require higher water contents than NC IMPBs to explain their FeO contents (a, b), the amount of water accreted to explain the FeO contents of NC IMPBs is also substantial.
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Grewal, D.S., Nie, N.X., Zhang, B. et al. Accretion of the earliest inner Solar System planetesimals beyond the water snowline. Nat Astron 8, 290–297 (2024). https://doi.org/10.1038/s41550-023-02172-w
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DOI: https://doi.org/10.1038/s41550-023-02172-w