Nucleosynthetic isotope variability among Solar System objects is often used to probe the genetic relationship between meteorite groups and the rocky planets (Mercury, Venus, Earth and Mars), which, in turn, may provide insights into the building blocks of the Earth–Moon system1,2,3,4,5. Using this approach, it has been inferred that no primitive meteorite matches the terrestrial composition and the protoplanetary disk material from which Earth and the Moon accreted is therefore largely unconstrained6. This conclusion, however, is based on the assumption that the observed nucleosynthetic variability of inner-Solar-System objects predominantly reflects spatial heterogeneity. Here we use the isotopic composition of the refractory element calcium to show that the nucleosynthetic variability in the inner Solar System primarily reflects a rapid change in the mass-independent calcium isotope composition of protoplanetary disk solids associated with early mass accretion to the proto-Sun. We measure the mass-independent 48Ca/44Ca ratios of samples originating from the parent bodies of ureilite and angrite meteorites, as well as from Vesta, Mars and Earth, and find that they are positively correlated with the masses of their parent asteroids and planets, which are a proxy of their accretion timescales. This correlation implies a secular evolution of the bulk calcium isotope composition of the protoplanetary disk in the terrestrial planet-forming region. Individual chondrules from ordinary chondrites formed within one million years of the collapse of the proto-Sun7 reveal the full range of inner-Solar-System mass-independent 48Ca/44Ca ratios, indicating a rapid change in the composition of the material of the protoplanetary disk. We infer that this secular evolution reflects admixing of pristine outer-Solar-System material into the thermally processed inner protoplanetary disk associated with the accretion of mass to the proto-Sun. The identical calcium isotope composition of Earth and the Moon reported here is a prediction of our model if the Moon-forming impact involved protoplanets or precursors that completed their accretion near the end of the protoplanetary disk’s lifetime.
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Financial support for this project was provided to M.B. by the Danish National Research Foundation (DNRF97) and the European Research Council (ERC Consolidator Grant Agreement 616027—STARDUST2ASTEROIDS). V.A.F. acknowledges financial support from a DFG-Eigenstelle FE 1523/3-1 and the Royal Society for the purchase of Dhofar 287. We thank Å. Nordlund, A. Johansen and F. Moynier for discussion on the paper, as well as J. Day for comments that helped improve the quality of our paper.
The authors declare no competing financial interests.
Reviewer Information Nature thanks J. Day and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
Extended Data Figure 1 μ48Ca values of planetary bodies versus mass for different masses of the ureilite parent body and Earth’s precursor.
a–c, Regressions (solid lines) and associated correlation coefficients through the data (squares) by assuming an ureilite parent body with a radius of 50 km and masses of MEarth, 0.5MEarth and 0.25MEarth for Earth’s precursor. d–f, Regressions through the data but assuming a ureilite parent body with a radius of 105 km and masses of MEarth, 0.5MEarth and 0.25MEarth for Earth’s precursor. The masses for the angrite parent body, Vesta and Mars are the same as in Fig. 2a.
Extended Data Figure 2 Three-isotope plot of the average δ42/44Ca versus δ43/44Ca for Earth, meteorite parent bodies and chondrite groups relative to the standard SRM 915b.
The solid line shows the mass-dependent fractionation predicted by kinetic mass fractionation. Uncertainties shown for δ42/44Ca and δ43/44Ca are two times the standard error of the mean per group of analysed samples. For groups containing a single sample (ordinary chondrites, CI, CM and C2-ung), the error represents either the external reproducibility (0.05 and 0.03 for δ42/44Ca and δ43/44Ca, respectively) or the analytical uncertainty of the measurement; whichever is larger.
Extended Data Figure 4 Comparison of μ48Ca values determined for desert and non-desert finds or falls.
Data are shown for martian (a), angrite (b) and ureilite (c) meteorites. The grey shaded area indicates the external reproducibility of individual sample analyses. Uncertainties shown are two standard errors of the mean.
Extended Data Figure 5 Correlation between parent-body mass and nucleosynthetic anomalies for 50Ti, 54Cr, 62Ni and 145Nd.
The data are from refs 1, 2, 6, 46, 47, 48, 49, 50, 51, 52, 53, 54. The masses are shown relative to the mass of Earth, MEarth. Arrows indicate the effects of mixing CI-like matter with the inner-disk reservoir on the isotope composition, as predicted on the basis of measured nucleosynthetic signatures of CI chondrites. Error bars indicate the 95% confidence level of the mean.
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Schiller, M., Bizzarro, M. & Fernandes, V. Isotopic evolution of the protoplanetary disk and the building blocks of Earth and the Moon. Nature 555, 507–510 (2018). https://doi.org/10.1038/nature25990
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