# The origin of s-process isotope heterogeneity in the solar protoplanetary disk

## Abstract

Rocky asteroids and planets display nucleosynthetic isotope variations that are attributed to the heterogeneous distribution of stardust from different stellar sources in the solar protoplanetary disk. Here we report new high-precision palladium isotope data for six iron meteorite groups. The palladium data display smaller nucleosynthetic isotope variations than the more refractory neighbouring elements. Based on this observation, we present a model in which thermal destruction of interstellar dust in the inner Solar System results in an enrichment of s-process-dominated stardust in regions closer to the Sun. We propose that stardust is depleted in volatile elements due to incomplete condensation of these elements into dust around asymptotic giant branch stars. This led to the smaller nucleosynthetic variations for Pd reported here and the lack of such variations for more volatile elements. The smaller magnitude variations measured in heavier refractory elements suggest that material from high-metallicity asymptotic giant branch stars is the dominant source of stardust in the Solar System. These stars produce fewer heavy s-process elements (proton number Z ≥ 56) compared with the bulk Solar System composition.

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## Data availability

The authors declare that the original data supporting the findings of this study are available within the paper and its Supplementary Information. Original Pd and Pt data points for individual meteorites are also available from the EarthChem library (https://doi.org/10.1594/IEDA/111416). All other data are available from the corresponding author on reasonable request.

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## Acknowledgements

This work was supported by the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007–2013)/ERC grant agreement no. 279779 awarded to M.S. and by the Lendület grant (LP17-2014) of the Hungarian Academy of Sciences awarded to M.L. The authors acknowledge funding from ETH, the National Center for Competence in Research ‘PlanetS’, supported by the Swiss National Science Foundation (SNSF) and project funding from the SNSF (200020_179129). We are grateful to D. Farsky and D. Cook for their assistance in acquiring the Rh/Pd ratios used in this study. We thank C. Smith and D. Cassey (Natural History Museum, London), J. Hoskin (Smithsonian Institute) and P. Heck (Field Museum) for the loan of meteorite materials used in this study. Comments from M. Rehkämper helped improve an early version of this manuscript.

## Author information

M.S. designed the research project. M.E. prepared the samples for isotope analyses and conducted the measurements with assistance from A.C.H. M.E. did the data interpretation and wrote the first draft of the manuscript with important input from M.S., A.C.H. and M.L. All authors contributed equally to subsequent revisions of the manuscript.

Correspondence to Mattias Ek.

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## Extended data

### Extended Data Fig. 1 Palladium isotope composition of iron meteorites from the IAB, IIAB, IID, IIIAB, IVA and IVB groups.

All epsilon values are reported relative to 105Pd and internally normalised to 108Pd/105Pd. Uncertainties on data points reflects the 2 standard error of the mean.

### Extended Data Fig. 2 Nuclear field shift effects on Pd isotopes.

(a) The Pd isotope pattern produced by nuclear field shift effects, internally normalised to 108Pd/105Pd (short dashed line) and 108Pd/106Pd (solid line), calculated using the equations from Ref. 89 and the charge radii from Ref. 90. (b) ε105Pd against ε110Pd (internally normalised to 108Pd/106Pd; Supplementary Table 1) for five individually processed aliquots of Toluca (IAB), Odessa (IAB), the other IAB meteorites, Rodeo (IID) and four aliquots of Carbo (IID) sampled at different locations within the meteorite. The solid line shows the nuclear field shift trend, internally normalised to 108Pd/106Pd, and the dashed line shows an s-process deficit/excess trend calculated using the s-process yields of Ref. 20. Uncertainties are shown as the 2 standard error of the mean.

### Extended Data Fig. 3 Cosmic ray effects (CRE) on Pd isotopes in iron meteorites.

Regressions of (a) ε102Pd, (c) ε106Pd, and (d) ε110Pd against ε196Pt for the IAB, IID and IVB groups. Panel (b) shows regressions of ε104Pd versus ε196Pt multiplied by the Rh/Pd ratio of the sample, to account for varied CRE contributions from 103Rh(n,β)104Pd, for the same three groups. Individual samples and the slope of the regression are normalised to the intercept for each isotope/group such that the slopes can be compared directly. The black line shows the modelled CRE trend for each isotope, taken from Ref. 72. The slope of the regressions for ε102Pd, ε104Pd, ε106Pd and ε110Pd overlap for all three groups and agree well with the modelled slope. Uncertainties on individual data points given as the 2 standard error of the mean. Uncertainty envelope around regressions represents the 2 standard deviations of the regression calculated using the equation from Ref. 91.

### Extended Data Fig. 4 Isotopic dichotomy between carbonaceous (CC) and non-carbonaceous (NC) meteorites in ε100Mo-ε92Mo and ε110Pd-ε102Pd.

(a) The dichotomy reported in Mo is characterised by an enrichment in ε92Mo for the CC meteorites (blue) relative to the NC group (grey). A small addition of supernova derived material to the stardust and/or ISM dust fraction coupled with thermal processing of ISM dust mantles can explain this offset. (b) Only the IVB irons of the two analysed CC-type iron meteorite groups (IID and IVB) show the negative shift in ε102Pd predicted by the isotopic dichotomy. Given the typical uncertainty on ε102Pd for individual meteorites (~ 1 ε; Supplementary Table 1) due to the large Ru correction on 102Pd (Ref. 62), it is barely possible to resolve the expected effect. The dashed lines indicate a mixing line between an s-process endmember33 and the terrestrial composition. The blue dashed line represents a mixing line between an s-process endmember33 and the terrestrial composition with a 0.008% enrichment in the residual r-process component, estimated based on the Mo data. Mo data from Ref. 11,14 and Pd data from Table 1. Uncertainties on Pd data points reflect either the 2 standard error of the mean or the 2 standard deviation of the x-axis intercept of a regression against ε196Pt (See Table 1). Uncertainties on Mo data points reflect either the 2 standard error of the mean (data from Ref. 11) or the 95 % confidence interval (data from Ref. 14).

### Extended Data Fig. 5 Elemental ratios as a function of metallicity for FRUITY, Monash and NuGrid s-process models for AGB stars with an initial mass between 1.5 – 4 Mʘ.

The relative proportion of light s-process elements (Y, Zr, Mo, Ru, Pd and Cd) vary little with different metallicities and are independent of the initial stellar mass and nucleosynthetic model. All models show a clear trend with the yield of heavy s-process elements (Ba, Ce, Nd, Hf, W, Pt, Os) decreasing, relative to the light s-process elements, as the metallicity increases. Shown in panel Ce/Y are the observational data for Ba stars51,92 in grey. These also indicate a decrease in the Ce/Y ratio as a function of increasing metallicity51. FRUITY data93,94,95 correspond to the total yield for non-rotating stars with a metallicity of 0.006, 0.08, 0.010, 0.014 and 0.020 and mass of 1.5, 2.0, 2.5, 3.0 and 4.0 Mʘ. Monash data81,96,97,98 depict the yields of stars with a metallicity of 0.007, 0.014 and 0.030 and mass of 1.5, 2.5, 3.0, 3.5 and 4.0 Mʘ computed with a mass extension of the mixing leading to the formation of the main neutron source 13C of 2 × 10−3 (Mʘ ≤ 3) and 1 × 10−3 (Mʘ > 3). NuGrid data99 represent the final surface composition for stars with a metallicity of 0.01 and 0.02 and mass of 2.0 and 3.0 Mʘ. Elemental ratios are shown in standard spectroscopic notation where [Elx/Ely] = log(Elx/Ely)*-log(Elx/Ely)ʘ, where Elx and Ely are abundances by number.

## Supplementary information

### Supplementary Information

Caption for Supplementary Data 1

### Supplementary Data

Supplementary Table 1, Supplementary Table 2 and a copy of Table 1.

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Ek, M., Hunt, A.C., Lugaro, M. et al. The origin of s-process isotope heterogeneity in the solar protoplanetary disk. Nat Astron (2019). https://doi.org/10.1038/s41550-019-0948-z

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