The dissipation of the solar nebula constrained by impacts and core cooling in planetesimals

Rapid cooling of planetesimal cores has been inferred for several iron meteorite parent bodies on the basis of metallographic cooling rates, and linked to the loss of their insulating

mantles during impacts. However, the timing of these disruptive events is poorly constrained. Here, we used the short-lived 107Pd–107Ag decay system to date rapid core cooling by determining

Pd–Ag ages for iron meteorites. We show that closure times for the iron meteorites equate to cooling in the time frame ~7.8–11.7 Myr after calcium–aluminium-rich inclusion formation, and

that they indicate that an energetic inner Solar System persisted at this time. This probably results from the dissipation of gas in the protoplanetary disk, after which the damping effect

of gas drag ceases. An early giant planet instability between 5 and 14 Myr after calcium–aluminium-rich inclusion formation could have reinforced this effect. This correlates well with the

timing of impacts recorded by the Pd–Ag system for iron meteorites.

All data are available within this Article and its Supplementary Information, or available from the authors on request.

This work was supported by the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007–2013/ERC grant agreement 279779, M.S.). We gratefully acknowledge

funding from STFC (ST/F002157/1 and ST/J001260/1, M.R.; ST/J001643/1, M.S.) and funding from the Swiss National Science Foundation (project 200020_179129, M.S.). A.C.H. wishes to thank M. Ek

and M. Fehr for laboratory assistance at ETH Zürich during this study. M.S. would like to thank R. Carlson and M. Horan (DTM, Carnegie Institution) for their support and the opportunity to

analyse Ag isotopes in May 2006. We also thank C. Smith and D. Cassey (Natural History Museum, London) and J. Hoskin (Smithsonian Institution National Museum of Natural History) for the loan

of samples used in this work.

Present address: The Photon Science Institute, The University of Manchester, Manchester, UK

Institute of Geochemistry and Petrology, ETH Zürich, Zürich, Switzerland

School of Earth and Environmental Sciences, The University of Manchester, Manchester, UK

Department of Earth Science & Engineering, Imperial College London, London, UK

Space Science and Technology Centre, School of Earth and Planetary Sciences, Curtin University, Perth, Western Australia, Australia

Department of Earth and Planetary Sciences, Western Australian Museum, Perth, Western Australia, Australia

Department of Geoscience, Aarhus University, Aarhus, Denmark

M.S. designed the study. A.C.H., K.J.T. and M.S. prepared samples for isotope analyses and conducted the isotopic measurements. All authors were involved in the data interpretation and

writing of the manuscript.

Nature Astronomy thanks James Van Orman 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.

The GCR model calculations10 are shown for Ir/Pt ratios measured for these groups (dashed lines; Ir/Pt ratios for individual samples are given in Supplementary Materials Table 1).

Uncertainties are 2 S.D.

In each case, ε107Ag is plotted against 108Pd/109Ag for both GCR-corrected (filled symbols) and GCR-uncorrected data (unfilled symbols). Uncertainties on GCR-uncorrected data are 2 S.D. and

within the size of the symbol (Table 1), while uncertainties for ε107Ag for GCR-corrected data represent the propagated 2 S.D. uncertainties of the GCR correction (Table 2). Isochrons are

determined using GCR-corrected data and ISOPLOT55. *denotes GCR-corrected data from source11, shown for comparison but not included in the regressions.

In both scenarios the parent body accreted relatively late at ~1.4 Myr and then underwent limited metal-silicate differentiation at ~6.0 Myr after CAI14. For metal cooling at ~12.8 Myr, the

catastrophic impact event occurred in the timeframe ~11–13.6 Myr. This was followed by fast cooling and closure of the Pd–Ag system at ~12.8 +3.1/-4.6 Myr. For metal cooling at ~7.9 Myr

after CAI the body was disrupted at about 6 Myr, while at or near its peak temperature. It then cooled quickly, leading to closure of the Pd–Ag system at 7.9 +1.0/-1.1 Myr.

Summary of concentration and isotope data for Pd–Ag and Pt for IAB, IIAB and IIIAB iron meteorites.

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