In our Solar System, the planets formed by collisional growth from smaller bodies. Planetesimals collided to form Moon-to-Mars-sized protoplanets in the inner Solar System in 0.1–1 Myr, and these collided more energetically to form planets1. Insights into the timing and nature of collisions during planetary accretion can be gained from meteorite studies. In particular, iron meteorites offer the best constraints on early stages of planetary accretion because most are remnants of the oldest bodies, which accreted and melted in <1.5 Myr, forming silicate mantles and iron-nickel metallic cores2,3,4. Cooling rates for various groups of iron meteorites suggest that if the irons cooled isothermally in the cores of differentiated bodies, as conventionally assumed, these bodies were 5–200 km in diameter5,6. This picture is incompatible, however, with the diverse cooling rates observed within certain groups, most notably the IVA group7,8, but the large uncertainties associated with the measurements do not preclude it. Here we report cooling rates for group IVA iron meteorites that range from 100 to 6,000 K Myr-1, increasing with decreasing bulk Ni. Improvements in the cooling rate model, smaller error bars, and new data from an independent cooling rate indicator9 show that the conventional interpretation is no longer viable. Our results require that the IVA meteorites cooled in a 300-km-diameter metallic body that lacked an insulating mantle. This body probably formed ∼4,500 Myr ago in a ‘hit-and-run’ collision between Moon-to-Mars-sized protoplanets10. This demonstrates that protoplanets of ∼103 km size accreted within the first 1.5 Myr, as proposed by theory, and that fragments of these bodies survived as asteroids.
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Financial support from the NASA Cosmochemistry programme is acknowledged. We thank T. J. McCoy, L. Welzenbach, D. S. Ebel and J. Boesenberg for providing the meteorite samples; M. J. Jercinovic, J. R. Michael and P. Kotula for assistance and advice in obtaining electron microprobe and microscopy data; G. J. Taylor, E. Asphaug, W. F. Bottke and A. Ruzicka for discussions; and H. Haack, J. T. Wasson and J. A. Wood for critically reading the manuscript.
Author Contributions J.Y., J.I.G. and E.R.D.S. contributed equally to this work. J.Y. determined the metallographic cooling rates and established the thermal model. J.I.G. measured the cloudy zone particle size. E.R.D.S. provided the planetary science perspective. All authors discussed the results, wrote portions of the paper and commented on the manuscript.
This file contains Supplementary Table S1, Supplementary Notes and additional references. Supplementary Table provides the bulk P content of IVA irons. Supplementary Notes discuss the potential errors in determining the metallographic cooling rates in addition to that discussed in text.