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Iron meteorites as remnants of planetesimals formed in the terrestrial planet region


Iron meteorites are core fragments from differentiated and subsequently disrupted planetesimals1. The parent bodies are usually assumed to have formed in the main asteroid belt, which is the source of most meteorites. Observational evidence, however, does not indicate that differentiated bodies or their fragments were ever common there. This view is also difficult to reconcile with the fact that the parent bodies of iron meteorites were as small as 20 km in diameter2,3 and that they formed 1–2 Myr earlier than the parent bodies of the ordinary chondrites4,5,6. Here we show that the iron-meteorite parent bodies most probably formed in the terrestrial planet region. Fast accretion times there allowed small planetesimals to melt early in Solar System history by the decay of short-lived radionuclides (such as 26Al, 60Fe)7,8,9. The protoplanets emerging from this population not only induced collisional evolution among the remaining planetesimals but also scattered some of the survivors into the main belt, where they stayed for billions of years before escaping via a combination of collisions, Yarkovsky thermal forces, and resonances10. We predict that some asteroids are main-belt interlopers (such as (4) Vesta). A select few may even be remnants of the long-lost precursor material that formed the Earth.

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Figure 1: A snapshot of inner Solar System planetesimals and planetary embryos after 10 Myr of dynamical evolution.
Figure 2: The fraction of inner Solar System planetesimals scattered into the main-belt zone by gravitational interactions with planetary embryos.
Figure 3: The fraction of inner Solar System planetesimals that survive the first 10 Myr of collisional evolution.


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We thank S. Bus, J. Chambers, D. Durda, M. Gounelle, H. Haack, H. Levison, T. McCoy, D. Mittlefehldt, E. Scott, J. Sunshine, D. Vokrouhlicky and M. Zolensky for discussions and comments. The project was supported by NASA's Origins of Solar System and Planetary Geology and Geophysics programmes.

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Correspondence to William F. Bottke.

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Supplementary Discussion

This file contains an extended discussion of several issues that could not be addressed in the main text. There are two Supplementary Figures. Supplementary Figure 1 shows the delivery efficiency of test bodies from various main belt resonances striking the Earth. Supplementary Figure 2 shows how a hypothetical population of olivine-rich A-type asteroids placed in the inner main asteroid belt undergo collisional and dynamical evolution. (PDF 919 kb)

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Bottke, W., Nesvorný, D., Grimm, R. et al. Iron meteorites as remnants of planetesimals formed in the terrestrial planet region. Nature 439, 821–824 (2006).

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