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Hf–W–Th evidence for rapid growth of Mars and its status as a planetary embryo

Nature volume 473, pages 489–492 (2011)Cite this article

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Abstract

Terrestrial planets are thought to have formed through collisions between large planetary embryos1 of diameter ∼1,000–5,000 km. For Earth, the last of these collisions involved an impact by a Mars-size embryo that formed the Moon 50–150 million years (Myr) after the birth of the Solar System2,3. Although model simulations of the growth of terrestrial planets can reproduce the mass and dynamical parameters of the Earth and Venus, they fall short of explaining the small size of Mars4,5. One possibility is that Mars was a planetary embryo that escaped collision and merging with other embryos1. To assess this idea, it is crucial to know Mars’ accretion timescale6, which can be investigated using the 182Hf–182W decay system in shergottite-nakhlite-chassignite meteorites6,7,8,9,10. Nevertheless, this timescale remains poorly constrained owing to a large uncertainty associated with the Hf/W ratio of the Martian mantle6 and as a result, contradicting timescales have been reported that range between 0 and 15 Myr (refs 6–10). Here we show that Mars accreted very rapidly and reached about half of its present size in only Myr or less, which is consistent with a stranded planetary embryo origin. We have found a well-defined correlation between the Th/Hf and 176Hf/177Hf ratios in chondrites that reflects remobilization of Lu and Th during parent-body processes. Using this relationship, we estimate the Hf/W ratio in Mars’ mantle to be 3.51 ± 0.45. This value is much more precise than previous estimates, which ranged between 2.6 and 5.0 (ref. 6), and lifts the large uncertainty that plagued previous estimates of the age of Mars. Our results also demonstrate that Mars grew before dissipation of the nebular gas when ∼100-km planetesimals, such as the parent bodies of chondrites, were still being formed. Mars’ accretion occurred early enough to allow establishment of a magma ocean powered by decay of 26Al.

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Figure 1: Determination of the Th/Hf ratio of CHUR.
Figure 2: Accretion timescale of Mars inferred from 182 Hf– 182 W systematics.

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Acknowledgements

Discussions with M. Chaussidon, H. Kobayashi, F. J. Ciesla, D. J. Stevenson, T. W. Dahl, R. Yokochi and G. Coutrot were appreciated. Comments from A. D. Brandon helped to improve the quality of the manuscript. We thank H. Kobayashi for sharing the digital outputs of his model simulations with us. The meteorite samples were provided by the Field Museum, the Smithsonian, the Muséum National d’Histoire Naturelle and R. N. Clayton. F. Marcantonio and P. J. Patchett gave us solutions of standards and spikes that were used to calibrate the measurements. This work was supported by a Packard fellowship, NASA and the NSF through grants NNX09AG59G and EAR-0820807 to N.D.

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Authors and Affiliations

  1. Department of the Geophysical Sciences and Enrico Fermi Institute, Origins Laboratory, The University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637, USA,

    N. Dauphas & A. Pourmand

  2. Division of Marine Geology and Geophysics, Rosenstiel School of Marine & Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149, USA,

    A. Pourmand

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  1. N. Dauphas
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Contributions

Both authors contributed equally to this work. N.D. and A.P. devised the method for purification and analysis of U, Th, Lu and Hf; A.P. performed the meteorite measurements; N.D. did the modelling; N.D. and A.P. wrote the paper.

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Correspondence to N. Dauphas.

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The authors declare no competing financial interests.

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This file contains Supplementary Text and Data 1-5, Supplementary Table 1, Supplementary Figure 1 and legend, and additional references. (PDF 271 kb)

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Dauphas, N., Pourmand, A. Hf–W–Th evidence for rapid growth of Mars and its status as a planetary embryo. Nature 473, 489–492 (2011). https://doi.org/10.1038/nature10077

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