Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Mercury and other iron-rich planetary bodies as relics of inefficient accretion

Subjects

Abstract

Earth, Venus, Mars and asteroids such as Vesta and, perhaps, Lutetia1 have chondritic bulk compositions with massive silicate mantles surrounding iron cores. Anomalies include Mercury with its abundant metallic iron (about 70% by mass2), the Moon with its small iron core, and metal-dominated asteroids. Although a giant impact with proto-Earth can explain the Moon’s small core3, a giant impact origin for Mercury is problematic. Such a scenario requires that proto-Mercury was blasted apart with far greater specific energy than required for lunar formation4, yet retained substantial volatile elements5 and did not reaccrete its ejected mantle6. Here we present numerical hydrocode simulations showing that proto-Mercury could have been stripped of its mantle in one or more high-speed collisions with a larger target planet that survived intact. A projectile that escapes the planet-colliding orbit in this hit-and-run scenario7 ultimately finds a permanent sink for its stripped mantle silicates. We show that if Mars and Mercury are derived from two planetary embryos that randomly avoided being accreted into a larger body, out of 20 original embryos (the rest having accreted into Venus and Earth), then it is statistically probable that one of those had its mantle stripped in one or two hit-and-run collisions. The same reasoning applies to pairwise accretion of planetesimals in the early Solar System, in which the relic bodies, which avoided becoming accreted, would be expected to have a wide diversity of compositions as a consequence of hit-and-run processes.

This is a preview of subscription content, access via your institution

Relevant articles

Open Access articles citing this article.

Access options

Rent or buy this article

Prices vary by article type

from$1.95

to$39.95

Prices may be subject to local taxes which are calculated during checkout

Figure 1: Mercury-like planetary remnants can be formed in a single energetic hit-and-run collision (HRC) (h = 1).
Figure 2: One of the solutions forming a Mercury-like remnant in a single HRC, from Fig. 1.
Figure 3: Distribution of h for survivors of an original population, averaged over many realizations of a simple statistical model of accretion where hit and run (h) and perfect merger occur with equal probability.
Figure 4: Mechanical and gravitational interactions scale with size, but shocks do not.

References

  1. Elkins-Tanton, L. T., Weiss, B. P. & Zuber, M. T. Chondrites as samples of differentiated planetesimals. Earth Planet. Sci. Lett. 305, 1–10 (2011).

    Article  Google Scholar 

  2. Hauck, S. A. et al. The curious case of Mercury’s internal structure. J. Geophys. Res. 118, 1204–1220 (2013).

    Article  Google Scholar 

  3. Asphaug, E. Impact origin of the moon? Annu. Rev. Earth Planet. Sci. 42, 551–578 (2014).

    Article  Google Scholar 

  4. Benz, W., Anic, A., Horner, J. & Whitby, J. A. The origin of Mercury. Space Sci. Rev. 132, 189–202 (2007).

    Article  Google Scholar 

  5. Peplowski, P. N. et al. Radioactive elements on Mercury’s surface from MESSENGER: Implications for the planet’s formation and evolution. Science 333, 1850–1852 (2011).

    Article  Google Scholar 

  6. Gladman, B. & Coffey, J. Mercurian impact ejecta: Meteorites and mantle. Meteorit. Planet. Sci. 44, 285–291 (2009).

    Article  Google Scholar 

  7. Asphaug, E., Agnor, C. B. & Williams, Q. Hit-and-run planetary collisions. Nature 439, 155–160 (2006).

    Article  Google Scholar 

  8. Morbidelli, A., Bottke, W. F., Nesvorný, D. & Levison, H. F. Asteroids were born big. Icarus 204, 558–573 (2009).

    Article  Google Scholar 

  9. Kleine, T., Mezger, K., Palme, H., Scherer, E. & Münker, C. Early core formation in asteroids and late accretion of chondrite parent bodies: Evidence from 182Hf–182W in CAIs, metal-rich chondrites, and iron meteorites. Geochim. Cosmochim. Acta 69, 5805–5818 (2005).

    Article  Google Scholar 

  10. Safronov, V. S. Evolution of the Protoplanetary Cloud and Formation of the Earth and Planets (NASA Tech. Transl. F-677, 1972).

    Google Scholar 

  11. Agnor, C. & Asphaug, E. Accretion efficiency during planetary collisions. Astrophys. J. 613, L157–L160 (2004).

    Article  Google Scholar 

  12. Asphaug, E. Similar-sized collisions and the diversity of planets. Chem. Erde Geochem. 70, 199–219 (2010).

    Article  Google Scholar 

  13. Stewart, S. T. & Leinhardt, Z. M. Collisions between gravity-dominated bodies. II. The diversity of impact outcomes during the end stage of planet formation. Astrophys. J. 751, 32–49 (2012).

    Article  Google Scholar 

  14. Reufer, A. Collisions in Planetary Systems. PhD thesis, Univ. of Bern (2011)

  15. Asphaug, E. & Reufer, A. Late origin of the Saturn system. Icarus 223, 544–565 (2013).

    Article  Google Scholar 

  16. Canup, R. M. Dynamics of lunar formation. Annu. Rev. Astron. Astrophys. 42, 441–475 (2004).

    Article  Google Scholar 

  17. Reufer, A., Meier, M. M. M., Benz, W. & Wieler, R. A hit-and-run giant impact scenario. Icarus 221, 296–299 (2012).

    Article  Google Scholar 

  18. Benz, W., Slattery, W. L. & Cameron, A. G. W. Collisional stripping of Mercury’s mantle. Icarus 74, 516–528 (1988).

    Article  Google Scholar 

  19. O’Brien, D. P., Morbidelli, A. & Levison, H. F. Terrestrial planet formation with strong dynamical friction. Icarus 184, 39–58 (2006).

    Article  Google Scholar 

  20. Stewart, S. T., Leinhardt, Z. M. & Humayun, M. Giant impacts, volatile loss, and the K/Th ratios on the Moon, Earth, and Mercury. 44th Lunar and Planetary Science Conference Abstract 2306 (2013)

  21. Chambers, J. E. Late-stage planetary accretion including hit-and-run collisions and fragmentation. Icarus 224, 43–56 (2013).

    Article  Google Scholar 

  22. Burbine, T. H., Meibom, A. & Binzel, R. P. Mantle material in the main belt: Battered to bits? Meteorit. Planet. Sci. 31, 607–620 (1996).

    Article  Google Scholar 

  23. Farinella, P., Paolicchi, P. & Zappala, V. The asteroids as outcomes of catastrophic collisions. Icarus 52, 409–433 (1982).

    Article  Google Scholar 

  24. Davis, D. R., Chapman, C. R., Weidenschilling, S. J. & Greenberg, R. Collisional history of asteroids—evidence from Vesta and the Hirayama families. Icarus 62, 30–53 (1985).

    Article  Google Scholar 

  25. Yang, J., Goldstein, J. I. & Scott, E. R. D. Iron meteorite evidence for early formation and catastrophic disruption of protoplanets. Nature 446, 888–891 (2007).

    Article  Google Scholar 

  26. Moskovitz, N. A. & Walker, R. J. Size of the group IVA iron meteorite core: Constraints from the age and composition of Muonionalusta. Earth Planet. Sci. Lett. 308, 410–416 (2011).

    Article  Google Scholar 

  27. Asphaug, E., Jutzi, M. & Movshovitz, N. Chondrule formation during planetesimal accretion. Earth Planet. Sci. Lett. 308, 369–379 (2011).

    Article  Google Scholar 

  28. Wetherill, G. W. Formation of the terrestrial planets. Annu. Rev. Astron. Astrophys. 18, 77–113 (1980).

    Article  Google Scholar 

  29. Bottke, W. F., Nesvorný, D., Grimm, R. E., Morbidelli, A. & O’Brien, D. P. Iron meteorites as remnants of planetesimals formed in the terrestrial planet region. Nature 439, 821–824 (2006).

    Article  Google Scholar 

  30. Melosh, H. J. A hydrocode equation of state for SiO2 . Meteorit. Planet. Sci. 42, 2079–2098 (2007).

    Article  Google Scholar 

Download references

Acknowledgements

This research is sponsored by NASA NNX13AR66G, Collisional Accretion of Similar-Sized Bodies. Computing resources and travel by A.R. were provided by Arizona State University. We thank M. Kreslavsky (UCSC) for valuable discussions.

Author information

Authors and Affiliations

Authors

Contributions

A.R. ran and reduced the suites of SPH simulations. E.A. constructed the statistical analysis and wrote the paper.

Corresponding author

Correspondence to E. Asphaug.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Rights and permissions

Reprints and Permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Asphaug, E., Reufer, A. Mercury and other iron-rich planetary bodies as relics of inefficient accretion. Nature Geosci 7, 564–568 (2014). https://doi.org/10.1038/ngeo2189

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/ngeo2189

This article is cited by

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing