Letter | Published:

Remote sensing evidence for an ancient carbon-bearing crust on Mercury

Nature Geoscience volume 9, pages 273276 (2016) | Download Citation


Mercury’s global surface is markedly darker than predicted from its measured elemental composition. The darkening agent, which has not been previously identified, is most concentrated within Mercury’s lowest-reflectance spectral unit, the low-reflectance material1. This low-reflectance material is generally found in large impact craters and their ejecta2,3, which suggests a mid-to-lower crustal origin. Here we present neutron spectroscopy measurements of Mercury’s surface from the MESSENGER spacecraft that reveal increases in thermal-neutron count rates that correlate spatially with deposits of low-reflectance material. The only element consistent with both the neutron measurements and visible to near-infrared spectra4 of low-reflectance material is carbon, at an abundance that is 1–3 wt% greater than surrounding, higher-reflectance material. We infer that carbon is the primary darkening agent on Mercury and that the low-reflectance material samples carbon-bearing deposits within the planet’s crust. Our findings are consistent with the formation of a graphite flotation crust from an early magma ocean5, and we propose that the heavily disrupted remnants of this ancient layer persist beneath the present upper crust. Under this scenario, Mercury’s globally low reflectance results from mixing of the ancient graphite-rich crust with overlying volcanic materials via impact processes or assimilation of carbon into rising magmas during secondary crustal formation.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.


  1. 1.

    et al. Reflectance and color variations on Mercury: regolith processes and compositional heterogeneity. Science 321, 66–69 (2008).

  2. 2.

    et al. The evolution of Mercury’s crust: a global perspective from MESSENGER. Science 324, 613–618 (2009).

  3. 3.

    et al. Exposure of spectrally distinct material by impact craters on Mercury: implications for global stratigraphy. Icarus 209, 210–223 (2010).

  4. 4.

    et al. Orbital multispectral mapping of Mercury with the MESSENGER Mercury Dual Imaging System: evidence for the origin of plains units and low-reflectance material. Icarus 254, 287–305 (2015).

  5. 5.

    & Exotic crust formation on Mercury: consequences of a shallow, FeO-poor mantle. J. Geophys. Res. 120, 195–209 (2015).

  6. 6.

    et al. A comparison of the mercurian reflectance and spectral quantities with those of the Moon. Icarus 129, 217–231 (1997).

  7. 7.

    et al. The major-element composition of Mercury’s surface from MESSENGER X-ray spectrometry. Science 333, 1847–1850 (2011).

  8. 8.

    et al. Major-element abundances on the surface of Mercury: results from the MESSENGER Gamma-Ray Spectrometer. J. Geophys. Res. 117, E00L07 (2012).

  9. 9.

    et al. Variations in the abundance of iron on Mercury’s surface from MESSENGER X-Ray Spectrometer observations. Icarus 235, 170–186 (2014).

  10. 10.

    Space weathering from Mercury to the asteroid belt. J. Geophys. Res. 106, 10039–10073 (2001).

  11. 11.

    & Mercury’s albedo from Mariner 10: implications for the presence of ferrous iron. Icarus 197, 239–246 (2008).

  12. 12.

    & Relative rates of optical maturation of regolith on Mercury and the Moon. J. Geophys. Res. 118, 1903–1914 (2013).

  13. 13.

    et al. The MESSENGER Gamma-Ray and Neutron Spectrometer. Space Sci. Rev. 131, 339–391 (2007).

  14. 14.

    et al. The X-Ray Spectrometer on the MESSENGER spacecraft. Space Sci. Rev. 131, 393–415 (2007).

  15. 15.

    et al. Evidence for geochemical terranes on Mercury: global mapping of major elements with MESSENGER’s X-Ray Spectrometer. Earth Planet. Sci. Lett. 416, 109–120 (2015).

  16. 16.

    et al. Identification and measurement of neutron-absorbing elements on Mercury’s surface. Icarus 209, 195–209 (2010).

  17. 17.

    et al. Geochemical terranes of Mercury’s northern hemisphere as revealed by MESSENGER neutron measurements. Icarus 253, 346–363 (2015).

  18. 18.

    et al. Constraints on the abundance of carbon in near-surface materials on Mercury: results from the MESSENGER Gamma-Ray Spectrometer. Planet. Space Sci. 108, 98–107 (2015).

  19. 19.

    et al. Large impact basins on Mercury: global distribution, characteristics, and modification history from MESSENGER orbital data. J. Geophys. Res. 117, E00L08 (2012).

  20. 20.

    et al. Evidence for water ice near Mercury’s north pole from MESSENGER neutron spectrometer measurements. Science 339, 292–296 (2013).

  21. 21.

    & The Planetary Scientist’s Companion (Oxford Univ. Press, 1998).

  22. 22.

    et al. Stratigraphy of the Caloris basin, Mercury: implications for volcanic history and basin impact melt. Icarus 250, 413–429 (2015).

  23. 23.

    & Impact-induced compositional variations on Mercury. Earth Planet. Sci. Lett. 391, 234–242 (2014).

  24. 24.

    , & Darkening of Mercury’s surface by cometary carbon. Nature Geosci. 8, 352–356 (2015).

  25. 25.

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

  26. 26.

    & Equilibrium condensation from chondritic porous IDP enriched vapor: implications for Mercury and enstatite chondrite origins. Planet. Space Sci. 59, 1888–1894 (2011).

  27. 27.

    & MESSENGER Neutron Spectrometer Calibrated and Derived Data Record Software Interface Specification (NASA Planetary Data System Geosciences Node, 2013);

  28. 28.

    et al. Comprehensive survey of energetic electron events in Mercury’s magnetosphere with data from the MESSENGER Gamma-Ray and Neutron Spectrometer. J. Geophys. Res. 120, 2851–2876 (2015).

  29. 29.

    et al. Global distribution and spectral properties of low-reflectance material on Mercury. In 47th Lunar Planet. Sci. Conference Abstract 1195 (Lunar and Planetary Institute, 2016); .

  30. 30.

    et al. Enhanced sodium abundance in Mercury’s north polar region revealed by the MESSENGER Gamma-Ray Spectrometer. Icarus 228, 86–95 (2014).

  31. 31.

    et al. MESSENGER mission design and navigation. Space Sci. Rev. 131, 219–246 (2007).

  32. 32.

    & A Doppler filter technique to measure the hydrogen content of planetary surfaces. Nucl. Instrum. Methods A 245, 182–190 (1986).

  33. 33.

    & Data Reduction and Error Analysis for the Physical Sciences 2nd edn (WCB/McGraw-Hill, 1992).

  34. 34.

    Pelowitz, D. B. (ed.) MCNPX User’s Manual v. 2.5.0. Report LA-UR-94-1817 (Los Alamos National Laboratory, 2005).

  35. 35.

    et al. MCNPX benchmark for cosmic ray interactions with the Moon. J. Geophys. Res. 111, E06004 (2006).

  36. 36.

    et al. Gravitational effects on planetary neutron flux spectra. J. Geophys. Res. 94, 513–525 (1989).

  37. 37.

    et al. Evidence for young volcanism on Mercury from the third MESSENGER flyby. Science 329, 668–671 (2010).

Download references


We thank the entire MESSENGER team for their invaluable contributions to the development and operation of the spacecraft. The MESSENGER mission is supported by the NASA Discovery Program under contracts NAS5-97271 to The Johns Hopkins University Applied Physics Laboratory and NASW-00002 to the Carnegie Institution of Washington. D.J.L. acknowledges support from the MESSENGER Participating Scientist Program.

Author information


  1. The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland 20723, USA

    • Patrick N. Peplowski
    • , Rachel L. Klima
    • , David J. Lawrence
    • , Carolyn M. Ernst
    • , Brett W. Denevi
    • , John O. Goldsten
    •  & Scott L. Murchie
  2. Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA

    • Elizabeth A. Frank
    • , Larry R. Nittler
    •  & Sean C. Solomon
  3. Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA

    • Sean C. Solomon


  1. Search for Patrick N. Peplowski in:

  2. Search for Rachel L. Klima in:

  3. Search for David J. Lawrence in:

  4. Search for Carolyn M. Ernst in:

  5. Search for Brett W. Denevi in:

  6. Search for Elizabeth A. Frank in:

  7. Search for John O. Goldsten in:

  8. Search for Scott L. Murchie in:

  9. Search for Larry R. Nittler in:

  10. Search for Sean C. Solomon in:


P.N.P. led the data reduction and interpretation, as well as the development of this manuscript. R.L.K. led the spectral analysis of LRM. D.J.L. produced the data sets used in this analysis and developed the modelling codes. C.M.E. led the analysis and interpretation of LRM stratigraphy. J.O.G. led the design and assembly of the GRNS. L.R.N. and E.A.F. provided XRS data and interpretation. All authors assisted with the interpretation of the data and manuscript development, particularly B.W.D., S.L.M. and S.C.S.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Patrick N. Peplowski.

Supplementary information

PDF files

  1. 1.

    Supplementary Information

    Supplementary Information

About this article

Publication history






Further reading