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

Abstract

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.

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Figure 1: Visible to near-infrared spectral properties of LRM.
Figure 2: NS time-series measurements acquired at altitudes <1,000 km over study area C.
Figure 3: Histograms of residual neutron measurements for each study area.

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Acknowledgements

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.

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Authors and Affiliations

Authors

Contributions

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.

Corresponding author

Correspondence to Patrick N. Peplowski.

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The authors declare no competing financial interests.

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Peplowski, P., Klima, R., Lawrence, D. et al. Remote sensing evidence for an ancient carbon-bearing crust on Mercury. Nature Geosci 9, 273–276 (2016). https://doi.org/10.1038/ngeo2669

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