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Less than one weight percent of graphite on the surface of Mercury

Nature Astronomy volume 8, pages 280–289 (2024)Cite this article

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Abstract

The surface of Mercury is enriched in carbon compared with the other terrestrial bodies. Up to 4 wt% of carbon in the form of graphite has been invoked to explain its mysteriously low surface reflectance. However, the exact abundance and phase of carbon on Mercury were loosely constrained by orbital observations. Here we show that the oldest and darkest colour units on Mercury, comprising low-reflectance materials, consist of two spatially coherent subunits that do and do not have spectral characteristics attributed to graphite enrichment, respectively. We find from spectral modelling that a combination of less than 1 wt% of microcrystalline graphite and similar amounts of metallic iron is adequate for explaining the overall reflectances of various colour units on Mercury. Our results indicate that most carbon on Mercury may occur in forms other than intergrain graphite and that carbon did not entirely drain from the mantle during magma ocean crystallization.

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Fig. 1: Global distribution and reflectance spectra of the two types of LRM.
Fig. 2: Occurrence of LRMBD600− (green) on Mercury and its context relationship with LRMBD600+ (translucent yellow).
Fig. 3: Observed (dashed curves) and modelled (solid curves with the same colour) reflectance spectra for various colour units on Mercury.

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Data availability

The shapefiles of global LRM and BD600 maps on Mercury are available in the Zenodo data release60. The MDIS global eight-band colour and monochrome mosaics are available at https://messenger.jhuapl.edu/Explore/Images.html#global-mosaics. The spectrum of graphite in the RELAB sample CASC80 is available at http://www.planetary.brown.edu/relab/.

Code availability

The codes used for analyses and figures are available in the Zenodo data release60.

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Acknowledgements

This research is funded by the National Natural Science Foundation of China (Grant Nos. 42273040, 41773063 and 42241108 to Z.X.) and the B‐type Strategic Priority Program of the Chinese Academy of Sciences (Grant No. XDB41000000 to Z.X.). D. Trang is acknowledged for his suggestions. Y. Yang from the Chinese Academy of Sciences provided suggestions for the radiative transfer model.

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Authors and Affiliations

  1. Planetary Environmental and Astrobiological Research Laboratory, School of Atmospheric Sciences, Sun Yat-sen University, Zhuhai, China

    Rui Xu, Zhiyong Xiao, Yichen Wang & Jun Cui

  2. CAS Center for Excellence in Comparative Planetology, Hefei, China

    Zhiyong Xiao & Jun Cui

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Z.X. and R.X. both contributed to the conceptualization, visualization, methodology, modelling, analysis and writing of the paper. Y.W. and J.C. joined the discussions.

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Correspondence to Zhiyong Xiao.

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Nature Astronomy thanks Christian Huber, Rachel Klima, Brendan Anzures and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Xu, R., Xiao, Z., Wang, Y. et al. Less than one weight percent of graphite on the surface of Mercury. Nat Astron 8, 280–289 (2024). https://doi.org/10.1038/s41550-023-02169-5

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