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Darkening of Mercury's surface by cometary carbon

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Abstract

Mercury’s surface is darker than that of the Moon1,2. Iron-bearing minerals and submicroscopic metallic iron produced by space weathering are the primary known darkening materials on airless bodies. Yet Mercury’s iron abundance at the surface is lower than the Moon’s3,4; another material is therefore likely to be responsible for Mercury’s dark surface1,2,5,6,7,8. Enhanced darkening by submicroscopic metallic iron particles under intense space weathering at Mercury’s surface9,10,11,12 is insufficient to reconcile the planet’s low reflectance with its low iron abundance12. Here we show that the delivery of cometary carbon by micrometeorites provides a mechanism to darken Mercury’s surface without violating observational constraints on iron content. We calculate the micrometeorite flux at Mercury and numerically simulate the fraction of carbonaceous material retained by the planet following micrometeorite impacts. We estimate that 50 times as many carbon-rich micrometeorites per unit surface area are delivered to Mercury, compared with the Moon, resulting in approximately 3–6 wt% carbon at Mercury’s surface (in graphite, amorphous, or nanodiamond form). Spectroscopic analysis of products of hypervelocity impact experiments demonstrates that the incorporation of carbon effectively darkens and weakens spectral features, consistent with remote observations of Mercury1,2,5,6,7,8,12. Carbon delivery by micrometeorites provides an explanation for Mercury’s globally low reflectance and may contribute to the darkening of planetary surfaces elsewhere.

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Figure 1: Retention of micrometeorite impacts at Mercury.
Figure 2: Impact-generated agglutinates collected from crater floors.
Figure 3: Visible/near-infrared spectra of impact agglutinates and different terrain types at Mercury.

Change history

  • 16 April 2015

    In the version of this Letter originally published online, the caption of Fig. 2 should have read 'Agglutinates generated in the presence of complex organics (right column) are visually darker than organics-free agglutinates (left column)'. This error has been corrected in all versions of the Letter.

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Acknowledgements

This research was supported by NASA’s Planetary Geology & Geophysics (NNX13AB75G) and NESSF (NNXC12AL79H) programs. The authors thank T. Daly and T. Hiroi for assistance with sample analysis and gratefully acknowledge the technical team at the Ames Vertical Gun Range for supporting the impact experiments.

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M.B.S. wrote the manuscript and performed the calculations, impact experiments and sample analyses. P.H.S. assisted with impact experiments, development of ideas and manuscript edits. M.A.R. analysed remote sensing data for comparison with experimental results.

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Correspondence to Megan Bruck Syal.

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The authors declare no competing financial interests.

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Syal, M., Schultz, P. & Riner, M. Darkening of Mercury's surface by cometary carbon. Nature Geosci 8, 352–356 (2015). https://doi.org/10.1038/ngeo2397

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