The Charon-forming giant impact as a source of Pluto’s dark equatorial regions


Pluto exhibits complex regional diversity in its surface materials1,2. One of the most striking features is the dark reddish material, possibly organic matter, along Pluto’s equator coexisting with the H2O-rich crust2. Little is known, however, about the surface process responsible for the dark equatorial regions. Here, we propose that Pluto’s dark regions were formed through reactions in elongated pools of liquid water near the equator, generated by the giant impact that formed Charon35. Our laboratory experiments show that dark reddish organic matter, comparable to Pluto’s dark materials, is produced through polymerization of simple organic compounds6,7 that would have been present in proto-Pluto (for example, formaldehyde) by prolonged heating at temperatures ≥50 °C. Through hydrodynamic impact simulations, we demonstrate that an impactor, one-third the mass of Pluto, colliding with proto-Pluto—with an interior potential temperature of 150–200 K—could have generated both a Charon-sized satellite and high-temperature regions around Pluto’s equator. We also propose that high-velocity giant impacts result in global or hemispherical darkening and reddening, suggesting that the colour variety of large Kuiper belt objects812 could have been caused by frequent, stochastic giant impacts in a massive outer protoplanetary disk in the early Solar System1316.

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Figure 1: Visible transmittance spectra and visual appearance of solutions after heating.
Figure 2: Formation of a Charon-sized satellite and elongated post-impact heated area near Pluto’s equator.
Figure 3: Latitudinal distributions of heated-area fraction in the simulations, and observed dark-area fraction on Pluto.
Figure 4: Surface temperature distributions of the post-impact targets in the Mercator projection after high-velocity collisions.


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Y.S. thanks S. Tachibana for providing the methods to produce the organic matter from formaldehyde solution. This study was supported by Grant-in-Aids for Scientific Research from the Japan Society for Promotion of Science (26707024, 16001111, 16K13873 and 15K13562), from the JGC-S Scholarship Foundation, and from the Astrobiolgy Center of the National Institutes of Natural Sciences (NINS).

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Y.S. developed the idea for the study, and Y.S. and T.F. performed the experiments. H.G. performed hydrodynamic simulations. S.K. performed interior temperature calculation and image analysis. All the authors contributed to the interpretation of the results.

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Correspondence to Yasuhito Sekine.

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The authors declare no competing financial interests.

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Sekine, Y., Genda, H., Kamata, S. et al. The Charon-forming giant impact as a source of Pluto’s dark equatorial regions. Nat Astron 1, 0031 (2017).

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