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Vigorous convection as the explanation for Pluto’s polygonal terrain

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Pluto’s surface is surprisingly young and geologically active1. One of its youngest terrains is the near-equatorial region informally named Sputnik Planum, which is a topographic basin filled by nitrogen (N2) ice mixed with minor amounts of CH4 and CO ices1. Nearly the entire surface of the region is divided into irregular polygons about 20–30 kilometres in diameter, whose centres rise tens of metres above their sides. The edges of this region exhibit bulk flow features without polygons1. Both thermal contraction and convection have been proposed to explain this terrain1, but polygons formed from thermal contraction (analogous to ice-wedges or mud-crack networks)2,3 of N2 are inconsistent with the observations on Pluto of non-brittle deformation within the N2-ice sheet. Here we report a parameterized convection model to compute the Rayleigh number of the N2 ice and show that it is vigorously convecting, making Rayleigh–Bénard convection the most likely explanation for these polygons. The diameter of Sputnik Planum’s polygons and the dimensions of the ‘floating mountains’ (the hills of of water ice along the edges of the polygons) suggest that its N2 ice is about ten kilometres thick. The estimated convection velocity of 1.5 centimetres a year indicates a surface age of only around a million years.

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Figure 1: New Horizon’s image of Sputnik Planum on Pluto.
Figure 2: Calculated convection for Sputnik Planum polygons.
Figure 3: The N2–CO phase diagram.

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We thank all of the New Horizons team members, without whom none of this work would have been possible. We also thank T. Bowling, D. Minton, B. Hogan, J. Kendall, B. Link and C. Milbury for discussions. A.J.T. thanks the Fredrick N. Andrews Fellowship for funding.

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A.J.T. and H.J.M. conceived this work, developed the parameterized convection model, and conducted Rayleigh number calculations for this paper. J.K.S. developed Maxwell time arguments for ruling out thermal contraction, computed the surface and subsurface temperatures of Pluto, and calculated atmospheric pressures. A.M.F. advised A.J.T., and helped to edit and revise the manuscript.

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Correspondence to A. J. Trowbridge.

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The authors declare no competing financial interests.

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Reviewer Information Nature thanks G. Schubert and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Trowbridge, A., Melosh, H., Steckloff, J. et al. Vigorous convection as the explanation for Pluto’s polygonal terrain. Nature 534, 79–81 (2016).

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