Washboard and fluted terrains on Pluto as evidence for ancient glaciation

Abstract

Distinctive landscapes termed ‘washboard’ and ‘fluted’ terrains1,2, which border the N2 ice plains of Sputnik Planitia along its northwest margin, are among the most enigmatic landforms yet seen on Pluto. These terrains consist of parallel to sub-parallel ridges that display a remarkably consistent east-northeast–west-southwest orientation—a configuration that does not readily point to a simple analogous terrestrial or planetary process or landform. Here, we report on mapping and analysis of their morphometry and distribution as a means to determine their origin. Based on their occurrence in generally low-elevation, low-relief settings adjacent to Sputnik Planitia that coincide with a major tectonic system, and through comparison with fields of sublimation pits seen in southern Sputnik Planitia, we conclude that washboard and fluted terrains represent crustal debris that were buoyant in pitted glacial N2 ice that formerly covered this area, and which were deposited after the N2 ice receded via sublimation. Crater surface age estimates indicate that this N2 ice glaciation formed and disappeared early in Pluto’s history, soon after formation of the Sputnik Planitia basin. These terrains constitute an entirely new category of glacial landform.

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Fig. 1: WFT to the northwest of Sputnik Planitia.
Fig. 2: Maps of discernible WFT coverage in uplands northwest of Sputnik Planitia.
Fig. 3: Comparison of the textures of elongate sublimation pits in southern Sputnik Planitia and washboard terrain.
Fig. 4: Schematic of the hypothesized mechanism for WFT ridge formation.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

This work was supported by NASA’s New Horizons project.

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O.L.W. performed the mapping that is the basis of the research and wrote the majority of the main text, as well as the ‘Factors affecting ridge mapping’ and ‘Surface albedo, colour and composition’ sections of the Methods. J.M.M., A.D.H. and W.B.M. aided O.L.W. in interpretation of the mapping and morphometric analysis, and editing of the manuscript. J.T.K. quantitatively compared the washboard mapping results with TPW simulations (Supplementary Fig. 6) and provided much of the text in the ‘Do washboard ridges record a palaeopole?’ section of the Methods. K.N.S. provided much of the text in the ‘Crater density-based terrain age estimates’ section of the Methods, as well as editing the manuscript. T.B. provided text discussing climatological considerations of the glaciation hypothesis for WFT, as well as editing the manuscript. S.J.R. provided the pixel scale, solar incidence angle and emission angle maps in Supplementary Fig. 2b–d, as well as editing the manuscript. P.M.S. provided the topography data used in the mapping and analysis (including the DEM shown in Fig. 2b and Supplementary Fig. 1) and associated text. B.S. provided the composition maps in Supplementary Fig. 1c–f and associated text. B.J.B. provided the normal reflectance map in Supplementary Fig. 1b. Per New Horizons team requirements, the New Horizons Science Team principal investigator (S.A.S.) and four project scientists (K.E., C.B.O., H.A.W. and L.A.Y.) must be included as co-authors on all peer-reviewed papers authored by New Horizons team members and affiliates. All authors approved the final version of the manuscript.

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Correspondence to Oliver L. White.

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The authors declare no competing interests.

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Supplementary Figures 1–7

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White, O.L., Moore, J.M., Howard, A.D. et al. Washboard and fluted terrains on Pluto as evidence for ancient glaciation. Nat Astron 3, 62–68 (2019). https://doi.org/10.1038/s41550-018-0592-z

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