<figure><img src="image_1.png" /></figure>

HAL
open science

# Washboard and fluted terrains on Pluto as evidence for ancient glaciation

Oliver L White, Jeffrey M Moore, Alan D Howard, William B McKinnon, James T Keane, Kelsi N Singer, Tanguy Bertrand, Stuart J Robbins, Paul M Schenk, Bernard Schmitt, et al.

<figure><img src="image_2.png" /></figure>

To cite this version:

Oliver L White, Jeffrey M Moore, Alan D Howard, William B McKinnon, James T Keane, et al. Washboard and fluted terrains on Pluto as evidence for ancient glaciation. Nature Astronomy, 2019, 3 (1), pp.62-68. 10.1038/s41550-018-0592-z. hal-02415153

HAL Id: hal-02415153

https://hal.science/hal-02415153v1

Submitted on 2 Oct 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_3.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Washboard and fluted terrains on Pluto as evidence for ancient glaciation

Oliver L. White<fnref n="1" />,²*, Jeffrey M. Moore², Alan D. Howard³, William B. McKinnon⁴, James T. Keane⁵,
Kelsi N. Singer ID⁶, Tanguy Bertrand², Stuart J. Robbins⁶, Paul M. Schenk⁷, Bernard Schmitt ID⁸,
Bonnie J. Buratti⁹, S. Alan Stern⁶, Kimberly Ennico ID², Cathy B. Olkin⁶, Harold A. Weaver¹⁰,
Leslie A. Young⁶ and New Horizons Geology, Geophysics and Imaging Theme Team¹¹

Distinctive landscapes termed ‘washboard’ and ‘fluted’ terrains<fnref n="1" />,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.

The ridges of ‘washboard’ and ‘fluted’ terrain<fnref n="1" />,² (WFT) emboss portions of the uplands adjacent to Sputnik Planitia. Washboard terrain (Fig. 1a) consists of sets of mostly east-northeast-west-southwest-trending ridges spaced ~1–2 km crest to crest. This terrain occurs in level topographical settings within uplands and valley floors to the northwest and west of Sputnik Planitia, as well as exterior to and within Burney crater. Fluted terrain (Fig. 1b) is seen on high-relief (>2 km) spurs and massifs that separate basins and valleys to the northwest of Sputnik Planitia, the slopes of which feature ridges spaced at ~2–3 km intervals². These ridges tend to orient downslope at up to 20° (ref. ³). A notable type of fluted terrain is the ‘fluted craters’ (Fig. 1c), the walls of which display a pattern of radially aligned, downslope-orientated ridges and troughs. The albedo of WFT surfaces matches that of nearby non-ridged terrain and is brighter in massif and upland plateau settings than valley floor settings (Supplementary Fig. 1b).

Figure 2 presents our mapping of the boundaries of WFT overlain on a global mosaic and stereo digital elevation model (DEM)³.

Supplementary Fig. 2a shows our mapping of individual WFT ridges overlain on the base map, a mosaic of Long-Range Reconnaissance Imager (LORRI) images drawn from four separate observations (Methods). The vertical precision achieved by stereo topography in these regions is ~100 m, and because the WFT ridges are not resolved in the topography, they must have relief of <100 m³. The elevation range of WFT ridges extends from -2.9 km (roughly level with Sputnik Planitia along its northwestern boundary) to +1.9 km (on the upland plateau southeast of Burney), with a mean elevation of -0.5 ± 0.9 km. They are never seen in topographical settings that are both high relief and high elevation. WFT mostly concentrates within a system of deep basins and sinuous valleys extending radially outwards from the northwest margin of Sputnik Planitia (Fig. 2b). This section of Sputnik Planitia’s perimeter is where elevations are lowest and slopes leading into the uplands are gentlest (on a scale of hundreds of kilometres). Topographical profiles taken along the complete >250 km lengths of these valleys yield mean slopes of less than 0.4°—shallower than the ~1° mean slopes from the edge of Sputnik Planitia to the uplands around the rest of its perimeter across a similar lateral scale. These basins and valleys form part of a 3,200-km-long, north-south-orientated tectonic system seen in global topography between the 145°E and 165°E meridians³ (Fig. 2b inset), which intersects the perimeter of Sputnik Planitia.

There are several instances where fluted ridges on massif flanks extend onto valley floors and thus transition to washboard ridges (for example, the white arrow in Fig. 1a), implying that both terrains are different manifestations of the same geological process. Whereas the ridges of fluted terrain tend to orient downslope, like washboard ridges they typically maintain an east-northeast-west-southwest orientation, although this trend is not as constrained as it is for washboard ridges (Supplementary Fig. 3). The mean azimuths of WFT ridges are 70.9 ± 17.0° and 76.5 ± 20.5°, respectively. The corresponding mean slopes (as determined using the DEM) are 2.3 ± 2.1° and 4.5 ± 3.2°, respectively (the mean length of all ridges is 3.8 ± 2.3 km, and the pixel scale of the DEM is 300 m pixel⁻<fnref n="1" />). We interpret the higher standard deviation of azimuths of fluted ridges to be a consequence of existing within steeper topographical settings relative to the washboard ridges.

<footnote n="1">¹The SETI Institute, Mountain View, CA, USA. ²Space Science Division, NASA Ames Research Center, Moffett Field, CA, USA. ³Department of Environmental Sciences, University of Virginia, Charlottesville, VA, USA. ⁴Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St Louis, Saint Louis, MI, USA. ⁵California Institute of Technology, Pasadena, CA, USA. ⁶Southwest Research Institute, Boulder, CO, USA. ⁷Lunar and Planetary Institute, Houston, TX, USA. ⁸Université Grenoble Alpes, CNRS, IPAG, Grenoble, France. ⁹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA. ¹⁰Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA. ¹¹A full list of members and their affiliations appears at the end of the paper. *e-mail: owhite@seti.org</footnote>
<figure><img src="image_4.png" /><figcaption>**Fig. 1** | WFT to the northwest of Sputnik Planitia. **a**, Oblique image (76 m pixel⁻¹; emission angle -55°) centred at 43.7° N and 147.8° E. Illumination is from the top at an angle of 13°. Washboard ridges occupying level terrain on high albedo plateaus and in low albedo valleys can be seen. The white arrow indicates a ridge that extends from a valley floor up the slope of a neighbouring massif. **b**, Equirectangularly projected image (386 m pixel⁻¹) centred at 50.7° N and 155.2° E. Illumination is from the top-left at an angle of 12°. White arrows indicate the ridge and trough morphologies of fluted massifs and spurs, which are separated by sinuous, lower-albedo valleys that display occurrences of washboard texture on their floors (black arrow). **c**, Equirectangularly projected image (386 m pixel⁻¹) centred at 47.7° N and 151.5° E. Illumination is from the top-left at an angle of 11°. White arrows indicate the fluted walls of impact craters.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 2** | Maps of discernible WFT coverage in uplands northwest of Sputnik Planitia. **a**, Map of WFT coverage overlain on a visible mosaic. **b**, Map of WFT boundaries overlain on a stereo DEM. Insets indicate the occurrence in the context of the entirety of Sputnik Planitia. The white dotted lines in the inset of **b** indicate the approximate boundaries of the north-south-orientated trough-ridge system that has been identified in the global topography of Pluto³.</figcaption></figure>

The common occurrence of the WFT at low elevations bordering Sputnik Planitia (in both level and steep settings), and their less frequent occurrence at higher elevations further inland (and then only in level settings), supports the hypothesis that they derived from a former high stand of glacial N₂ ice that filled the low, steep-sided valleys and basins adjacent to Sputnik Planitia, and also extended to more elevated, gently sloping plateau areas. The coincidence of WFT with the large tectonic system can explain why the ridges are restricted to the uplands northwest of Sputnik Planitia and not seen in other areas bounding Sputnik Planitia that might have formerly hosted glaciation (for example, along its northeastern and southwestern edges³). The tectonism would have fragmented the H₂O ice crust, allowing portions of it to be dislodged and ascend to the surface of the denser N₂ ice, where they can be manipulated by glacial flow. The mountain range of Al-Idrisi Montes, located proximal to the WFT along the western edge of Sputnik Planitia, is thought to have formed in a similar manner². We hypothesize that the WFT ridges are a deposit consisting of these buoyant debris left behind after retreat of the formerly extensive glaciation by sublimation. They are not a texture stemming from glacial erosion.

The ridges do not have a blocky appearance, so the sizes of individual debris fragments must be below the resolution of the best New Horizons imaging (a few hundred metres). The areal extent of WFT does not indicate the only regions where former glaciation reached, but rather those where such glaciation was characterized by the entrainment of a dense concentration of debris liberated from the fragmented H₂O ice crust. Compositional mapping⁴ does not indicate that the WFT region correlates to a particularly elevated H₂O ice abundance, but since the maps are sensitive only to the topmost centimetres of the surface at most, we consider them to be more a representation of seasonal and longer-term climatic cycling and topographical effects, rather than the composition of the underlying geology (Methods and Supplementary Fig. 1d).

The age of the WFT is an important component of the glacial hypothesis. We derive a crater size-frequency distribution (SFD) for the WFT by mapping impact craters ≥3 km in diameter that superpose the ridged texture (Methods and Supplementary Figs. 4 and 5). Based on cratering rates estimated for Pluto⁵, this distribution corresponds to an age of ~4 Gyr. WFT occupies the rim of the basin containing Sputnik Planitia. This basin is the oldest geological feature
<figure><img src="image_6.png" /><figcaption>**Fig. 3** | Comparison of the textures of elongate sublimation pits in southern Sputnik Planitia and washboard terrain. **a**, Equirectangularly projected image (117 m pixel⁻¹), centred at 5.2° S and 182.6° E, showing elongate pits in southern Sputnik Planitia. Illumination is from the top-left at an angle of 75°. **b**, Enhanced-contrast, polar-projected image (76 m pixel⁻¹), centred at 46.7° N and 144.9° E, showing the washboard texture east of Burney, re-projected such that the ridges are aligned vertically. Illumination is from the left at an angle of 10°.</figcaption></figure>

identified on Pluto¹, and modelling of volatile behaviour in response to topography has shown that infilling of the basin with all available surface N₂ ice would be complete by tens of millions of years after its formation⁶. During this period, substantial N₂ ice glaciation would have existed within high-latitude, low-elevation uplands adjacent to Sputnik Planitia, such as the WFT region⁶, but would be unstable both due to N₂ preferentially accumulating on the floor of the newly formed Sputnik Planitia basin⁷, as well as Pluto's very high obliquity causing higher latitudes to experience higher annual mean surface temperatures than lower latitudes⁸. Glacial recession from the WFT region therefore took place as the N₂ ice migrated to the very deep, lower-latitude Sputnik Planitia basin, leaving behind the WFT ridges in their wake.

The existence of the deposits as aligned ridges defies simple explanation. Their very consistent east-northeast-west-southwest azimuth, regardless of latitude or location relative to Sputnik Planitia, suggests that they are not former margins of the N₂ ice glaciation, which would be expected to conform more to local topography and Sputnik Planitia's boundaries. Instead, this orientation is consistent with a global-scale process, yet it is unclear what process would have imparted an original east-northeast-west-southwest orientation, and modelling of true polar wander (TPW) of Pluto in response to infilling of Sputnik Planitia with N₂ ice⁹ indicates that the ridges were never aligned parallel to lines of latitude or longitude before reorientation (Methods and Supplementary Fig. 6). However, an origin tied to sublimation of N₂ ice is probable, based on morphological comparisons with sublimation pits in Sputnik Planitia. Fields of elongated sublimation pits are seen at low latitudes in Sputnik Planitia (Fig. 3a), the texture and scale of which bear a noticeable resemblance to the WFT. The sub-parallel, elongate pits in Sputnik Planitia are ~1 km wide and separated by a few km, comparable to the washboard ridges (Fig. 3b). The origin of the alignment of Sputnik Planitia's sublimation pits is uncertain, but may be related to structural anisotropies in the ice (that is, stress fields associated with flow paths)¹⁰,¹¹. We hypothesize that sublimation of N₂ ice that formerly covered the WFT region (Fig. 4a) produced a pitted texture comparable to what is seen in southern Sputnik Planitia today (despite large excursions in its atmospheric pressure, the surface environment of Pluto has overwhelmingly favoured sublimation, rather than melting, of N₂ ice throughout its history⁶,¹²). During sublimation, buoyant debris covering the ice would move downslope and concentrate within these elongate pits (Fig. 4b). Pit enlargement via sublimation must compete with infilling via viscous

<figure><img src="image_7.png" /><figcaption>**Fig. 4** | Schematic of the hypothesized mechanism for WFT ridge formation. **a**, Tectonic fracturing of Pluto's H₂O ice crust creates an abundance of loose pulverized fragments that ascend to the surface of denser, low-viscosity glacial N₂ ice covering the landscape. **b**, Sublimation of N₂ ice produces elongate pits into which the refractory crustal fragments collect via movement downslope. **c**, After the eventual recession of the glacial N₂ ice via sublimation, the fragments are left as residual ridges that record the elongate pitted texture of the sublimating N₂ ice. Ridges are absent where the underlying crust was not originally fractured.</figcaption></figure>

flow¹¹,¹³, causing the surface of the ice to be dynamic as pits open, relax away and open again. The timescale of this process is uncertain due to the rheology of N₂ ice being poorly constrained¹¹ (an upper limit of 2.8 million years—the length of a Plutonian obliquity cycle⁸—can be set, as surface N₂ ice will be subject to climatic changes over the course of the cycle), but the concentration of debris on the floors of the elongate pits will persist throughout. Once all the N₂ ice has withdrawn, parallel ridges of debris would be left as a surviving record of the sublimation texture (Fig. 4c). Deposition of the ridge material on level terrain allowed it to preserve its original orientation, producing washboard ridges, while deposition on steep slopes caused the orientation of the ridge to diverge from this common orientation, producing fluted ridges. Depending on its integrity, some shearing of the material may occur as it is deposited on steeper slopes, which may explain the wider spacing of fluted ridges (~2–3 km) relative to washboard ridges (~1–2 km). Transition of a single ridge from one type to the other would occur with changing slope conditions. Isolated instances where the orientation of the ridges diverges greatly from east-northeast-west-southwest may be a consequence of localized environmental factors affecting pit orientation (Supplementary Fig. 7).

We acknowledge that the precise mechanism that elongated the sublimation pits and defined their strikingly consistent orientation is elusive. Future laboratory studies and landform evolution modelling of the behaviour of nitrogen ice under Pluto conditions should advance our understanding of which factors are important in controlling the scale and configuration of the pitted sublimation textures observed in glacial nitrogen ice on Pluto. While there are other regions bordering Sputnik Planitia that probably hosted widespread glaciation in Pluto's distant past², the existence of WFT as a distinctive marker left behind after glacial recession allows the extent of the former glaciation to be mapped in this region. Such glaciation would have extended across 40° of latitude and covered at least 70,000 km² of Pluto's uplands to locations as far as >350 km from the current boundary of Sputnik Planitia. Recession would have taken place as a consequence of redistribution of surface N₂ ice from the WFT region to the newly formed Sputnik Planitia basin⁶,⁷ via sublimation and re-condensation as well as glacial flow into the basin, but a general decline of Pluto's N₂ budget in its early history by loss to space¹⁴ may also have contributed.

## Methods

**Factors affecting ridge mapping.** Given the small scale and often subtle texture of ridges of the WFT, our mapping is very sensitive to three qualities of New Horizons imaging: pixel scale, solar incidence angle and emission angle. Supplementary
Fig. 2a presents our mapping of individual ridge crests of the WFT, overlain on the LORRI base map, and is accompanied by three maps presenting the pixel scale (Supplementary Fig. 2b), solar incidence angle (Supplementary Fig. 2c) and emission angle (Supplementary Fig. 2d; the angle between the surface normal and a vector pointing towards the spacecraft) across our mapping area. The most crucial property that determines suitability for mapping is the pixel scale. The frequency of the washboard texture means that it can only be detected in imaging of ~400 m pixel⁻¹ or better. For our mapping, we have therefore used 4 LORRI observations that represent some of the highest-quality imaging obtained by New Horizons, ranging in pixel scale from 76 to 386 m pixel⁻¹ (Supplementary Fig. 2b). This imaging is concentrated near the centre of the encounter hemisphere, so the washboard and fluted texture cannot be detected in the far-eastern, -northern and -western regions of the encounter hemisphere, if it exists. There are a few instances where our mapping of WFT abuts the edge of the mapping area, indicating that the total coverage of these terrains actually extends beyond the area that we have mapped. Yet, based on our observations of how the distribution of WFT correlates to generally low-elevation, low-relief topographical settings bordering Sputnik Planitia, we do not think its coverage extends significantly beyond the area we have mapped. WFT (or some sort of variant) may potentially exist in other formerly glaciated areas of Pluto away from the northwestern rim of Sputnik Planitia that are poorly resolved or in darkness, but the coincidence of such glaciation with tectonics is necessary to generate the requisite crustal fragments that can be manipulated by the flowing ice and deposited on its sublimation.

During the New Horizons flyby, WFT was located within a region of Pluto that was illuminated at very low solar incidence angles, reaching a maximum of 33° (Supplementary Fig. 2c). The lack of shadows and shading attributed to surface relief makes identification of WFT challenging in some areas, especially within Burney crater, where solar incidence angles are <10°. However, the low solar incidence angles are compensated for to an extent by the high emission angles of the high-resolution mosaics that cover Burney crater (Supplementary Fig. 2d), which range between ~40 and ~70° for the central and northern areas of Burney. These high emission angles are instrumental in accentuating the relief of the ridges in these very low-contrast illumination conditions such that the boundaries of washboard terrain in Burney can be outlined, which would otherwise probably be impossible if the emission angles were less than 40°, as they are for most of the high-resolution imaging covering WFT elsewhere. However, while a faint east-northeast-west-southwest-aligned fabric can be perceived in the imaging, the crests of individual ridges cannot generally be confidently mapped, especially when re-projected to an equirectangular projection, which is why our mapping of individual ridges within Burney in Supplementary Fig. 2a covers a significantly lesser area than our mapping of the total washboard coverage indicated by the black outlines in Supplementary Fig. 2b-d. Where WFT is illuminated at higher solar incidence angles, the identification of ridges is easier, and the disparity between mapping of individual ridges and the total extent of WFT is lessened.

Solar incidence is also an important factor when considering potential bias in our mapping of ridge orientations, specifically whether the strong clustering of washboard and fluted ridge azimuths around 72° is an artefact resulting from the prevailing solar azimuth at the time of observation, which can over-represent slope segments orientated orthogonally to the solar azimuth. Given the illumination conditions during flyby, such an over-representation is very unlikely. The low solar incidence angle for WFT means that preferential illumination of their slopes is a very weak effect. In addition, given the proximity of WFT to the sub-solar point, the solar azimuth varies dramatically across their expanse. In their southern reaches, the solar azimuth is orientated north-northwest-south-southeast, whereas in their northeastern reaches, the solar azimuth is orientated east-northeast-west-southwest, and in Burney crater the Sun is nearly directly overhead, but the east-northeast-west-southwest trend of the ridges prevails under all of these illumination conditions, indicating that solar azimuth is not a significant factor influencing the azimuths we have measured.

A final factor that potentially influences our mapping in the southern reaches of the WFT is the transition to the dark equatorial band informally named Cthulhu Regio (Supplementary Fig. 2a). Cthulhu Regio is defined by dark mantling material, superposed on various geological terrains, that is interpreted to form due to atmospheric tholin deposition¹,¹⁵,¹⁶. In this transitional zone, the mantle is discontinuous, with some ridge fields clearly displaying dark material filling the troughs between the ridges, while the ridge crests remain uncovered. The presence of the mantle in low-lying areas here raises the possibility that some washboard ridges, which we consider to display relief not exceeding ~100 m based on their non-detection in even the highest-resolution stereo DEMs, may be completely buried underneath the mantle where it is thick enough and therefore cannot be detected by our mapping. Our mapping of washboard terrain within valleys marginal to Sputnik Planitia south of ~35°N may therefore represent a minimum due to this effect.

**Surface albedo, colour and composition.** Ridges of the WFT do not appear to possess an intrinsic albedo or colour signature, instead adopting the albedo and colour characteristics of the terrain in which they occur. The surface albedo and colour within this area of Pluto are strongly dependent on surface elevation, as indicated in Supplementary Fig. 1a-c. Supplementary Fig. 1a reproduces the stereo

DEM in Fig. 2b, while Supplementary Fig. 1b,c shows, respectively, the normal reflectance¹⁷ and raw spectral indicator of dark red material across the same area¹. Low-elevation, low-relief areas (in particular, the wide, interconnected valleys and basins extending north from the edge of Sputnik Planitia, as well as the interior of Burney crater) exhibit moderate normal reflectance and reddish colouration (relative to the very dark, red terrain of Cthulhu Regio). Accordingly, WFT ridges, which tend to concentrate at low elevations, naturally occur within these moderate-albedo, reddish settings, avoiding the higher-elevation, higher-relief, higher-albedo and paler massifs in the northern portion of the map as well as the rim of Burney crater. Exceptions include some fluted ridges that are seen on bright massif slopes adjacent to Sputnik Planitia, and the dark tholin mantle in the south that is blanketing washboard ridges.

Supplementary Fig. 1d-f shows maps of the surface abundance of H₂O, CH₄ and N₂ ice in this area¹. There is a positive correlation between CH₄ band depth and normal reflectance, indicating that the bright, pale surfaces of high-relief, high-elevation massifs in the north and the rim of Burney crater are preferred precipitation locations of CH₄ ice. In contrast, the darker, washboard-hosting valleys and basins that separate these massifs, as well as the floor of Burney crater, tend to display a moderately elevated N₂ band depth (but not as high as the bright, blocky, low-elevation portion of the adjacent Al-Idrisi Montes). This elevation-dependent compositional dichotomy is explained by the precipitation behaviour of CH₄ and N₂ on Pluto. Both will precipitate at low elevations, but since the abundance of N₂ in the atmosphere is much higher than that of CH₄, the N₂ ice will dominate at these low elevations, while at high elevations the atmosphere is too warm for N₂ to precipitate, so only CH₄ can do so¹⁷,¹⁸. The H₂O spectral indicator map indicates minimal abundance of surface H₂O ice across most of the area covered by WFT. Higher abundances are seen to the south, where they correlate to the dark mantling material.

Based on the variations in surface albedo, colour and composition, as seen in Supplementary Fig. 1, we interpret the surface composition to be related to volatile distribution controlled by seasonal and longer-term climatic cycling and topographical effects, rather than an indication of the underlying geology. These maps are therefore not especially revealing with respect to providing information on the composition of the ridges themselves. We interpret the ridges to be formed of H₂O and/or CH₄ ice based on evidence for them being composed of material that was manipulated by denser, glacially flowing N₂ ice. The maps do, however, provide some indication of where washboard terrain may be located beyond the boundaries of New Horizons imaging in which it can be detected. For instance, Supplementary Fig. 1a indicates the location of a low-elevation zone to the northeast of Burney crater that displays a moderate normal brightness and concentration of red material and N₂, as well as a low concentration of CH₄. These are characteristics of valleys and basins north of Sputnik Planitia, as well as the floor of Burney crater, where washboard terrain has been mapped in imaging of 400 m pixel⁻¹ or better.

**Crater density-based terrain age estimates.** WFT appears to be superposed on very ancient landscapes: both washboard and fluted ridges can be seen on the floors and walls of large (diameters (*D*) of several tens of kilometres), eroded impact craters (Supplementary Fig. 4a). Yet, smaller (generally, *D* < 10 km), sharply defined craters do not display such patterning (Supplementary Fig. 4b), meaning that the process that gave rise to the WFT was not recent.

We have identified craters that superpose the washboard and fluted texture in order to derive a crater SFD for the terrain and constrain its age. Our base map for this purpose is 320 m pixel⁻¹ New Horizons Multispectral Visible Imaging Camera imaging that covers all of the mapped WFT, which we have used to identify and measure craters with *D* > 3 km (that is, around ten times the pixel scale of the imaging, and the minimum reliable diameter for identifying and accurately measuring craters¹⁹). We have included all craters that superpose the ridges, including those with areas that are not surrounded entirely by WFT. In total, there are 61 superposed craters with *D* = 3–16 km across an area of 71,684 km² (Supplementary Fig. 5a). The crater SFD for the terrain is shown in the relative differential, or R-plot²⁰, form in Supplementary Fig. 5b. This plots differential crater counts (dN) relative to (divided by) a –3 power-law distribution (dN = *D*-b/D⁻³, where *k* and *b* are the relevant power law parameters), and it has the benefit of highlighting SFD slope differences between different diameter ranges²⁰. Supplementary Fig. 5b also includes surface crater density and age predictions for the 'knee' or broken power-law Kuiper belt object distribution⁵, the slope of which matches the Pluto and Charon crater data for *D* > ~10–15 km²¹. For craters on Pluto and Charon with *D* < ~10–15 km, the SFD slope is seen to break again to an even shallower slope²¹ and that new break (termed the 'elbow' break) is superimposed over the knee distribution in Supplementary Fig. 5b as an approximate adjusted prediction. Because most of the craters superimposed on washboard terrain are smaller than the 'elbow' break in slope, this new slope break is included to guide the eye in determining the surface age estimate. The SFD for craters superimposed on Burney crater, the largest impact feature on the encounter hemisphere after the Sputnik basin, and which is partly colonized by washboard terrain, is also shown for comparison²¹. Neither the WFT, nor Burney, are crater saturated. The R-plot for WFT indicates an age of ~4 Gyr. Burney appears to marginally predate the WFT, which could indicate that the washboard ridges were emplaced within Burney soon after the impact that formed it.
Do washboard ridges record a palaeopole? TPW is the reorientation of a planet's surface with respect to its angular momentum vector. TPW occurs when geological processes redistribute mass within or on the planet, perturbing the planet's inertia tensor and changing the orientation of the principal moments of inertia. The resulting effect is the displacement of the polar axis with respect to the surface geography. TPW of Pluto has been previously inferred on the basis of two key observations. The first is the geographical location of Sputnik Planitia—the single largest geological structure on Pluto—very near the anti-Charon point (180° E and 0°N). This alignment is unlikely to be based on chance and hints at a dynamical cause. TPW would displace mass excesses towards the equator and tidal axis, and thus provides a natural explanation for its location²². The second observation is the coherent tectonic pattern observed across Pluto. When a planet undergoes TPW, each geographical location experiences a change in the tidal-rotational potential, which builds stress in the lithosphere. This stress results in a global network of faults with a characteristic pattern²³. A coherent network of extensional faults criss-crosses Pluto's surface¹, the pattern of which closely replicates the tectonic pattern predicted from TPW due to volatile loading within Sputnik Planitia⁴.

The unusual orientation of the ridges of the washboard terrain might represent a third independent line of evidence for TPW of Pluto⁴. Sublimation often generates linear features that align N–S, including terrestrial penitentes²⁴⁻²⁶, Pluto’s own bladed terrain of the informally named Tartarus Dorsa¹⁸,²⁷ and possibly the sublimation pits in southern Sputnik Planitia¹⁶,¹¹. While the WFT ridges share similarities with these other sublimation terrains on Pluto, they are not aligned N–S, and instead trend east-northeast-west-southwest (Supplementary Fig. 6a). It is difficult to envisage how a consistent azimuth of this sort could be a primary feature, and it is possible that this misalignment is the result of TPW, whereby the WFT ridges used to be aligned with the spin pole. Thus, the ridges may point towards a Plutonian palaeopole⁴.

To evaluate this hypothesis, we performed a geometric analysis of the spatial distribution and orientation of the WFT ridges. We considered two scenarios: one in which the WFT ridges were originally aligned N–S (in which case Pluto’s palaeopole would be located at the intersection of the great circles aligned parallel to each of the WFT ridges); and another in which the ridges were originally aligned E–W (in which case, the palaeopole would be located at the intersection of the great circles aligned orthogonal to each of the WFT ridges). The great circles effectively represent palaeo-meridians in each case. We performed this analysis for washboard ridges only, and not fluted ridges, as we hypothesized that the orientations of fluted ridges have been significantly modified during the deposition process by underlying topography. Supplementary Fig. 6b,c shows the palaeo-meridians for the N–S and E–W scenarios, respectively. In both cases, the palaeo-meridians form a band several tens of degrees across that does not converge to a single point. This lack of convergence would only be consistent with the ridges pointing towards a palaeopole if they were located at or proximal to the palaeo-equator, where the palaeo-meridians would be parallel to each other. Additionally, the lack of convergence might also be attributed to azimuthal variation introduced by geological complexities associated with the formation, elongation and infilling of the sublimation pits, and the deposition of the ridges once the N₂ ice has sublimated away. For comparison, the bladed terrain of Tartarus Dorsa, which is believed to represent sublimation features that are partially analogous to terrestrial penitentes¹⁸,²⁷, are aligned predominantly north-south but have significant spread in the orientation of the blades (s.d.: ~15°). We performed a latitude-longitude grid search across Pluto to identify possible palaeopoles. For each candidate palaeopole, we determined what fraction of the washboard ridges (weighted by the length of each washboard ridge) pointed to the palaeopole to within 15°. Contours of the palaeopole quality-of-fit are shown in Supplementary Fig. 6d,e. Even allowing for such a large misfit, no single point on the surface of Pluto is consistent with the orientation of all of the washboard ridges. At most, some candidate palaeopoles can explain ~70% of the washboard azimuths.

As a decisive test of the validity of the TPW hypothesis, we compared the results of this geometric analysis with the TPW solutions of ref. ⁹ (Supplementary Fig. 6f-i). There are two kinds of TPW solutions—'small' solutions (Supplementary Fig. 6f,g) and 'large' solutions (Supplementary Fig. 6h,i)—arising from whether the intermediate and minimum principal axes of inertia swap during reorientation. For both the N–S and E–W washboard scenarios, the washboard palaeo-meridians intersect several of the modelled TPW solutions, but the TPW solutions do not pass through the core of our best-fitting palaeopoles. The 'small' and 'large' TPW solutions that are most consistent with the N–S scenario can only explain ~40% of the washboard ridges (that is, the TPW palaeopole paths only intersect the 40% contour in Supplementary Fig. 6f,h). The 'small' TPW solutions do not yield a better match to the E–W scenario than the N–S washboard scenarios (Supplementary Fig. 6g), although the 'large' TPW solutions do pass somewhat closer to the core of the E–W washboard palaeopoles, with some solutions being capable of matching nearly 70% of all washboard ridges (Supplementary Fig. 6i).

Nearly all TPW solutions, however, would result in the washboard ridges being at similar or higher latitude than today (that is, the TPW palaeopole paths in Supplementary Fig. 6f-i are closer to the washboard ridges than the present-day pole). At high latitude, solar incidence angles are low, thereby inhibiting

preferential sublimation of north- and south-facing pit walls over others, and so reducing the likelihood that solar incidence alone would cause pits in sublimating N₂ ice at high latitude to be elongate. While we consider the orientations of WFT ridges to record the orientation of elongate sublimation pits that harboured the WFT material, the lack of converging palaeo-meridians (Supplementary Fig. 6b,c), well-fitting palaeopoles (Supplementary Fig. 6d,e), difficulty in matching modelled TPW paths (Supplementary Fig. 6f-i) and consistently high latitude of the WFT ridges throughout Pluto's history suggest that these features cannot be considered to represent independent evidence for TPW on Pluto.

## Data availability

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

Received: 8 August 2018; Accepted: 7 September 2018;
Published online: 12 November 2018

## References

1. Moore, J. M. et al. The geology of Pluto and Charon through the eyes of New Horizons. *Science* **351**, 1284–1293 (2016).

2. Howard, A. D. et al. Present and past glaciation on Pluto. *Icarus* **287**, 287–300 (2017).

3. Schenk, P. M. et al. Basins, fractures and volcanoes: global cartography and topography of Pluto from New Horizons. *Icarus* **314**, 400–433 (2018).

4. Schmitt, B. et al. Physical state and distribution of materials at the surface of Pluto from New Horizons LEISA imaging spectrometer. *Icarus* **287**, 229–260 (2017).

5. Greenstreet, S. et al. Impact and cratering rates onto Pluto. *Icarus* **258**, 267–288 (2015).

6. Bertrand, T. et al. The nitrogen cycles on Pluto over seasonal and astronomical timescales. *Icarus* **309**, 277–296 (2018).

7. Bertrand, T. & Forget, F. Observed glacier and volatile distribution on Pluto from atmosphere-topography processes. *Nature* **540**, 86–89 (2016).

8. Earle, A. M. et al. Long-term surface temperature modeling of Pluto. *Icarus* **287**, 37–46 (2017).

9. Keane, J. T. et al. Reorientation and faulting of Pluto due to volatile loading within Sputnik Planitia. *Nature* **540**, 90–93 (2016).

10. White, O. L. et al. Geological mapping of Sputnik Planitia on Pluto. *Icarus* **287**, 261–286 (2017).

11. Moore, J. M. et al. Sublimation as a landform-shaping process on Pluto. *Icarus* **287**, 320–333 (2017).

12. Stern, S. A. et al. Past epochs of significantly higher pressure atmospheres on Pluto. *Icarus* **287**, 47–53 (2017).

13. Wei, Q. et al. Young surface of Pluto's Sputnik Planitia caused by viscous relaxation. *Astrophys. J. Lett.* **856**, L14 (2018).

14. Gladstone, G. R. et al. The atmosphere of Pluto as observed by New Horizons. *Science* **351**, aa8866 (2016).

15. Grundy, W. M. et al. Surface compositions across Pluto and Charon. *Science* **351**, aa9189 (2016).

16. Grundy, W. M. et al. Pluto's haze as a surface material. *Icarus* **314**, 232–245 (2018).

17. Buratti, B. J. et al. Global albedos of Pluto and Charon from LORRI New Horizons observations. *Icarus* **287**, 207–217 (2017).

18. Moore, J. M. et al. Bladed terrain on Pluto: possible origins and evolution. *Icarus* **300**, 129–144 (2018).

19. Robbins, S. J. et al. Craters of the Pluto-Charon system. *Icarus* **287**, 187–206 (2017).

20. Crater Analysis Techniques Working Group. Standard techniques for presentation and analysis of crater size-frequency data. *Icarus* **37**, 467–474 (1979).

21. Singer, K. N. et al. Craters on Pluto and Charon—surface ages and impact populations. In *Proc. 47th Lunar and Planetary Science Conference* 2310 (LPI, 2016).

22. Nimmo, F. et al. Reorientation of Sputnik Planitia implies a subsurface ocean on Pluto. *Nature* **540**, 94–96 (2016).

23. Matsuyama, I. & Nimmo, F. Tectonic patterns on reoriented and despun planetary bodies. *Icarus* **195**, 459–473 (2008).

24. Matthes, F. E. Ablation of snow-fields at high altitudes by radiant solar heat. *Trans. Am. Geophys. Union* **15**, 380–385 (1934).

25. Lliboutry, L. The origin of penitents. *J. Glaciol.* **2**, 331–338 (1954).

26. Amstutz, G. C. On the formation of snow penitentes. *J. Glaciol.* **3**, 304–311 (1958).

27. Moores, J. E. et al. Penitentes as the origin of the bladed terrain of Tartarus Dorsa on Pluto. *Nature* **541**, 188–190 (2017).

## Acknowledgements

This work was supported by NASA's New Horizons project.
## Author contributions

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.

## Competing interests

The authors declare no competing interests.

## Additional information

**Supplementary information** is available for this paper at https://doi.org/10.1038/s41550-018-0592-z.

**Reprints and permissions information** is available at www.nature.com/reprints.

**Correspondence and requests for materials** should be addressed to O.L.W.

**Publisher’s note:** Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

© The Author(s), under exclusive licence to Springer Nature Limited 2018
New Horizons Geology, Geophysics and Imaging Theme Team

Jeffrey M. Moore², William B. McKinnon⁴, John R. Spencer⁶, Ross A. Beyer¹,², Marc W. Buie⁶,
Bonnie J. Buratti⁹, Andrew F. Cheng¹⁰, Dale P. Cruikshank², Cristina M. Dalle Ore¹,², G. Randy Gladstone⁶,
William M. Grundy<fnref n="12" />, Alan D. Howard³, Tod R. Lauer¹³, Ivan R. Linscott¹⁴, Francis Nimmo<fnref n="15" />,
Cathy B. Olkin⁶, Joel W. Parker⁶, Simon B. Porter⁶, Harold J. Reitsema⁶, Dennis C. Reuter¹⁶,
James H. Roberts¹⁰, Stuart J. Robbins⁶, Paul M. Schenk⁷, Mark R. Showalter¹, Kelsi N. Singer⁶,
Darrel F. Strobel¹⁷, Michael E. Summers¹⁶, G. Len Tyler¹⁴, Harold A. Weaver¹⁰, Oliver L. White¹,²,
Orkan M. Umurhan¹,², Maria Banks¹⁶, Olivier S. Barnouin¹⁰, Veronica J. Bray¹⁸, Brian Carcich¹⁹,
Andrew L. Chaikin²⁰, Carrie L. Chavez¹,², Cindy A. Conrad⁶, Douglas P. Hamilton<fnref n="21" />, Carly J. A. Howett⁶,
Jason D. Hofgartner⁹, Joshua A. Kammer⁶, Carey M. Lisse¹⁰, Annie Marcotte¹⁰, Alex H. Parker⁶,
Kurt D. Retherford⁶, Magda Saina¹⁰, Kirby Runyon¹⁰, Eric Schindhelm⁶, John Stansberry²²,
Andrew J. Steffl⁶, Ted Stryk²³, Henry B. Throop²⁴, Constantine C. C. Tsang⁶, Anne J. Verbiscer³,
Helene Winters¹⁰, Amanda M. Zangari⁶, Richard P. Binzel²⁵, Alissa M. Earle²⁵, S. Alan Stern⁶,
Harold A. Weaver¹⁰, Leslie A. Young⁶ and Kimberly Ennico²

<footnote n="12">¹²Lowell Observatory, Flagstaff, AZ, USA. ¹³National Optical Astronomy Observatory, Tucson, AZ, USA. ¹⁴Stanford University, Stanford, CA, USA.</footnote>

<footnote n="15">¹⁵University of California Santa Cruz, Santa Cruz, CA, USA. ¹⁶NASA Goddard Space Flight Center, Greenbelt, MD, USA. ¹⁷Johns Hopkins University, Baltimore, MD, USA. ¹⁸University of Arizona, Tucson, AZ, USA. ¹⁹Cornell University, Ithaca, NY, USA. ²⁰Independent science writer, Arlington, VT, USA.</footnote>

<footnote n="21">²¹University of Maryland, College Park, MD, USA. ²²Space Telescope Science Institute, Baltimore, MD, USA. ²³Roane State Community College, Harriman, TN, USA. ²⁴Planetary Science Institute, Tucson, AZ, USA. ²⁵Massachusetts Institute of Technology, Cambridge, MA, USA.</footnote>