Large-scale cryovolcanic resurfacing on Pluto

The New Horizons spacecraft returned images and compositional data showing that terrains on Pluto span a variety of ages, ranging from relatively ancient, heavily cratered areas to very young surfaces with few-to-no impact craters. One of the regions with very few impact craters is dominated by enormous rises with hummocky flanks. Similar features do not exist anywhere else in the imaged solar system. Here we analyze the geomorphology and composition of the features and conclude this region was resurfaced by cryovolcanic processes, of a type and scale so far unique to Pluto. Creation of this terrain requires multiple eruption sites and a large volume of material (>10⁴ km³) to form what we propose are multiple, several-km-high domes, some of which merge to form more complex planforms. The existence of these massive features suggests Pluto’s interior structure and evolution allows for either enhanced retention of heat or more heat overall than was anticipated before New Horizons, which permitted mobilization of water-ice-rich materials late in Pluto’s history.

Pluto’s surface has experienced considerable and ongoing resurfacing through both endogenic and exogenic processes^1,2,3. Pluto is the largest body in the Kuiper belt with a radius (R) of 1188.3 ± 1.6 km⁴ and bulk density constraints for a differentiated Pluto indicate the outer ~300 km of Pluto are water-ice-rich overlying a rocky core⁵, with a poorly constrained carbonaceous component⁶. Based on this rock abundance, Pluto is expected to have maintained relatively low levels of radiogenic heating (≲5 mW m⁻²) throughout much of its history^7,8. Pluto’s largest moon Charon (R = 606.0 ±1.0 km) likely formed through a large, grazing impact with Pluto^9,10. Models predict the tidal evolution of Pluto and Charon progressed rapidly after the impact, and any tidal heating should have ended very early in their history (<100 Myrs after the impact)¹¹. Despite these constraints, modelling suggests a subsurface water-rich ocean could potentially persist into the present on Pluto^{8,12,13,14,15}. Any ocean is generally predicted to exist 100–200 km or more below the surface of Pluto, at the base of the icy shell¹⁶.

Typical surface temperatures on Pluto are ~35–60 K^17,18,19,20, with cooler temperatures for the brighter, volatile-rich surfaces. Pluto’s atmospheric surface pressure in 2015 was ~10 μbar^21,22, and no liquid can exist on the surface of Pluto for long owing to this pressure being far below the triple point of the observed ice species (N₂, CO, CH₄, NH₃, CH₃OH, and H₂O)^23,24. At these low temperatures pure water ice should generally form an immobile bedrock, as it is also far from its melting temperature of ~273 K. The addition of ammonia or other anti-freeze components (e.g., salts) to the water ice can lower the melting temperature somewhat. The freezing temperature can be depressed by up to ~100 K for high concentrations of ammonia at low pressure e.g.,²⁵. Additional antifreeze components could potentially lower the melting temperatures even further²⁶, but the surface temperatures on Pluto are so cold and the atmospheric pressure so low that freezing of a fluid on the surface would still occur on relatively short geologic timescales²³. On Pluto’s surface, nitrogen ice (N₂) is much closer to its melting temperature (63 K) than water ice, and can flow or viscously relax over relatively short timescales^27,28. Volatile ices (N₂, CO, CH₄) also play a role in resurfacing areas of Pluto through sublimation, physical erosion, and/or deposition/mantling^{24,29,30,31,32}.

Here we show that the potential icy volcanic (or cryovolcanic) constructs and their surrounding terrain discussed here (Fig. 1) have many morphological traits that are distinct from any other area on Pluto. These geologic features do not appear to be formed predominantly by erosion nor do they appear to be constructed primarily of volatile ices. Here we refer to cryovolcanism as the collection of processes that cause mobile subsurface material to extrude onto the surface and either partially or fully resurface the existing terrain. We propose a large volume of material has erupted from multiple sources (and likely in more than one episode over time) to form the many large domes and rises found in this region.

a Wright Mons region with features labelled (see text), b, high-resolution topography for Wright Mons³⁶, c, zoom of region with smaller dome named Coleman Mons (label “D”; also see Fig. 4), undulating, hummocky terrain on the flanks of Wright Mons and the superposed smaller-scale (1–2 km) ridges or boulders, d, topographic profile of Wright Mons and adjacent rise as shown by the line A to A’ in panel a. All images are from the new Horizons observation PEMV_P_MVIC_LORRI_CA (315 m px⁻¹; see Supplementary Table 1) on a simple cylindrical projection. The large arrow in the upper left of panel a indicates the direction of incoming sunlight and is repeated in subsequent figures. All figures in the main text and supplement are shown with north up. The longitude and latitude extents of the image are as follows: panel a ~163–182°E and ~16–28°S; panel b ~166–177°E and ~17–24°S; panel c ~167–171°E and ~22–25°S.

Full size image

Morphological characteristics

The region of putative cryovolcanic terrains discussed here lies to the southwest of the Sputnik Planitia ice sheet (Supplementary Fig. 1), which fills an ~1000-km-diameter ancient impact basin^{2,33,34,35,36}. The most prominent and largest-scale structures in the cryovolcanic region are large rises or mounds of material separated by broad depressions (Figs. 1–3 and Supplementary Fig. 1). The configuration of the large rises gives the impression of annular features with deep central depressions in two cases. These features are named Wright and Piccard Montes. However, further inspection suggests these features may or may not be annular, and instead may simply have arisen from the merging of several adjacent rises (discussed below). The main topographic rise of Wright Mons (Fig. 1) stands ~4–5 km high (relative to the lower areas of surrounding terrain) and spans ~150 km, and Piccard Mons (Supplementary Fig. 2) is ~7 km high at its tallest points and ~225 km wide. The inferred volume of the main topographic rise of Wright Mons alone is ~2.4 x 10⁴ km³ (similar to the volume of Mauna Loa, see Supplementary Note 8).