Large-scale cryovolcanic resurfacing on Pluto

The study investigates unusual terrains on Pluto revealed by New Horizons, characterized by very few impact craters and dominated by enormous rises with hummocky flanks near Sputnik Planitia. Pluto, with radius 1188.3 ± 1.6 km and likely differentiated with a ~300 km thick water-ice-rich outer shell over a rocky core, is expected to have had modest radiogenic heating (~5 mW m−2) and lost tidal heating early after the Pluto-Charon impact. Despite low surface temperatures (~35–60 K) and a tenuous atmosphere (~10 µbar) that preclude long-lived surface liquids, models suggest a subsurface ocean may persist at depths >100–200 km. Volatile ices (N2, CO, CH4) can flow or be redistributed by seasonal and astronomical cycles, but water ice should be immobile at the surface. The research question is whether the distinctive large rises and associated hummocky terrains southwest of Sputnik Planitia are constructional cryovolcanic features, and if so, what materials and emplacement mechanisms are responsible and what this implies about Pluto’s internal heat and evolution.

Prior work established Pluto’s geological diversity and ongoing resurfacing (e.g., Moore et al. 2016; Stern et al. 2018), its differentiated interior and potential long-lived subsurface ocean (Robuchon & Nimmo 2011; Kamata et al. 2019; Bierson et al. 2020), and that volatile ices undergo complex cycles shaping the surface (Grundy et al. 2016; Bertrand et al. 2016–2019). Sputnik Planitia’s nitrogen-ice convection demonstrates volatile-driven activity (McKinnon et al. 2016). Ammonia-bearing materials have been detected elsewhere on Pluto, potentially indicating cryofluid eruptions along fractures (Dalle Ore et al. 2019). Comparisons to volcanic morphologies on Earth and Mars, as well as cryovolcanic domes on Europa and features on Ceres, provide context but do not match the morphology observed at Wright and Piccard Montes. Explosive cryovolcanism prerequisites appear unmet, and classic shield caldera analogs are inconsistent with the deep, broad central depressions seen on Pluto’s features. These studies frame the novelty of Pluto’s large-scale, possibly water-ice-rich cryovolcanic constructs.

Data and mapping: The team used New Horizons imaging and spectral datasets. High-resolution panchromatic images (best 234–315 m px−1) and MVIC color data characterized morphology and color. LEISA infrared spectra mapped CH4, N2, and H2O ice distributions using band-depth and index maps. Topography was derived by stereogrammetry from multiple stereo image pairs (e.g., PELR_P_LEISA_HIRES at 240 m px−1 and PEMV_P_MVIC_LORRI_CA at 315 m px−1), integrated into a global DEM with effective horizontal resolutions of 945–1440 m px−1 and vertical precisions ~90–230 m. Shadow measurements cross-validated heights. Geomorphologic analysis: The authors mapped large rises (Wright Mons, Piccard Mons, medial montes) and measured dimensions, slopes, and central depressions. They quantified the hummocky texture (typical wavelengths 6–12 km; heights a few hundred meters to ~1 km) and assessed smaller superposed ridges/blocks (1–2 km scale). Crater counts provided an upper bound on surface age (~1–2 Ga), noting uncertainties from small-number statistics and impactor flux. Compositional analysis: LEISA maps were examined to separate CH4-dominated veneers (more prevalent at higher elevations) from N2-rich patches in local lows and to identify exposures of H2O ice mixed with a dark red organic (tholins) on darker, warmer slopes. Shadowed regions were excluded in compositional mapping due to low SNR. Mechanical and emplacement modeling: For the smaller dome Coleman Mons (~25 km diameter, ~1.5 km high), the authors applied a lava dome growth model (Bridges & Fink) to estimate basal yield strength from the observed aspect ratio (A ≈ 0.06). Using ρ ≈ 920 kg m−3 and g = 0.62 m s−2 for Pluto, they estimated σ_b (yield strength) ~6.4 × 10^4 Pa (range 2.4 × 10^4 to 2.1 × 10^5 Pa), consistent with ductile strengths for mobile water or ammonia-water ices. They evaluated alternative formation mechanisms for the hummocky texture: (1) numerous small domes, (2) pillow-like extrusion of rapidly cooled lavas, and (3) contractional folding of a thin frozen skin over viscous material (funiscular/pahoehoe analogy). A folding analysis using H = (L/2π) ln(R) with L ≈ 10 km and plausible interior temperatures (150–273 K) yielded required upper-layer thicknesses H ≈ 8–13 km, judged unrealistic for the scale, and no clear compressional source was identified. Fracture mapping found few obvious deep conduits (notably Ride Rupes scarp and one other scarp), possibly masked by later emplacement. Stratigraphic/temporal indicators: Color/albedo variations and textural differences north and west of Wright Mons, superposed fractures on adjacent plains, and the distinct Coleman Mons dome were interpreted as possible evidence for multiple emplacement episodes.

- The region southwest of Sputnik Planitia hosts enormous rises with undulatory/hummocky flanks, unlike terrains elsewhere on Pluto or other bodies. Key edifices include Wright Mons (~4–5 km high, ~150 km across; volume ~2.4 × 10^4 km^3) and Piccard Mons (~7 km high, ~225 km across). Multiple adjacent rises in the medial montes region appear continuous with these edifices. - Central depressions are very large (Wright’s ~40–50 km wide, ~4 km deep; Piccard’s larger and U-shaped), lack collapse terraces, and are inconsistent with typical calderas; their depth/width implies that a simple summit collapse would require removal of >50% of the edifice volume, deemed unlikely. - Flank slopes are gentle (~3–5°, up to ~10°; central depression walls up to ~20° locally). Hummocky wavelengths are typically 6–12 km with relief of a few hundred meters to ~1 km; smaller superposed ridges/blocks are 1–2 km wide. - Compositionally, much of the area is mantled by thin CH4 frost (enhanced at higher elevations); N2-rich ice occurs as small smooth patches in lows and as a thin component at lower elevations. Dark, redder patches (tholins) and H2O ice signatures are exposed mainly on warmer, north-facing slopes. The morphology and rheology indicate the bulk material of the rises is not volatile N2/CH4 ice (which cannot support large relief) but is predominantly water-ice-rich material. - The features are interpreted as constructional cryovolcanic constructs produced by extrusion from multiple subsurface sources and likely multiple episodes. Many large rises may have merged, producing complex planforms and giant central depressions without requiring massive collapse. - The smaller dome Coleman Mons implies basal yield strengths ~6 × 10^4 Pa (range 2.4 × 10^4–2.1 × 10^5 Pa), compatible with mobile water or ammonia-water ice rheologies. - No clear flow fronts, levees, or vent alignments are resolved at New Horizons image scales (best 234–315 m px−1), and no evidence for explosive volcanism is observed. The crater scarcity suggests a relatively young surface (upper limit ~1–2 Ga), with many features potentially younger. - The required erupted volume is large (>10^4 km^3), and the scale and morphology of these cryovolcanic constructs are unique in the imaged solar system.

The findings support large-scale, late-stage cryovolcanic resurfacing on Pluto by water-ice-rich materials, despite Pluto’s low surface temperatures and modest expected internal heat flux. The morphology, composition, and topography argue against erosional origins or construction primarily by volatile N2/CH4 ices. Multiple rises appear to have formed from subsurface sources located beneath each construct, with some merging to generate complex shapes and deep, broad central depressions without invoking extraordinary summit collapse. The absence of diagnostic flow morphologies may reflect image resolution limits or subsequent modification. Thermally, mobilizing water-ice-rich material late in Pluto’s history implies enhanced internal heat or effective heat retention, potentially via interior stratigraphy (e.g., insulating clathrate layers) and/or a long-lived subsurface ocean. These cryovolcanic features complement evidence from volatile-ice activity (e.g., Sputnik Planitia) and localized ammonia-bearing effusions elsewhere, collectively indicating a more geologically and thermally active Pluto than anticipated pre–New Horizons. The unique environmental conditions on Pluto (very low T, low P, low g, abundant volatiles) may lead to cryovolcanic expressions unlike those on other bodies, requiring adaptation of terrestrial cryovolcanic models.

This work identifies and characterizes a vast, unique cryovolcanic province on Pluto, dominated by large, water-ice-rich constructional rises (e.g., Wright and Piccard Montes) with hummocky flanks and deep central depressions. The morphology and compositional mapping indicate that thin CH4/N2 veneers mantle features whose bulk is not volatile ice, and the large erupted volumes (>10^4 km^3) and merging of multiple rises imply multiple subsurface sources and likely multiple emplacement episodes. These results suggest Pluto retained or generated more internal heat than expected, enabling late-stage mobilization of water-ice-rich materials and adding key constraints on Pluto’s interior structure and thermal evolution. Future research directions include: higher-fidelity modeling of cryomagma ascent, extrusion, cooling, and dome growth under Pluto conditions; improved constraints on rheologies (including ammonia/salt brines) and yield strengths; relaxation and timescale analyses; searches for additional examples and higher-resolution topography; and integrated thermal-structural models exploring insulating layers and ocean longevity.

- Imaging resolution (best 234–315 m px−1) and lighting (terminator proximity; haze illumination for Piccard Mons) limit detection of subtle flow features, vents, and fine-scale structures; shadowed regions were excluded from spectral mapping. - The full southward extent of resurfaced terrain is unknown due to limited visibility in haze-light. - Crater-based age estimates are uncertain due to small-number statistics and impactor flux uncertainties; the ~1–2 Ga upper limit may not tightly constrain emplacement timing. - Spectral detection of ammonia is absent here, potentially obscured by overlying methane; compositional inferences for the bulk are indirect. - Few obvious large fractures are visible in the region (aside from major scarps), complicating identification of conduits; possible burial by subsequent extrusion is inferred but unproven. - Mechanical analogs (pillow lavas, pahoehoe/funiscular folding) and terrestrial-derived models may not directly apply under Pluto’s extreme conditions, and some tested mechanisms (folding) require unrealistic parameters (e.g., very thick upper layers).