Pluto is the largest body in the Kuiper Belt, with a diameter of 2377 km and a density which suggests a structure comprising about 300 km of water-rich ice overlaying a rocky core. It is not predicted to have high levels of internal radiogenic heating, and, while it is thought that the moon Charon was formed in a collision event, any residual heating from this event, or subsequent tidal interactions, should have dissipated billions of years ago. Surface temperatures on Pluto range between 35 and 60K (between -238 and -213°C). The surface pressure on Pluto is about 10 μbar, precluding the existence of any form of liquid there. At the temperatures and pressures found on the surface of Pluto water should be an immobile bedrock. Adding sufficient amounts of ammonia to water can lower its freezing point by about 100 degrees, and other substances could conceivably lower it slightly further, but depressing the freezing point of water enough for it to retain any fluidity on the surface of Pluto is highly unlikely. Nevertheless, when the New Horizons space probe visited the dwarf planet in 2015, it found abundant signs of resurfacing, which are difficult to explain without the presence of liquid water beneath the surface of Pluto.
In a paper published in the journal Nature Communications on 29 March 2021, a team of scientists led by Kelsi Singer of the Southwest Research Institute discuss the possibility of cryovolcanism (ice volcanoes) as an explanation for the observed features on the surface of Pluto.
Singer et al. examine an area to the southwest of the Sputnik Planitia ice sheet (an ancient impact basin with a diameter of about 1000 km), which they view as a possible volcanic terrain. The most prominent features of this region are a series of large rises or mounds of material separated by broad depressions, the two largest of which form annular features with deep central depressions, and are named Wright Mons and Picard Mons. Wright Mons stands roughly 4-5 km high, and has a diameter of about 150 km, giving it a volume of roughly 24 000 km³, while Picard Mons appears to be about 7 km high, and to have a diameter of about 225 km.
Wright Mons is surrounded, and partially covered, by a hummocky terrain, which has a typical wavelength of 6-12 km, but reaches up to 20 km between hummocks in places. The hummocks are generally interconnected rather than singular, and have flattened or rounded tops. In the images the northern flank of Wright Mons, and the plain to the north of the mountain, appear to be smoother and less hummocky, but Singer et al. suggest that this is probably an artifact of the lighting direction. The hummocks appear to have no preferred direction with regard to the summit, and closer examination shows smaller structures, such as boulders, blocks, slabs, and ridges, with scales of 1-2 km, superimposed on top of the hummocks.
The slopes of the flanks of Wright Mons average about 3-5°, but reach 10° in places. The mountain has a central depression roughly 40-50 km across and about 4 km deep (i.e. taking it down to the level of the surrounding terrain). The central depression of Picard Mons appears to be larger still, with a more u-shaped profile. The structure of these bodies is unlike anything seen in terrestrial or Martian volcanoes, with the central caldera(?) taking up a third of the structure's area and being as deep as the mountain is tall. The inner slopes of the crater appear to have the same hummocky profile as the outer slopes, with no collapse terraces, or similar structures, as might be expected on a volcano in the Inner Solar System. The northern and southern faces of Wright Mons, and the nearby ridge of the Medial Montes all appear to have similar profiles, with a steeper northern face and a more shallowly inclined southern face, both of which are covered by hummocks.
If the interior depressions of Wright and Picard montes are collapse structures, formed within formerly dome-shaped structures, then they would represent a loss of about half the mountains' total masses, which seems highly implausible.
A few other depressions of varying size are scattered across the area around Wright and Picard montes. These are generally not circular in shape, which rules out an impact origin. Some of these structures may be related to collapse at fault faces, while others might be depressions between areas of uplift.
No vents or fractures from which effusive material might escape can be seen in this landscape, not any sign of flow through which the presence of such structure might be inferred, although this might be an artefact of the resolution of the images (234–315 m per pixel), which prevents the detection of small structures. Nor are there any signs of explosive eruptions, such as fall deposit patterns (either radial or directional), or steeper cones. The extent of this terrain is unclear, as it continues southward until it disappears within a low-light haze. The absence of any cratering on or around Wright Mons gives it a maximum age of 1-2 billion years, and potentially considerably younger.
Methane, nitrogen, and water ice have all been observed on the surface of Pluto, in high-volume, concentrated deposits, so Singer et al. consider whether these might make up the structures of Wright and Picard montes, and the surrounding terrain. Data provided by the Linear Etalon Imaging Spectral Array instrument on New Horizons suggests that nitrogen- and methane-rich complexes of nitrogen, methane, and carbon monoxide cover much of the surface of Pluto. These are likely to sublimate and redeposit on a seasonal cycle (which on Pluto implies a 248 Earth year cycle), or a multi-million-year cycle determined by obliquity/precession cycles. In darker areas with low albedos these volatiles are either not being deposited, or not being retained, with the upshot that these areas are dominated by the non-volatile water ice 'bedrock'. Such areas include the dark equatorial band of Pluto, as well as the terrain around Wright and Picard montes. In this later area, deeper areas are dark, implying water ice, whereas higher elevations are more reddish, suggesting the presence of methane ice. This is probably a thin layer of snow, with the bulk of the structures composed of water ice. The area shows a very different topographic profile to the 'bladed terrain' found at high altitudes close to Pluto's equator, which is thought to be formed by the condensation and sublimation of thick methane deposits.
Patches of nitrogen ice can also be seen in the area around Wright and Picard montes, both in thin layers on the peaks, similar to the distribution of methane, and in depressions where it has apparently pooled. Although conditions on Pluto are cold enough to make water ice an immobile bedrock, nitrogen here is much closer to its melting point, and therefore likely to be capable of some sort of glacier-like plastic flow, which means that nitrogen ice is unlikely to be able to form tall topographic features.
For these reasons, Singer et al. assume that the bulk of the Wright and Picard montes structures, and the surrounding topographic features, are composed of water ice, although these is a potential for other materials to have played a role in their formation and evolution. Notably, ammonia has been detected in areas in Pluto's Northern Hemisphere where cryofluids are thought to have erupted through extensional fractures. No ammonia has been detected in the study area, but its presence could potentially be masked by the presence of methane ice. There are also some darker areas on some north-facing slopes, which may indicate the presence of tholins (disordered and insoluble carbon-rich macromolecular materials).
The ices of the Wright Mons region have a slightly reddish tinge to them, indicating the presence of methane, while the terrain to the north is significantly redder, and that to the west is much less red. This may indicate that these regions are of different ages, having been emplaced at different times, from varying source reservoirs, although their precise age-relationship is impossible to determine.
Singer et al. note a number of features which they believe indicate suggest the study area was not formed by the erosion of an older, elevated terrain. The area around Wright and Picard montes represents a large area of terrain lacking in craters, suggesting that the area was resurfaced in a single or series of events rather than as a consequence of gradual evolution. The distinct hummocky pattern seen in this terrain covers both the plains and the raised topographic features, and is unlike anything seen elsewhere on Pluto. The raised topographic features of the region are variable in elevation, whereas topographic features made from similar materials and exposed to the same conditions tend to erode down to similar heights.
Examination of the available data leads Singer et al. to conclude that Wright Mons, and probably also Picard Mons, formed by the merging of multiple separate rises into a single structure, and that it shares a structure essentially similar to the other topographic features of the area. They note that a smaller dome-like feature, named Coleman Mons, may provide an example of the emplacement of how such structures form. This dome is about 25 km in diameter and 1.5 km high, a structure which could be achieved by water or water-ammonia ice being deposited around a central vent, but would be beyond the ductile strength of softer ices such as nitrogen or methane.
Smaller dome-like feature named Coleman mons. (a) Topography overlain on base image of feature, (b) base image alone, (c) topography alone, (d) perspective view of dome and pit with no vertical exaggeration, (e) view inside the pit, (f) topographic profiles as shown in panels (a)-(c) with 3× vertical exaggeration. Singer et al. (2022).The hummocky profile of the flanks of Wright Mons and the surrounding terrain suggests some form of viscous flow is occurring. Singer et al. suggest three different possibilities for the formation of these features, namely; the (1) creation of individual small volcanic domes, the (2) viscous extrusion of rapidly cooled lavas analogous to pillow lavas, or the (3) compression of viscous material with a frozen skin analogous to pahoehoe, viscous pressure ridges, or funiscular terrain on Enceladus.
The formation of ridges from pillow-lava like extrusions would require lava ice to be extruded at similar rates and for similar durations, both on the flanks of Wright Mons and on the surrounding plains. Such an even rate of extrusion across a varied terrain seems highly unlikely, and is at odds with observations of volcano behaviour both on Earth and elsewhere in the Solar System. It is possible that the hummocks were formed first, and then uplifted, but this would require an implausibly large mass of material being emplaced from below. Likewise, if the hummocks are contractional or compressional features, then this would imply a high-viscosity layer 8-13 km thick, which is in itself implausible, as well as a force capable of compressing such a layer, for which no obvious cause exists.
In addition, the extrusion of material to the surface to form any of these features would require a system of deep fractures through which the material could escape. Such structures have been seen elsewhere on the surface of Pluto, but not within the subject area, with the only possible fractures being along the large scarp which separates the Wright region from the plateau to the west, and another possible scarp further to the northwest, although other fractures could be hidden beneath extruded material on the surface.
None of the conventional models used to explain the emplacement of volcanic terrains on Earth and other Solar System bodies examined to date appears to fit well with the observed features on Pluto. Furthermore, the features of the area around Wright Mons appear to be quite different from anything seen elsewhere in the Solar System. No obvious vent regions can be seen, and though the surface does appear to be subject to viscous flow, no indication of directionality can be observed, making it difficult to explain the formation of the topology of the region. However, the constructs are compatible with a cryovolcanic origin, fuelled from multiple subsurface sources where the sources are below the constructs.
The low surface temperature on Pluto, combined with the low predicted heat flux within the Dwarf Planet's interior, makes it very hard to account for the mobilization of subsurface fluid comprised largely of water, but the relative youth of the terrain suggests that a heat-source must be available. While in itself apparently unlikely, such an unexplained heat-source is would help to explain other areas of Pluto with young surfaces comprised of volatile ices, such as Sputnik Planitia.
See also...
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