The Mysterious Morphology of Hekla Cavus, Pluto

Post contributed by Dr. Caitlin Ahrens, NASA Goddard Space Flight Center, USA.

Cryovolcanism involves the transfer of icy or gaseous subsurface materials either to the surface (eruptive) or through the subsurface (non-eruptive) of an icy planetary body. It differs from magmatism and volcanism on Earth, which involves the migration and eruption of molten rock. Cryovolcanism is thought to have operated on several icy bodies in the Solar System, including Enceladus, Triton, Pluto, and possibly Europa. Cryovolcanism results primarily from internal heat-producing processes, and excludes sublimation and condensation processes at the surface. In the case of Pluto, there is evidence for a subsurface fluid layer, the presence of cryovolcanoes, and cryovolcanic subsurface materials (called cryomagma) which can contain ammonia and methane. Due to the presence of a deformable subsurface layer, it is possible for the material to shift, causing uplift followed by a collapse-type event. This is a possible scenario at Hekla Cavus (Image 1), a large, elongated, and irregular depression situated within a much larger north-south (N-S) ridge-trough system outlined by mountain ranges.

Image 1: Image of Hekla Cavus taken from the LORRI instrument onboard the New Horizons.

Image 2 shows the orientation and placement of Hekla Cavus within the ridge-trough system just west of Sputnik Planitia. The ridge-trough system has numerous elongated depressions. The observation that these depressions are elongated in a similar direction to the ridge-trough system suggests that they could have formed by lateral erosion (based on the orientation of the cavus and placement of fissures) and/or explosive volcanism related to the evolution of the ridge-trough system, rather than as impact craters. However, it is not yet clear how these possible formation processes operate on Pluto. The volume of Hekla Cavus is difficult to explain by sublimation processes alone, suggesting that other processes were involved in its formation. The mass wasting feature at the fluted western wall (Image 1) suggests that the wall has been modified since its initial formation by the downward motion of slump material to the cavus floor. Hekla Cavus is also associated with arcuate ring fractures and fissures, suggesting that underlying geophysical processes could explain its formation.

Image 2: Context map of features. Note that the orientation of Hekla Cavus and direction of the ridge-trough system.  

Determining the process (or processes) for the origin and collapse evolution of Hekla Cavus is a challenge because the observations of calderas (or collapse-type events) on rocky bodies may not directly relate to the processes at work on an icy body. Our understanding of the origin of the Hekla Cavus depression itself is currently affected by a lack of information on Pluto’s interior. This makes it difficult to accurately analyze the subsurface dynamics of a collapse event. However, we hypothesize that basic dynamics involving the collapse of a caldera-like structure from a previous built-up mound (as illustrated in Image 3) is a viable explanation. We propose that the ridge-trough system, possibly with greater stresses near the observed cryovolcanic edifices of Wright and Piccard Mons, would have uplifted from subsurface forces of cryo-materials (e.g., lenses of cryo-liquids). The subsequent redistribution of subsurface materials away from the uplifted bulge could have depleted the cryo-magmatic materials, removing lithospheric support and causing a collapse event. South of Hekla Cavus are several similar large pitted formations, each possibly formed by a collapse event.

Image 3: Cross-section of a typical caldera evolution at Hekla Cavus. The observed bulging surrounding Hekla Cavus may result from magmatic pressure that gives rise to slight doming pre-collapse.

Further Reading

Ahrens, C., and Chevrier, V., (2020). Investigation of the morphology and interpretation of Hekla Cavus, Pluto. Icarus, 114108 (In Press).

Cruikshank, D., Umurhan, O., Beyer, R., et al., 2019. Recent cryovolcanism in Virgil Fossae on Pluto. Icarus 330, 155–168.

Howard, A., et al., 2017. Pluto: pits and mantles on uplands north and east of Sputnik Planitia. Icarus 293, 218–230.

Neveu, M., Desch, S., Shock, E., Glein, C., 2015. Prerequisites for explosive cryovolcanism on dwarf planet-class Kuiper belt objects. Icarus 246, 48–64.

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

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