A cryovolcano (sometimes informally referred to as an ice volcano) is a type of
volcano that erupts gasses and
volatile material such as liquid
water,
ammonia, and
hydrocarbons, collectively referred to as cryolava, from a reservoir of subsurface cryomagma. These eruptions can take many forms, such as fissure and curtain eruptions, effusive cryolava flows, and large-scale resurfacing, and can vary greatly in output volumes. Immediately after an eruption, cryolava quickly freezes, constructing geological features and altering the surface.
Although rare in the inner
Solar System, past and recent cryovolcanism is common amongst
planetary objects in the outer Solar System, especially on the icy moons of the
giant planets and potentially amongst the
dwarf planets as well. As such, cryovolcanism is important to the geological histories of these worlds.
Like
volcanism on the
terrestrial planets, cryovolcanism is driven by escaping internal heat, often supplied by extensive
tidal heating in the case of the moons of the giant planets. However, isolated dwarf planets are capable of retaining enough internal heat from formation and radioactive decay to drive cryovolcanism on their own, an observation which has been supported by both in situ and distant observations.[1][2]
Mechanisms
A major challenge in models of cryovolcanic mechanisms is that liquid water is substantially denser than water ice, in contrast to
silicates where liquid magma is less dense than solid rock. As such, cryomagma must overcome this in order to erupt onto a body's surface. It has been proposed that the
exsolvation of compounds more volatile than water or the inclusion of various compounds can help lower the density of the cryomagma, or that alternatively, impurities in the ice shell can increase its density.[3]: 182 Ammonia may be a common component of cryomagmas, and has been detected in the plumes of
Saturn's moon Enceladus. A partially frozen Ammonia-water
eutectic mixture can be positively buoyant with respect to the icy crust, enabling its eruption.[4]: 766-767 Alternatively, the progressive pressurization of a subsurface ocean may be enough to drive cryomagma to ascend to the surface due to water's unusual property of expanding upon freezing. Internal ocean pressurization does not necessitate the addition of other volatile compounds, potentially explaining cryovolcanism within
Jupiter's moons, where salt-dominated impurities likely serve to make cryomagma denser.[5][3]: 183
Reservoirs
The most common reservoir of cryomagma is subsurface oceans.[3]: 167 Subsurface oceans are widespread amongst the satellites of the
giant planets and are largely maintained by
tidal heating.[6] Evidence for subsurface oceans also exist on the dwarf planets
Pluto[7] and, to a lesser extent,
Ceres.[8][9] Fractures in the ice shell, caused by diurnal tides, ice shell libration, internal expansion or contraction, and other stresses, can form direct conduits whence cryomagma may ascend from.[3]: 185
Alternatively, cryomagma may upwell through solid convection or density differences within the ice shell, ascending without the need for fractures or a subsurface ocean. Cryomagma may then melt due to warm ice transferring heat to impure, lower melting point ice, which may then erupt or form
diapirs.[3]: 189-190
Composition
Besides water, cryomagma may contain additional impurities, drastically changing its properties.
Explosive cryovolcanism, or cryoclastic eruptions, is expected to be driven by the exsolvation of dissolved volatile gasses as pressure drops whilst cryomagma ascends, much like explosive volcanism works on terrestrial planets. Whereas terrestrial explosive volcanism is primarily driven by dissolved H2O, CO2, and SO2, explosive cryovolcanism may instead be driven by methane and carbon monoxide. Upon eruption, cryovolcanic material is pulverized in violent explosions much like
volcanic ash and
tephra, producing cryoclastic material.[4]: 768
Effusive eruptions
Effusive cryovolcanism takes place with little to no explosive activity and is instead characterized by widespread cryolava flows which cover the pre-existing landscape. The resulting structure depends on the viscosity of the erupted material. Effusive cryovolcanism can resurface large regions and form expansive, relatively flat plains, similar to
shield volcanoes and
flood basalt eruptions on terrestrial planets. More viscous erupted material does not travel as far, and instead can construct localized high-relief domes and edifices more akin to
lava domes. No instances of active effusive cryovolcanism have been observed.[3]: 199-200
Upon the arrival of Dawn, the dwarf planet
Ceres was discovered to have numerous
bright spots (designated as faculae) located within several major impact basins, most prominently in the center of
Occator Crater. These bright spots are composed primarily of various salts, and have been hypothesized to have formed from impact-induced upwelling of subsurface material and may indicate that Ceres had a subsurface ocean in its past.[12]Dawn also discovered
Ahuna Mons and
Yamor Mons (formerly Ysolos Mons), two prominent isolated mountains which are hypothesized to be young cryovolcanic domes.[13] Ceres remains the closest-known cryovolcanically active body; due to this, it is expected that cryovolcanic domes eventually subside after becoming extinct due to viscous relaxation.[13]
Europa receives enough tidal heating from Jupiter to sustain a global liquid water ocean, and as such surface is exceedingly young, at roughly 60 to 90 million years old.[14]: 452 [15] Its most striking features, a dense web of lineae, appear to be the sites of active resurfacing on Europa, proceeding in a manner similar to Earth's
mid-ocean ridges,[16] and Europa may experience a form of subduction in its icy crust.[15]
Despite its young surface age, few, if any distinct cryovolcanoes have been definitively identified on the Europan surface in the past.[3]: 193-194 Nevertheless, observations of Europa from the
Hubble Space Telescope hinted at the existence of weak, possibly cryovolcanic plumes.[17] More recent analyses of some Europan surface features have proposed cryovolcanic origins for them as well; Europa's chaos terrain has been interpreted as the site of very shallow cryomagma lakes,[18] and a field of cryovolcanic cones has been tentatively identified near Argadnel Regio.[19]
Ganymede
Ganymede's surface, like Europa's, is heavily tectonized yet appears to have few cryovolcanic features.[20] Ganymede's surface has several irregularly-shaped depressions (termed paterae) which have been identified as candidate cryovolcanic calderas.[21]
Saturn's moon Enceladus is host to the most dramatic example of cryovolcanism yet observed, with a series of vents erupting 250 kg of material per second that feeds Saturn's
E ring.[22][23] These eruptions take place across Enceladus's south polar region, sourced from four major ridges which form a region informally known as the
Tiger Stripes.[24] Enceladus's cryovolcanic activity is sustained by a global subsurface ocean.[25]
Other regions of Enceladus exhibit similar terrain to that of the Tiger Stripes, possibly indicating that Enceladus has experienced discrete periods of heightened cryovolcanism in the past.[25]
Saturn's moon
Titan has a dense
atmospheric haze layer which permanently obscures visible observations of its surface features, making the definitive identification of cryovolcanic structures especially difficult. Titan has an extensive subsurface ocean,[26] encouraging searches for cryovolcanic features, and several features have been proposed as candidate cryovolcanoes, most notably
Doom Mons.[27] Lakes and depressions in Titan's polar regions show morphological evidence of an explosive origin, leading to a hypothesis that they form from maar-like eruptions.[28]
Uranian moons
Of
Uranus's
five major satellites,
Miranda and
Ariel appear to have unusually youthful surfaces indicative of relatively recent activity. Miranda in particular has extraordinarily varied terrain.
Inverness Corona is located near Miranda's south pole and is estimated to be less than 1 billion years old,[29] and broad similarities between Miranda's
coronae and Enceladus's south polar region have been noted, leading to proposals of coronae being formed by the eruption of viscous cryomagma.[30] Ariel also exhibits widespread resurfacing, with large polygonal crustal blocks divided by chasmata as young as ~0.8 ± 0.5 billion years old, while relatively flat plains may have been the site of large flood eruptions.[30]
Evidence for relatively recent cryovolcanism on the other three round moons of Uranus is much less clear.
Titania hosts large grabens, and
Oberon has a massive ~11 km high mountain observed on its limb; the precise origins of the mountain is unclear, but a cryovolcanic origin has been proposed.[30]
With an estimated average surface age of 10-100 million years old, with some regions possibly being only a few million years old,
Triton is one of the most geologically active worlds in the Solar System.[31] Large-scale cryovolcanic landforms have been identified on Triton's young surface, including the massive
Leviathan Patera, one of the largest volcanic or cryovolcanic edifices in the Solar System.[32] Leviathan Patera borders two large, flat walled plains,
Ruach and
Tuonela Planitia, which have been hypothesized as young cryovolcanic lakes and may represent Triton's youngest cryovolcanic features.[33][34]: 870; 872 The regions around Ruach and Tuonela feature additional smaller subcircular depressions, some of which are partially bordered by walls and scarps. These depressions have been interpreted as diapirs, explosive craters, caldera-like structures, or impact craters filled in by cryolava flows.[35][33]
Triton's southern polar ice cap is marked by a multitude of dark streaks, likely composed of organic
tholins deposited by wind-blown plumes. At least two plumes, the Mahilani Plume and the Hili Plume, have been observed, with the two plumes reaching 8 kilometers in altitude.[34]: 873 These plumes have been hypothesized to be driven by subsurface sublimation of nitrogen ice under prolonged exposure to sunlight in the southern hemispheric summer; however, it has alternatively been proposed that the plume streaks may alternatively represent fallout from cryovolcanic eruptions.[36]
The surface of the dwarf planet
Pluto varies dramatically in age, and several regions appear to display relatively recent cryovolcanic activity. The most reliably identified cryovolcanic structures are
Wright Mons and Piccard Mons, two large mountains with central depressions which have led to hypotheses that they may be cryovolcanoes with peak calderas.[37][38] The two mountains are surrounded by an unusual region of hilly "hummocky terrain", and the lack of distinct flow features have led to alternative proposals that the structures may instead be formed by sequential dome-forming eruptions, with nearby Coleman Mons being a smaller independent dome.[39]
Virgil Fossae, a large fault within
Belton Regio, may also represent another site of cryovolcanism on Pluto. Large flows radiate away from Virgil Fossae and appear to have partially erased preceding terrain, with several nearby craters being partly infilled by material.[40] More recently, Hekla Cavus was hypothesized to have formed from a cryovolcanic collapse.[41] Similarly, Kiladze Crater has been proposed as a caldera formed from a voluminous eruption.[42]
Although
Sputnik Planitia represents the youngest surface on Pluto, it is not a cryovolcanic structure; Sputnik Planitia continuously resurfaces itself with the convective overturning of glacial nitrogen ice, fuelled by Pluto's internal heat and sublimation into Pluto's atmosphere.[43]
Charon's surface dichotomy indicates that a large section of its surface may have been flooded in large, effusive eruptions, similar to the
Lunar maria. These floodplains form
Vulcan Planitia and may have erupted as Charon's internal ocean froze.[44]
Other dwarf planets
In 2022, low-resolution near-infrared (0.7–5 μm)
spectroscopic observations by the
James Webb Space Telescope (JWST) detected light hydrocarbons and complex organic molecules on the dwarf planets
Quaoar,
Gonggong, and
Sedna, indicating that all three have experienced internal melting and planetary differentiation in their pasts. The presence of volatiles on their surfaces indicates that cryovolcanism may be resupplying methane.[2] JWST spectral observations of
Eris and
Makemake revealed that hydrogen-deuterium and carbon isotopic ratios indicated that both dwarf planets are actively replenishing surface methane as well, probably with the presence of a subsurface ocean.[45]
Gallery
Various examples of cryovolcanism in the Solar System
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