Cryovolcano
A cryovolcano is a type of volcano that erupts gases and volatile material such as liquid water, ammonia, and hydrocarbons. The erupted material is collectively referred to as cryolava; it originates from a reservoir of subsurface cryomagma. Cryovolcanic 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 on 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, constructing landforms or even resurfacing entire regions. Despite this, only a few eruptions have ever been observed in the Solar System. The sporadic nature of direct observations means that the true number of extant cryovolcanoes is contentious.
Like volcanism on the terrestrial planets, cryovolcanism is driven by escaping internal heat from within a celestial object, 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 observations by spacecraft and distant observations by telescopes.
Etymology and terminology
The term cryovolcano was coined in 1987 by Steven K. Croft, in a conference abstract for a presentation at the Geological Society of America meeting. The term is a compound of cryo-, from the Ancient Greek κρῠ́ος, and volcano.Other terminology used to describe cryovolcanism is analogous to volcanic terminology:
- Cryolava and cryomagma are distinguished in a manner similar to lava and magma. Cryomagma refers to the molten or partially molten material beneath a body's surface, where it may then erupt onto the surface. If the material is still fluid, it is classified as cryolava, which can flow in cryolava channels, analogs to lava channels. Explosive eruptions may pulverize the material into a fine "ash" termed cryoclastic material. Cryoclastic material flowing downhill produces cryoclastic flows, analogs to pyroclastic flows.
- A cryovolcanic edifice is a landform constructed by cryovolcanic eruptions. These may take the form of shields, cones, or domes. Cryovolcanic edifices may support secondary landforms, such as caldera-like collapse structures, cryovolcanic flow channels, and cryovolcanic fields and plains.
Types of cryovolcanism
Explosive eruptions
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 the mechanisms of explosive volcanism on terrestrial planets. Whereas terrestrial explosive volcanism is primarily driven by dissolved water, carbon dioxide, and sulfur dioxide, 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.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. In contrast to explosive cryovolcanism, no instances of active effusive cryovolcanism have been observed. Structures constructed by effusive eruptions depend on the viscosity of the erupted material. Eruptions of less viscous cryolava 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 features such as cryovolcanic domes.Mechanisms
For cryovolcanism to occur, three conditions must be met: an ample supply of cryomagma must be produced in a reservoir, the cryomagma must have a force driving ascent, and conduits need to be formed to the surface where cryomagma is able to ascend.Ascent
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. Therefore, cryomagma must overcome this in order to erupt onto a body's surface. Planetary scientists have proposed several hypotheses to explain how cryomagma erupts onto the surface:- Compositional buoyancy: the introduction of impurities such as ammonia, which is expected to be common in the outer Solar System, can help lower the densities of cryomagmas. However, the presence of impurities in cryomagma alone is unlikely to succeed in overcoming the density barrier. Conversely, the density of the ice shell can be increased through impurities as well, such as the inclusion silicate particles and salts. In particular, objects that are only partially differentiated into a rocky core and icy mantle are likely to have ice shells rich in silicate particles.
- Gas-driven buoyancy: besides affecting density, the inclusion of more volatile impurities may help decrease the density of cryomagma as it ascends by the formation of gas bubbles. The volatile compounds are fully dissolved in the cryomagma when pressurized deep beneath the surface. Should the cryomagma ascend, the cryomagma is depressurized. This leads to the exsolution of the volatiles out of the cryomagma, forming gas bubbles that help lower the density of the bulk solution.
- Internal pressurization: the progressive pressurization of a subsurface ocean as it cools and freezes may be enough to force 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.
Eruption
An alternative model for cryovolcanic eruptions invokes solid-state convection and diapirism. If a portion of an object's ice shell is warm and ductile enough, it could begin to convect, much as the Earth's mantle does. As the ice convects, warmer ice becomes buoyant relative to surrounding colder ice, rising towards the surface. The convection can be aided by local density differences in the ice due to an uneven distribution of impurities in the ice shell. If the warm ice intrudes on particularly impure ice, the warm ice can lead to the melting of the impure ice. The melting may then go on to erupt or uplift terrain to form surface diapirs.
Cryomagma reservoirs
Cryovolcanism implies the generation of large volumes of molten fluid in the interiors of icy worlds. A primary reservoir of such fluid are subsurface oceans. Subsurface oceans are widespread amongst the icy satellites of the giant planets and are largely maintained by tidal heating, where the moon's slightly eccentric orbit allows the rocky core to dissipate energy and generate heat. Evidence for subsurface oceans also exist for the dwarf planets Pluto and, to a lesser extent, Ceres, Eris, Makemake, Sedna, Gonggong, and Quaoar. In the case of Pluto and the other dwarf planets, there is comparatively little, if any, long-term tidal heating. Thus, heating must largely be self-generated, primarily coming from the decay of radioactive isotopes in their rocky cores.Reservoirs of cryomagma can hypothetically form within the shell of an icy world as well, either from direct localized melting or the injection of cryomagma from a deeper subsurface ocean. A convective layer in the ice shell can generate warm plumes that spread laterally at the base of the brittle icy crust. The intruding warm ice can melt impure ice, forming a lens-shaped region of melting. Other proposed methods of producing localized melts include the buildup of stress within strike-slip faults, where friction may be able to generate enough heat to melt ice; and impact events that violently heat the impact site. Intrusive models, meanwhile, propose that a deeper subsurface ocean directly injects cryomagma through fractures in the ice shell, much like volcanic dike and sill systems.
Cryomagma composition
Water is expected to be the dominant component of cryomagmas. Besides water, cryomagma may contain additional impurities, drastically changing its properties. Certain compounds can lower the density of cryomagma. Ammonia in particular 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. Methanol can lower cryomagma density even further, whilst significantly increasing viscosity. Some impurities can increase the density of cryomagma. Salts, such as magnesium sulfate and sodium sulfate significantly increase density with comparatively minor changes in viscosity. Salty or briny cryomagma compositions may be important for cryovolcanism on Jupiter's icy moons, where salt-dominated impurities are likely more common. Besides affecting density and viscosity, the inclusions of impurities—particularly salts and especially ammonia—can encourage melting by significantly lowering the melting point of cryomagma.| Cryomagma composition, mass % | Melting point | Liquid density | Liquid viscosity | Solid density |
| Pure water 100% | 273 | 1.000 | 0.0017 | 0.917 |
| Brine 81.2%, 16%, 2.8% | 268 | 1.19 | 0.007 | 1.13 |
| Ammonia and water 67.4%, 32.6% | 176 | 0.946 | 4 | 0.962 |
| Ammonia, water, and methanol 47%, 23%, 30% | 153 | 0.978 | 4,000 | – |
| Nitrogen and methane 86.5%, 13.5% | 62 | 0.783 | 0.0003 | – |
| Basaltic lava | – | – | ~10–100 | – |