**1. Introduction**

All types of volcanism known until now are in the solar system. All considerations and models for volcanism in other stellar systems are built upon our knowledge from our own system. New types of volcanism still unthought of, might be a challenging research topic but may not be considered here.

The main aim of this chapter is to consider cryovolcanism powered by tidal heating and its potential in exosystems. As an introduction, for reference and to characterize

the main features as a base for better comparison, a rough overview of its counterpart silicate volcanism as well as underlying types of energy sources in the solar system are given.

The most prominent objects in our solar system harboring active volcanoes are both an example of what may be named high-temperature range volcanism as rocks are molten and are apparent in the form of glowing liquid lava on Earth and on the moon Io. This is generally better known under silicious-based/silicate volcanism because silicate is the most dominating component in liquid rocks. The known temperatures rise to about 1600 K on Io in the volcanoes on the surface [1], while on Earth about 1000 K to 1550 K temperature in the lava is reached [2] depending on the composition of the rocks. The temperature of magma below the surface may have still higher temperatures.

These two objects already show us also the main energy categories on which volcanism, as we know it, relies on. For the Earth, it is mainly based on the conserved accretion/contraction energy from its formation, decay of radioactive elements pulled into the mantle and center of the planet by gravity-induced differentiation, and also on friction rising from the resulting tectonic activity [3]. For Io, it is mainly based on tidal heating from the huge tidal forces raised by the gas giant it is orbiting in its crust and upper mantle [3]. Both may gain also energy from friction that is arising from the resulting tectonic activity. Some of the following general considerations may also apply to forms of energy resources. The energy retention behavior (and so also the duration of volcanic activity) is among other factors strongly depending on surfaceto-volume ratios regardless of energy source. Bigger objects with lower surface-tovolume ratios are tending to stay hotter for a longer period and are able to sustain volcanism longer. For Earth and Moon during the assumed collision of their precursor bodies Theia and Gaia, a transfer of the core of Theia into the forming core of the Earth may have increased also the amount of heavier and so radioactive elements, increasing the power for volcanism on Earth and by this decreasing it on the Moon. Also during this early phase, tidal heating may have played a much bigger role for both objects, as they have been much closer together [4].

Regarding ancient evolution steps in the solar system, it is important stressing that even much tinier impacts than Theia with Gaia were much more common and have played a stronger role in melting parts of a planet, asteroid, or moon, especially during the late heavy bombardment (LHB). As it may be perceived as an external energy source and is now of little relevance, volcanism by bombardment will not be discussed further.

Considering the long-term evolution of heating sources also leads us to inactive silicate volcanism as the bodies considered are too tiny to have been able to sustain volcanism until now, as on the Moon, Mercury [5], Venus, and Mars. They are covered with lava plains and show also volcanoes, for example, the highest of the solar system, Olympus Mons on Mars. Still, for all these objects, signs for stronger or lesser still ongoing or very recent volcanic/tectonic activity have been found or are discussed (Moon: [6–10]; Mercury: [11–13]; Venus: [14–17]; Mars: [18, 19]). On Mars also a connection to a known type of lower temperature volcanism may already be found as the melting of ice and/or its remnants under a volcano may have been found as well [20–23].

In the case of Venus, a relatively young surface [24] and its own type of tectonics [25] may also indicate a presence of modifying influences on silicate volcanism that are not well known until now. If the missing of water or other solvents (on Venus probably mostly after entering into a runaway greenhouse effect) is a cause for a changed plate tectonic and so volcanism [26–28], also availability and abundance of

*Cryovolcanism in the Solar System and beyond: Considerations on Energy Sources… DOI: http://dx.doi.org/10.5772/intechopen.105067*

#### **Figure 1.**

*Overview of types of volcanism identified or assumed on celestial bodies in the solar system (right panel) and the extrasolar planetary system TRAPPIST-1 (left panel). The horizontal axis corresponds to the mean semi-major axis of the orbits as distance to the sun (right panel) and the vertical axis corresponds to the mean semi-major axis of the orbits of moons or exoplanets as the distance from their central object (hosting planet or star TRAPPIST-1) [37–43]. The radii of the circles depicting each object are scaled logarithmically to the actual radii of the celestial bodies [37–39, 41–53]. Please note that close to the dwarf planets Pluto and Haumea the two further dwarf planets Orcus and Quaoar exist. These are shown in the zoomed-in inset panel. Charon is a moon of Pluto, and Namaka and Hi'iaka are moons of Haumea. The dwarf planet Sedna on the right side of the right panel orbits the sun at a mean distance of 506 au, so further away as it is shown here, which is indicated with an arrow. Colorcoded is the types of volcanism known or assumed on the objects. Table 1 Provides an overview of the respective references. The hollow cross marks objects in the solar system for which ongoing volcanic eruptions or geysers are known. The filled plus marks objects in the solar system for which at least former volcanic eruptions, geysers, or domes (remnants of extinct volcanic activity) are known or strongly assumed. Please note that only on Pluto at least former volcanic eruptions are assumed, but not on Orcus. An asterisk (\*) at the end of an object's name marks when one or several moons are present but not shown here.*

water, NH3 or CH4 have to be considered for modifying silicate volcanism, showing again a link to material and substances beyond rock.

Also, a discussed inhomogeneous distribution of radionuclides as a cause for volcanic activities, for example, on the Moon [6] further highlights a need for deep consideration of how volcanism may be sustained and be modified in behavior.

Moving on outward in the solar system brings us into ranges of asteroids, all of them being tinier than the aforementioned planets and so obviously have cooled and are not maintaining volcanism now. Accretion and radioactive energy seem to be nowadays not important for any type of volcanism in the asteroids. Still, ancient traces of volcanism may be found. The importance of meteorite impacts for melting gets relatively bigger on tinier objects. But also a differing composition of radioactive elements seems to play a bigger role as <sup>26</sup>*Al* [29] and <sup>60</sup>*Fe* [30] seem to have molten these tiny objects and given rise to silicate volcanism. This peroid has made a huge influence on these objects, even though this period may not have been very long, regarding the relatively short half-life of these isotopes.

Entering the realm of the gas giants opens new perspectives. The rocky objects that can show volcanism are now mainly moons, tinier in size but are orbiting much larger gas giants or maybe very close double systems orbiting each other, for example, some TNOs. These conditions open the possibility for tidal heating as the main energy source for volcanoes. Accretion and radioactive energy seem to be nowadays of lesser importance for any type of volcanism in the asteroids, gas giant moons, and beyond in the solar system.

Considering Io as an exception in this range, as we also will show, we encounter two other known examples of volcanism around gas giants that are based on tidal heating, but are now in the lower temperature ranges of cryovolcanism. The moons Enceladus and also Triton have been identified as cryovolcanic worlds [31, 32]. Others show signs of active geology and tectonics, for example, on Europa [33] or Ganymede [34], and are believed to have liquid layers or even oceans of solvents, such as water or NH3, in their depths and even deeper a basic silicate volcanism.

Regarding this, it becomes easily obvious that a real stable definition of cryovolcanism is not as easy. The aim is mostly trying to focus on volatiles, for example, molten water or methane are thrown out on the surface in an environment colder than their own melting temperature, also even if in greater depths rocks might be quite hot. Earth itself is mostly not being considered as a planet harboring cryovolcanism, even though any volcano under ice known (Iceland) or assumed (Antarctica) and geysers all over the world would fulfill such definitions in winter. Also, mud volcanism (also called "cold" volcanism) being based on mud diapirs and being generally associated with (silicate) volcanism [35, 36], is normally not considered under cryovolcanism.

All these ambiguities in defining cryovolcanism may result from a bias in detecting cryovolcanism on foreign worlds in astronomy or astrophysics. Big eruptions are much easier to observe by optic sensors (on or close to Earth or even on probes) as well as by mass analyzing probes in the proximity of these objects than by constant release of volatiles by tectonics of slowly moving ice shields covering deeper-lying liquids or even silicate volcanism. Also, old remnant structures of previous volcanism may still cover deeper active processes, which is much more problematic to investigate. If we improve our detection capabilities, also our definitions will evolve. Regarding detection and research on cryovolcanic worlds, this all illustrates the strong necessity of modeling based on easier accessible observations, either to understand where we might find such objects with cryovolcanism or what kind of cryovolcanism we might expect. This leads apart from the known active volcanic and cryovolcanic

**Object Primary Type of volcanism, etc. Reference** Mercury Sun At least former silicate volcanism [12, 13] Venus Sun At least former silicate volcanism [15–17] Earth Sun Silicate volcanism; Active volcanic eruptions Moon Earth At least former silicate volcanism [9, 10] Mars Sun At least former silicate volcanism; At least former volcanic eruptions/domes [18, 19] Ceres Sun At least former cryovolcanism; At least former volcanic eruptions/geysers/domes [54–56] Io Jupiter Silicate volcanism; Active volcanic eruptions [57] Europa Jupiter Cryovolcanism; At least former volcanic eruptions/geysers/domes [33, 57–60] Ganymede Jupiter Cryovolcanism; At least former volcanic eruptions/geysers/domes [34, 57, 61, 62] Callisto Jupiter Cryovolcanism [57] Mimas Saturn Cryovolcanism/At least former cryovolcanism (debated) [63–65] Enceladus Saturn Cryovolcanism; Active volcanic eruptions/geysers [31, 66] Tethys Saturn At least former cryovolcanism [67–69] Dione Saturn Cryovolcanism [66] Rhea Saturn Cryovolcanism [70] Titan Saturn Cryovolcanism; At least former volcanic eruptions/geysers/ domes [71–79] Iapetus Saturn At least former cryovolcanism [70, 80] Miranda Uranus At least former cryovolcanism [81–84] Ariel Uranus Potential candidate for at least former cryovolcanism [81, 82] Umbriel Uranus Potential candidate for at least former cryovolcanism [81] Titania Uranus Cryovolcanism [70] Oberon Uranus Cryovolcanism [70] Triton Neptune Cryovolcanism; Active volcanic eruptions/geysers [32, 70, 85–89] Pluto Sun At least former cryovolcanism; At least former volcanic eruptions/geysers/domes [70, 90] Charon Pluto At least former cryovolcanism [91] Orcus Sun At least former cryovolcanism [70, 92] Haumea Sun Potential candidate for at least former cryovolcanism [93] Hi'iaka Haumea Potential candidate for at least former cryovolcanism [93] Quaoar Sun Potential candidate for at least former cryovolcanism [94] Eris Sun At least former cryovolcanism [50, 70] Dysnomia Eris At least former cryovolcanism [50] Sedna Sun At least former cryovolcanism [70]

*Cryovolcanism in the Solar System and beyond: Considerations on Energy Sources… DOI: http://dx.doi.org/10.5772/intechopen.105067*

#### **Table 1.**

*Celestial objects in the solar system on which different types of volcanism are present or strongly assumed. The last column gives the respective references. References given here were also used to categorize the types of volcanism given in Figure 1. For each object, its orbited primary and the types of known or strongly assumed volcanism and eruptions (active or extinct) are listed.*

worlds to a huge list of strongly assumed, mainly cryovolcanic, active as well as inactive worlds (see **Figure 1** and **Table 1**).

All these models are strongly based on energy resources and energy transport. Reconsidering some basic parameters in these models may illuminate some specific aspects of cryovolcanic worlds and offers an insight into basic principles to find general concepts for application in far exoplanetary systems.
