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“The Melodic Interplay of Cryovolcanism: A Mix of Cold and Heat”


An artist’s depiction of a cryovolcano is erupting on Neptune’s moon Triton.

Otherworldly Oceans

Cover of the October 2023 issue of Eos

“Is it imaginable that there may be places in our solar system with more water than on Earth?” inquired Steve Vance, a planetary scientist at NASA’s Jet Propulsion Laboratory and project scientist for the Europa Clipper mission.

Although Earth is known for its striking blue appearance, other celestial bodies such as Jupiter’s moons Europa and Ganymede, Saturn’s moons Enceladus and Titan, and various objects in the outer solar system have proven to be surprisingly dynamic ocean worlds. These bodies exhibit signs of constant surface renewal, occurring at a faster rate than previously thought. Their interiors contain unique types of ice and vast bodies of water, potentially even with hydrothermal vents that could support ocean life. All of these features make these worlds strong candidates for being habitable environments.

Volcanism is the main driving force behind the dynamic nature of these moons. It is possible that deep within their interiors, there may be silicate volcanism similar to that found on Earth. However, these moons may also experience a unique type of volcanism that affects their icy surfaces. The combination of heat, water, ice, and rock on these moons is of great interest to both planetary and Earth scientists.

Is it possible for life to exist on these frozen, volcanic moons? How can we establish livable regions in the outer part of our solar system? Information gathered from these moons with vast oceans could change our approach to finding habitable planets and provide insight into the potential for life in the entire galaxy.

We all know the importance of staying warm, especially during the colder months.

The necessity of maintaining warmth is well-known, particularly during the winter season.

Heat is a necessary component for all oceanic environments, including our planet Earth.

Heat on our planet is generated by two primary sources. The Sun provides the majority of the heat that ensures our oceans remain in liquid form. The Earth’s core, however, is heated by the decay of radioactive elements and leftover heat from its formation. The significant amount of internal heat needs to be released somehow, and according to Vance, volcanism is an effective way to do so.

In the far reaches of the solar system, an alternative source is necessary to produce the necessary heat for liquid water and volcanic activity. Due to the vast distance, the Sun’s energy alone is insufficient to maintain liquid water. While the moons of gas giants are quite large, with some even larger than Mercury, they are still not large enough to sustain the level of radioactive decay or residual heat required for the same types of volcanism found on Earth.

A significant amount of warmth present in these aquatic worlds is a result of tidal forces. As it orbits a gas giant alongside other moons, the oceanic moon experiences regular stretching and distortion, leading to the conversion of some of this energy into heat.

Io, the innermost of Jupiter’s four Galilean moons, exhibits this phenomenon most prominently. Due to its orbit around Jupiter and interactions with Callisto, Europa, and Ganymede, Io endures extreme conditions that make it the most volcanically active object in the entire solar system.

The impact of tidal forces on Jupiter’s other moons is not as extreme, but still significant enough to potentially create liquid oceans beneath icy surfaces. In Europa’s case, the tidal heating may even generate enough heat to produce magma in its rocky layer.

There is no solid proof of silicate volcanism occurring on any of these moons, aside from Io. However, theoretical models suggest that their interiors may possess enough heat to melt rock.

Sarah Fagents, a planetary volcanologist at the University of Hawai‘i at Mānoa, stated that the extent of thermal and dynamic connection between silicate volcanism within the planet and its surface activity is still unknown.

Cryovolcanism

These moons could potentially be responsible for both silicate volcanism and cryovolcanism, two distinct types of volcanic activity.

Cryovolcanism refers to the eruption of volatiles, such as ammonia, from a volcano into an environment where they freeze due to the low temperatures. In the outer solar system, most objects have extremely cold surfaces, causing water to always be frozen. This is one reason why the discovery of potential cryovolcanism in images from NASA’s Voyager, Galileo, and Cassini missions was unexpected for scientists.

The jets on Enceladus are most likely evidence of cryovolcanism, potentially playing a role in the formation of Europa’s ridges and Titan’s mountains.

Icy plumes spill from Enceladus’s south pole in this sequence of images from the Cassini spacecraft.

Images from the Cassini spacecraft show icy plumes erupting from the south pole of Enceladus. Credit goes to NASA/JPL-Caltech/Space Science Institute, released to the public domain.

Fagents was researching basaltic lava and explosive events on Earth when her boss redirected her attention to studying surface features on Europa. It was a remarkable discovery. The surface of Europa is rich in a variety of features, including dome-like and flow-like structures that suggest the presence of fluids on its surface.

Volcanologists and Fagents noticed similarities between the dome-shaped and flow-like structures on Europa and the lava domes and flows found in the Cascades or Yellowstone in North America. However, unlike these places, Europa’s surface is composed entirely of ice and there are no rocks present for hundreds of kilometers beneath its icy layer according to current geophysical data.

Fagents stated that these bodies have surface features that do not have a comparable counterpart on Earth. Attempts have been made to create silicate volcanic imitations, but they may not be the most effective.

This alternative to silicate volcanism opens the door between disciplines. “There is a lot we can learn from the fields of glaciology and hydrology that might be more pertinent than volcanology,” Fagents said. “You need to bring in a diversity of expertise and viewpoints” to better understand the phenomenon.

Fracturing Theories

I discovered a connection between planetary science and my knowledge of physics while contemplating the geophysical fluid dynamics of Europa.

Lynnae Quick, a planetary geophysicist at NASA Goddard Space Flight Center and a mission scientist on Europa Clipper, was instrumental in connecting scientific observation with scientific process. During her time studying physics in graduate school, Quick was given an internship opportunity by planetary scientist Louise Prockter at APL (Johns Hopkins University Applied Physics Laboratory). This led to her spending a summer researching Europa, which sparked her passion for the subject. Through this experience, Quick discovered a connection between her background in physics and planetary science, specifically in exploring the geophysical fluid dynamics of Europa.

Quick found the mechanics of cryovolcanism fascinating because it seemed to go against what is typically seen in terrestrial volcanism. In traditional volcanoes, the magma is less dense and therefore floats to the surface to erupt. However, in cryovolcanism, liquid water is denser than its solid form, which is why ice floats in a drink. This made it difficult to envision how liquid water in Europa’s icy shell could rise and erupt above the ice.

Fagents explained that while volcanism is caused by internal forces and involves melting and upward movement, cryovolcanism is more complex. It requires cracks in the icy surface and a means for liquid to pass through them. Although the difference in density is not significant, ocean water, especially if it is salty and denser than regular water, would naturally want to stay in place.

There are two main explanations for the eruption of liquid water (cryomagma), and both involve the influence of tidal forces. The initial theory suggests that the stretching and squeezing of ocean worlds as they revolve around their central planet can cause pockets of water to become pressurized, potentially leading to eruptions.

A diagram of the interior of Enceladus and how its plumes may form.

A possible explanation for the formation of Enceladus’s plumes is based on the moon’s internally generated heat caused by tidal forces. Attribution: Surface image from NASA/JPL-Caltech/Space Science Institute, interior image from LPG-CNRS/Université de Nantes/Université d’Angers, graphic design by ESA.

The alternate concept, introduced by Fagents in 2003 and expanded upon by Elodie Lesage of the Jet Propulsion Laboratory, focuses on the solidification of water and its resulting expansion, which can exert significant force. This phenomenon, known as frost wedging, is capable of destroying mountains on Earth. On a moon with an oceanic surface like Europa, where pockets of liquid water may exist beneath the icy exterior, the expansion of ice would cause an increase in pressure within these pockets.

A diagram of how cold geysers (cryovolcanism) on Enceladus might work.

A different explanation for the functioning of cryovolcanism on Enceladus involves the solidification of water. When water freezes and turns into ice, its size and the force applied to it expand, causing cracks to form and release the pressure, similar to the plumes observed. Image credit: Cflm001/Wikimedia, CC BY-SA 3.0

Quick explained that there are pockets of melted substance in the Earth’s crust, and these pockets experience an excess of pressure. It is believed that these pockets cannot simply adjust to the pressure within the brittle ice, so the only solution is for fluid-filled cracks to form and release the pressure by rising to the surface.

According to Fagents, there are numerous types of features that could potentially be caused by cryovolcanism, with the exception of Enceladus which is our only clear evidence.

There are an abundance of questions but not enough answers.

There are a multitude of inquiries, yet insufficient responses.

According to Fagents, scientists currently have more inquiries than answers.

She stated that individual puzzle pieces are functional on their own, but it is unclear how they fit together and if they actually contribute to comprehending the process of volcanic activity from the depths to the surface.

Some of those puzzle pieces include how fractures in the ice are kept open to allow for eruptions to occur. “One of the sticking points is the fact that it’s so hard to get throughgoing fractures in the ice shell, especially for the thicknesses of ice shells we’re looking at,” explained Fagents. “Clearly Enceladus manages it, but with thicker ice shells, you have static pressure wanting to close any fractures that open up.”

One issue is the process of creating planetary characteristics that contain liquid water or icy slurries. The expert stated, “Liquid water has a low viscosity and cannot create relief.”

Another intriguing aspect of the moons is the age of their frozen surface features. Research has shown that the surfaces of these watery worlds are actually younger than initially expected when Voyager 1 and 2 were sent on their journey to the outer reaches of the solar system.

The giant volcanic mountain Doom Mons on Saturn’s moon Titan, suggestive of relatively recent cryovolcanism, dominates the lower portion of this image from Cassini. Credit: NASA, Public Domain

Quick stated that the subsurface liquid on Europa contains salts, minerals, and other impressive components. She then discussed how to analyze the surfaces of icy moons, stating that a low albedo feature, which appears dark, indicates a recently formed and youthful surface.

According to Quick, the Cassini-Huygens mission captured images on Titan that suggest a possible release of ammonia-rich liquid from the Doom Mons volcano. She noted that although the flow was probably solid when observed, it provided evidence of potential cryovolcanism activity in recent times.

Things may become even more strange. Vance believed there may be a form of volcanic activity caused by denser, salty fluids on planets with thick icy layers above a liquid ocean. This type of activity is referred to as “inverted volcanism” and is not entirely unfamiliar on Earth. While it is short-lived in Antarctica due to the small volumes of brine that can flow, it is interesting to consider the possibility of large amounts of brine accumulating in a 30-kilometer ice shell on Europa. This could be due to strong chemical gradients, particularly if there is oxygen-rich ice flowing into a relatively reducing ocean on Europa.

“Ice, volcanoes, and life are all interconnected in shaping the Earth’s surface and environment.”

The Earth’s surface and environment are shaped by the interconnected forces of ice, volcanoes, and life.

The speculation surrounding this topic led to the most intriguing inquiry: Is there potential for life? To address this, we must gain a comprehension of the interplay between the solid, liquid, and frozen elements in these underwater environments.

Quick explained that moons such as Europa and Enceladus, which have oceans on top of rocky mantles, also likely have a seafloor. If the rocky material is warm, it is probable that there are hydrothermal vents and similar chemical processes occurring at the seafloor, similar to those found on Earth.

A NASA illustration of hydrothermal vents on the ocean floor of Enceladus

This image shows the possible interaction between water and rock at the seafloor of Enceladus, Saturn’s frozen moon, according to NASA’s Cassini mission scientists. Credit: NASA/JPL-Caltech/Southwest Research Institute, Public Domain

This is the next frontier.

Quick stated that in order for life to exist, there must be a source of energy, liquid water, and organics. In the cases of Europa and Enceladus, organic compounds have been detected, along with evidence of liquid water and tidal heating.

It appears that the necessary conditions for supporting life, or at least making a place habitable, may exist in the outer regions of our solar system. The possibility of life developing on watery planets such as Enceladus, Europa, or Titan hinges on the intersections of various key factors.

According to Vance, the outer solar system lacks significant activity in the open ocean and mid-depth regions due to the absence of strong energy gradients. However, potential areas for further exploration could be in locations where chemical and energy gradients are present and can drive chemical reactions. These interfaces may include the boundaries between rocky interiors and oceans, or between oceans and icy shells on ocean worlds.

Jennifer Glass, a faculty member in the Department of Earth and Atmospheric Sciences at Georgia Institute of Technology, tackled the topic of potential habitability and extraterrestrial life from a unique perspective.

According to her, the primary method of increasing biomass is through respiration. This involves having a supply of electrons (known as a reductant), a destination for those electrons (an oxidant), and transporting the electrons across a membrane while also pumping protons.

This respiration process may have occurred on early Earth at hydrothermal vents, with volcanism and magmatism making it possible. Tidal heating may play that role on ocean worlds like Enceladus, Europa, and Titan. Without it or another “source of internal heat of some form, this is all off the table,” said Glass. “You need heat and a source of reduced compounds…. Environments like vents would also be advantageous because you get strong gradients.”

Glass believes that, besides Earth, ocean worlds may have the most potential for supporting life in our solar system. However, this idea becomes more complex when considering hydrothermal vents, which are dependent on oxidants from the surface even though they exist deep underwater.

A significant research conducted by Eric Gaidos and his team at the University of Hawai’i at Mānoa revealed the difficulty in establishing life on a frozen ocean world. The authors pointed out that even if Europa’s interior is geologically active, the energy-producing chemical reactions essential for terrestrial organisms, such as methanogenesis and sulfur reduction, would not be accessible to potential life forms. They also noted that carbon and sulfur would be released in reduced forms rather than oxidized forms.

Cryovolcanism plays a crucial role in recycling energy and organic materials within the shallow subsurface.

Possible rewording: Hypothetical creatures could potentially rely on sustenance from the sky rather than traditional food sources. According to Quick, essential elements such as water, energy, and organics are affected by cryovolcanism, which helps to redistribute them in the shallow subsurface.

The significance of cryovolcanism for the possibility of life on ocean worlds lies in its ability to maintain a warm environment on typically cold moons. This could potentially create habitats for life within their icy exteriors, distinct from the effects of silicate volcanism. As Quick explains, cryomagmas flowing through the ice shell create warm and briny pockets of water, making them potential locations for life.

Fagents concurred, stating that the transportation of fluids through an ice shell is crucial as it allows for nutrients to come into contact with habitable environments.

The water pockets may serve as a home for organisms. According to Glass, there are bacteria present in gas clathrates on our planet. These clathrates can be seen as a sort of frozen environment. On a planet with an ocean, any melted water pockets could potentially support these bacteria.

Exploring the Outer Solar System and Beyond by diving deeper

Venturing into the Depths of the Outer Solar System and Beyond

Quick stated that these worlds have significantly altered our perspective on the factors that determine a habitable planet and the location of such a planet in relation to its star.

Planets that are suitable for life can orbit around massive planets, creating enough heat for volcanic activity and the presence of liquid water.

Every day, the James Webb Space Telescope provides valuable information to astronomers about exoplanets. In the next ten years, several missions will be launched to explore the outer solar system. NASA’s Europa Clipper will orbit Europa, Dragonfly will land on Titan, and the European Space Agency’s JUICE will closely observe Callisto, Europa, and Ganymede with unprecedented detail.

Engineers and technicians lift the tall core of NASA’s Europa Clipper spacecraft.

The Europa Clipper, a mission set to launch in October 2024, is among the various missions going to explore the ocean worlds in the outer reaches of our solar system. Attribution: NASA/JPL-Caltech, Public Domain

Scientists are eager to compare the latest data with data obtained from previous missions. According to Quick, if Europa Clipper observes surface features that were previously identified as potential cryolava flows or plume deposits by the Galileo spacecraft, and these features have either grown in size or have maintained a low albedo, it could indicate that cryovolcanism has been a frequent event and eruptions have been happening periodically since the initial imaging.

Studying the frozen landscapes and active volcanoes of the outer regions of our solar system can also provide valuable insights into the history and formation of our own planet. According to Quick, the potential to explore destinations like Saturn’s moon Titan is like looking back in time on Earth, and it is a remarkable feat that we have the technology to do so.

—Science Writer Erik Klemetti (@eruptionsblog)

Reference: Klemetti, E. (2023), The Melodic Mixture of Ice and Fire in Cryovolcanism, Eos, 104, https://doi.org/10.1029/2023EO230365. Published on September 25, 2023.

Text © 2023. The authors. CC BY-NC-ND 3.0

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