The song of ice and fire of cryovulcanism.

This translation of an Eos article is approved.

“Can you imagine ‘more water than on Earth’ being a common occurrence in other parts of the solar system?” asked Steve Vance, planetary scientist at NASA’s Jet Propulsion Laboratory and project scientist for the agency’s Europa Clipper mission.

Models suggest that their interiors may contain enough heat to cause a rock to melt.

“What remains to be seen is the extent of the thermal and dynamic relationship between silicic volcanism in the interior and what happens above it,” explained Sarah Fagents, planetary volcanologist at the University of Hawaii at Mānoa.

Criovulcanismo

Actually, these moons can be the source of not only one but two types of vulcanism: silicic vulcanism and its distant cousin, cryovulcanism.

Cryovolcanism is a process in which a volcano erupts volatile substances such as ammonia (as opposed to silicates like feldspars) in an environment below the freezing point of the volatiles. In the outer solar system, the surfaces of most objects are so cold that water is always frozen. This is partly why images of potential cryovolcanism from NASA’s Voyager, Galileo, and Cassini missions were so surprising to scientists.

It is highly likely that the plumes of Enceladus are examples of cryovolcanism, which may contribute to the formation of structures such as the ridges of Europa and the mountains of Titan.

Fagents was studying terrestrial “basaltic lavas and explosive objects” when he was reminded by his boss to observe the surface characteristics of Europa. It was amazing. Looking at Europa’s surface, you can see such a diverse range of features… There are these small dome-shaped elements and flows that really look like there were fluids on the surface.

For Fagents and other vulcanologists, the dome and flow forms on Europa are reminiscent of the well-known lava domes and flows found in places like the Cascades or Yellowstone in North America. However, the surface of Europa is entirely composed of ice, and according to current geophysical data, there are no rocks for hundreds of kilometers beneath its icy crust.

“There are features on the surfaces of these bodies that do not have a good terrestrial analogue,” explained Fagents. “We are trying to create silicic volcanic analogues, but I don’t think they are necessarily the best.”

This alternative to silicic volcanism bridges the gap between disciplines. “There are many things we can learn from the fields of glaciology and hydrogeology that may be more relevant than vulcanology,” Fagents stated. “It is necessary to incorporate a diversity of experiences and perspectives” in order to better understand this phenomenon.

Teorías de fracturamiento

Lynnae Quick, a planetary geophysicist at NASA’s Goddard Space Flight Center and a scientist for the Europa Clipper mission, was one of the individuals whose experience helped connect scientific observation with the scientific process. “While studying for my graduate degree in physics, Louis Prockter [planetary scientist] offered me an internship at APL [Applied Physics Laboratory at Johns Hopkins University], so I spent a summer studying Europa,” Quick recalled. “I really fell in love with it. I found a connection between planetary science and my knowledge of physics by considering the dynamics of geophysical fluids on Europa.”

Quick was intrigued by the mechanisms of cryovolcanism because, when compared to terrestrial volcanism, the phenomenon can seem counterintuitive. Silicate magma tends to be less dense and more buoyant than the surrounding rock, causing it to rise and erupt. Liquid water, on the other hand, is denser than its solid counterpart. (Anyone who has observed ice in a drink has seen this contradiction in action: ice floats.) Therefore, the idea that liquid water in Europa’s icy crust could rise and erupt over the ice was difficult to imagine.

According to Fagents, vulcanism is a purely endogenous process driven by melting and buoyant ascent, but cryovulcanism is a bit more complex than that. There must be fractures in the ice layer and then a mechanism that allows liquid to flow through those fractures. The difference in density may not be significant, but it would be enough to keep ocean water in place, especially if it is saltier and denser than regular water.

Two main theories have emerged to explain how liquid water (cryomagma) can erupt, and once again tidal forces come into play. The first theory involves the stretching and contracting of tides experienced by ocean worlds as they orbit their parent planet. These tidal processes can cause water reservoirs to become pressurized at certain points in the moon’s orbit, potentially triggering eruptions.

The second theory, proposed by Fagents in 2003 and subsequently developed by Eloide Lesage from the Jet Propulsion Laboratory, suggests that the transition of water to a solid state and the resulting increase in volume can be a powerful force. On Earth, for example, it can destroy mountains through frost weathering. In an ocean world like Europa, where pockets of liquid water may be present in the icy layer, the expansion of ice would increase the pressure in these pockets.

“According to Quick, there are small areas of melting in the crust, with excessive pressure acting upon them. We assume that these spots will not necessarily deform within the brittle ice to accommodate the excessive pressure. The only way to alleviate the pressure is by having fractures filled with liquid that are formed by the pressurization of these spots as they rise to the surface.”

Fagents stated, “It is surprising how many types of structures we see that could potentially be cryovolcanic. However, the only irrefutable evidence we have is from Enceladus.”

More questions than answers.

According to Fagents, scientists currently have more questions than solutions.

“We know that the puzzle pieces function independently,” he said, but “we don’t know how they fit together, or even if they do, in order to understand vulcanism from the depth to the surface.”

Some of these pieces include how fractures are kept open in the ice to allow for eruptions to occur. “One of the controversial points is the fact that it is very difficult to have fractures that penetrate through the ice layer, especially for the thicknesses of ice layers that we are observing,” explained Fagents. “Enceladus clearly achieves this, but with thicker ice layers, there is a static pressure that wants to close any fracture that opens up.”

Another issue is the mechanism by which planetary structures form with liquid water or liquid ice. “Liquid water has a very low viscosity,” he added. “You can’t create relief with water.”

Another puzzling characteristic of moons is the age of their icy surfaces. Data has revealed that the surfaces of these oceanic worlds are much younger than scientists anticipated when Voyager 1 and 2 first headed to the outer solar system.

“In Europe, we are aware that the liquid in the subsurface contains salts, minerals, and all these wonderful things,” Quick explained as he described how to interpret the surfaces of icy moons. “If we see an element with a low albedo – meaning a relatively dark surface – we know that it is quite recent, quite young.”

Quick noted that, on Titan, Cassini-Huygens captured an image of what could potentially be a fluid rich in ammonia erupting from the volcano Doom Mons. This fluid was likely solid at the time, he mentioned, but still indicated recent cryovolcanism.

Things could get even stranger. Vance considered the possibility of “brine volcanism” driven by denser and saltier fluids on worlds with thick layers of ice over a liquid ocean. This is known as “inverse volcanism,” and it is not entirely unheard of on Earth. These phenomena are short-lived in Antarctica, but that is because the volumes of material that brines can penetrate are small. It is intriguing to imagine, in a 30-kilometer layer of ice on Europa, some mechanism for accumulating large quantities of brine – strong chemical gradients, especially if, as in the case of Europa, there is oxygen-rich ice flowing towards a relatively small ocean.”

en el planeta

Ice, volcanoes, and life on the planet.

All of this theorizing hinted at the most fascinating question of all: Could there be life? Answering this question requires us to understand how the components of rock, water, and ice are interconnected in these ocean worlds.

Quick explained, “If you think of moons like Europa and Enceladus, where there are oceans sitting on top of rocky mantles, that means they have a seafloor.” He continued, “If that rocky material is somehow hot, we expect there to be hydrothermal vents and the same type of chemistry we see at the bottom of Earth’s ocean.”

That is the next frontier.

“We are aware that a source of energy is necessary for life. We also require liquid water and organic compounds,” stated Quick. “For places like Europa and Enceladus, we have detected organic compounds, confirmed the presence of liquid water, and observed tidal heating.”

Therefore, the pieces can be in place for life, or at least for habitability, in the outer solar system. The question of whether life could arise on ocean worlds like Enceladus, Europa, or Titan boils down to where these different components come together.

According to Vance, in the outer solar system, not much happens in the open ocean and middle depths because that is not where the strongest energy gradients are found. More promising areas can be found where chemical and energy gradients occur, driving chemical reactions. In ocean worlds, these interfaces include the areas between rocky interiors and oceans, or even the boundary between oceans and ice layers.

Jennifer Glass, an associate professor at the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology, approached the issue of habitability and extraterrestrial life from a different perspective.

“The main way to accumulate biomass,” he explained, “is through respiration, that is, having a source of electrons (a reducer) and then a sink for those electrons (an oxidant), and then moving the electrons along a membrane and pumping the protons.”

This respiration process may have occurred in the early Earth at hydrothermal vents, with vulcanism and magmatism making it possible. This role can be fulfilled by tidal heating on ocean worlds like Enceladus, Europa, and Titan. Without this or another “internal heat source in some form, all of this is out of the question,” Glass said. “You need heat and a source of reduced compounds… Environments like hydrothermal vents would also be advantageous because strong gradients are obtained.”

Glass believes that after Earth, the ocean worlds are likely to host the most habitable conditions in our solar system. However, this becomes complicated. “We’re really thinking about a system that is essentially an island in the depths,” Glass mentioned regarding hydrothermal vents, “but it still heavily depends on surface oxidants.”

A significant study led by Eric Gaidos from the University of Hawaii at Manoa and his colleagues discussed the challenges that life would face in an icy ocean world. “Even if the interior of Europa was geologically active, energy-generating reactions such as methanogenesis and sulfur reduction used by terrestrial organisms would not be available for hypothetical life forms,” wrote Gaidos and his co-authors. “Carbon and sulfur would be released as reduced species rather than oxidized.”

Hypothetical life forms may rely on the mana of the surface. “Consider water, energy, and organic compounds,” Quick stated. “Cryovolcanism plays a significant role in allowing energy and organic compounds to circulate through the shallow subsurface.”

“Cryovolcanism is important [for potential life on ocean worlds] in a different way than silicic volcanism, as it keeps normally cold moons warm and could provide places for life within their icy layers,” stated Quick. “We have cryomagmas moving through the ice layers, creating zones of warm, muddy, salty water.”

Fagents consented. “The movement of fluids through an ice layer would also be very significant, as it brings nutrients in contact with habitable environments,” he stated.

These bodies of water could even be their own habitat. “There are bacteria in gas clathrates” on Earth, Glass explained. “You could think of them almost like an ice niche. If there are areas of melted water [in an ocean world], they could be habitable for them.”

Exploring the outer solar system and beyond

Quick mentioned that these worlds have truly altered our thinking about what makes a world habitable and where it must be located around the star.

Habitable worlds can orbit large planets and generate the necessary heat for vulcanism – and liquid water.

The data from the James Webb Space Telescope enhances astronomers’ knowledge about exoplanets every day, and a fleet of missions will head to the outer solar system over the next decade. NASA’s Europa Clipper will orbit Europa, Dragonfly will visit the surface of Titan, and the European Space Agency’s Jupiter Icy Moons Explorer (JUICE) will make unprecedented observations of Callisto, Europa, and Ganymede.

Scientists hope to compare the new data with that collected during previous missions. “If the Europa Clipper spacecraft observes features on the surface that were previously identified as possible cryolava flows or plume deposits when the Galileo spacecraft first took images, and if these features have since grown larger or still have a very low albedo, this could suggest that cryovolcanism has been a regular occurrence and that eruptions have been taking place, at least sporadically, since the features were first captured in images,” explained Quick.

Studying the ice and volcanoes of the outer solar system will also help scientists learn about our own planet and its origins. “When we think about destinations like Saturn’s moon, Titan,” said Quick, “it’s like being able to look back in time…at Earth’s past. And it’s amazing that we have the ability to do so.”

Datos de autor

Erik Klemetti is a science writer known as @eruptionsblog.

Claudia Isabel Sánchez Alva (@Clau_Sanchez48) collaborated with Planeteando and GeoLatinas to make this translation possible.

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

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