May 6, 2022: New Experiments Reveal The Potential Freezing Point Of Subsurface Oceans Like The Potential Subsurface Ocean Of Europa Or Titan

A new research work entitled "On the pressure dependence of salty aqueous eutectics", published on April 12, 2022 in Cell Reports Physical Science and proposed by a team of researchers involving Baptiste Journaux reveals the potential limits of subsurface oceans like the potential subsurface ocean found beneath the crust of several icy worlds present in the Outer Solar System. Moons like Europa, Ganymede, Mimas, Tethys or Titan may contain a subsurface ocean beneath their external crust. If the subsurface ocean is dominated by water, one has to determine the potential limits of that ocean or the right combination of pressure and environmental temperature to allow the stable presence of a liquid layer beneath the external crust. The level of pressure above the potential subsurface ocean must be extreme so that any potential liquid can remain liquid even if the environmental temperature is extremely low. The presence of salt in the liquid dominated by water makes the freezing point of the liquid lower and a higher level of pressure can lower the freezing point of the potential liquid. How deep will we have to dig in order to reach the potential subsurface ocean inside Europa, Ganymede or even Titan ?

The new experiments upon the physical limits of any potential subsurface ocean were led by scientists from the University of Washington and from the University of California, Berkeley. The identification of any potential subsurface ocean in the Outer Solar System is fundamental in terms of exobiology in particular. The potential subsurface ocean can be in direct contact with the surface like on the tiny moon Enceladus, a surprising world evolving around the Gas Giant Saturn. There are geysers in the fractures of the south polar region of Enceladus. Those geysers demonstrate that a subsurface ocean dominated by water is clearly possible. Tidal forces must play a key role in the level of stability of the potential subsurface ocean. Baptiste Journaux who is an acting assistant professor of Earth and space sciences at the University of Washington pointed out: "The more a liquid is stable, the more promising it is for habitability." He added: "Our results show that the cold, salty, high-pressure liquids found in the deep ocean of other planets' moons can remain liquid to much cooler temperature than they would at lower pressures. This extends the range of possible habitats on icy moons, and will allow us to pinpoint where we should look for biosignatures, or signs of life."

At first sight, the Solar System appears mostly dry in terms of liquid water. The Earth is clearly an oasis of life where liquid water is abundant at the level of the surface or the crust. However, beyond our planet, there are cratered worlds or dry worlds devoid of any atmosphere that may contain a subsurface ocean dominated by liquid water beneath their external crust. Baptiste Journaux advanced: "Despite its designation as the 'blue marble', Earth is remarkably dry when compared to these worlds." Researchers imagine that there are several types of oceans as well as various types of salts in the potential subsurface oceans. The presumed subsurface ocean of Europa, a major moon of the Gas Giant Jupiter, may be around 100 miles deep. By contrast, the potential subsurface ocean of Titan may be more than 400 miles deep. Baptiste Journaux pointed out: "We know that water supports life, but the major part of the oceans on these moons are likely below zero degrees Celsius and at pressures higher than anything experienced on Earth." Can we imagine extremophiles living in the harsh environment of those oceans where chemical reactions are limited and where the pressure is extremely high ?

Baptiste Journaux explained: "We needed to know how cold an ocean can get before entirely freezing, including in its deepest abyss." The internal structure of the icy or terrestrial worlds found in the Outer Solar System is a major research topic. Is there a core dominated by iron and nickel like inside the Earth for instance ? Is the icy body relatively homogeneous ? Is the icy body a differentiated world like Venus or the Earth ? Is there a viscous liquid layer beneath the external crust ? Many parameters must be taken into account. We must put forward some hypotheses in order to evaluate the potential chemistry or dynamics of the presumed internal ocean. The researchers focused their attention on eutectics or on the lowest temperature at which a salty mixture can remain liquid before completely freezing. The ocean of the Earth is not exclusively composed of water. It also contains sodium chloride (NaCl). That may also be the case in the internal ocean of extraterrestrial worlds like Europa, Ganymede or Titan. In the environment of the Earth, salty water can remain liquid below the freezing temperature of pure water. In the high-pressure environment of the presumed internal ocean of extraterrestrial worlds found in the Outer Solar System, the freezing point of the liquid may be much lower.

The experiments performed by the group of researchers mobilized the UC Berkeley equipment, an equipment whose first mission was the cryopreservation of organs for medical applications and for food storage. The simulations performed by the team bring a clear idea upon the potential characteristics of the presumed internal oceans found in the Solar System. Baptiste Journaux who is a planetologist and who is an expert on the physics of water and minerals collaborated with UC Berkeley engineers to evaluate solutions of five different types of salt at pressures up to 3,000 times the atmospheric pressure of the Earth at sea level that is to say 300 megapascals or approximately 3 times the pressure encountered in the deepest ocean trench of the Blue Planet. Matthew Powell-Palm who took part in the research work, who produced the work as a postdoctoral researcher at UC Berkeley and who is also co-founder and CEO of the cryopreservation company BioChoric, Inc. pointed out: "Knowing the lowest temperature possible for salty water to remain a liquid at high pressures is integral to understanding how extraterrestrial life could exist and thrive in the deep oceans of these icy ocean worlds."

Baptiste Journaux is now collaborating with the team of the Dragonfly mission proposed by NASA. The rotorcraft of that mission will explore the surface of Titan in the 2030s, in an area where dunes and a major crater can be found. Will it find clues of an internal ocean dominated by water ? A mission devoted to Europa is also planned for the coming years. That's the Europa Clipper mission, a mission that will start from Earth in 2024. The European Space Agency has also planned a new mission to the moons of Jupiter. That's the Jupiter Icy Moons Explorer or the mission of the JUICE Spacecraft. The probe will evolve in the system of Jupiter to study Ganymede, Callisto and Europa. Orbiters can tell us a lot regarding the surface of any world as well as regarding the dynamics or the composition of the interior of any world. Ganymede is the largest moon in the Solar System and that world is devoid or almost devoid of any significant atmosphere. Ganymede has its own magnetic field like the Earth. Callisto is heavily cratered and its internal dynamics may be more limited than the internal dynamics of Ganymede. The surface of Europa is surprisingly young with a complex network of fractures.

Titan appears more complex than Ganymede at first sight because Saturn's largest moon contains a deep and thick atmosphere that appears completely opaque from outer space in the visible spectrum. Titan looks like the Earth to a certain extent. Titan contains lakes, seas and rivers in its high latitudes but those lakes, seas or rivers are not composed of liquid water. Those lakes, seas and rivers are dominated by liquid methane. Methane can appear in its liquid form on the surface of Titan like water on the surface of the Earth. Is there a liquid layer dominated by methane beneath the external crust of Titan ? That's a major question because planetologists try to understand how Titan can keep a significant concentration of methane in its atmosphere in the long run. Methane tends to be destroyed by ultraviolet light from the Sun. The external crust of the giant moon may be relatively rich in water. In the harsh environment of Titan, water can form stones or pebbles on the surface. Planetologists would like to find cryovolcanoes on Titan. Those cryovolcanoes could tell us a lot regarding the composition of the potential subsurface ocean present beneath the external crust of that intriguing world.

We know that most moons of Saturn are dominated by water ice. Therefore, Titan may also be rich in water ice but beneath its external crust, the physical conditions in terms of pressure and temperature may be right for the stable presence of a liquid layer dominated by water. The exact composition of the presumed extraterrestrial oceans in the Solar System is clearly a major question in terms of planetology, chemistry, geology or exobiology. Baptiste Journaux argued: "The new data obtained from this study may help further researchers' understanding of the complex geological processes observed in these icy ocean worlds." The research work based on the experiments upon the physical limits of the presumed subsurface oceans in the Outer Solar System also mobilized Boris Rubinsky, Brooke Chang, Anthony Consiglio, Drew Lilley and Ravi Prasher who are from UC Berkeley. The investigation was financed by the National Science Foundation and NASA. Are oceans dominated by liquid water widespread in the Cosmos or even in the Solar System ? One can advance that the ocean worlds composed of a subsurface ocean are more frequent than ocean worlds composed of an ocean on the surface because that type of world implies a relatively significant atmosphere and a relatively limited gravity.

The image above reveals, from the left to the right, Ganymede, Europa and Titan at scale. Those worlds evolving in the Outer Solar System may contain a subsurface ocean. Planetologists try to determine the nature, the dynamics or the characteristics of those hypothetical internal oceans. What is the exact composition of the liquid or what is the freezing point of the liquid ? Key questions must be answered ! Credit for the original view of Ganymede: NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill. Credit for the original view of Europa: NASA/JPL/DLR. Credit for the original view of Titan: NASA/JPL-Caltech/Space Science Institute. Montage credit: Marc Lafferre, 2022.

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