THE WATERY POTENTIAL OF ICY WORLDS
Research into the lowest temperature at which a liquid freezes under varying pressures and concentrations could offer new insight into the habitability of distant moons.
Written by Agam Shah
Matthew Powell-Palm, assistant professor of mechanical engineering at Texas A&M University
Baptiste Journaux, assistant professor in the Department of Earth and Space Sciences at the University of Washington
LIQUID WATER IS CONSIDERED one of the most important ingredients for a planet to support life, but there are many variables involved. As scientists continue to search for frozen moons and planets that might have the potential to support life, understanding how and why water freezes on them is a vital question.
In pursuit of an answer, researchers at Texas A&M and University of Washington in Seattle have come up with a concept to determine the physical limits at which liquid water destabilizes and turns into ice. That concept is called “cenotectic,” a thermodynamic point at which liquid water changes its phase state when all planetary conditions are considered.
Planetary scientist Baptiste Journaux, an assistant professor at the University of Washington’s Department of Earth and Space Sciences, first reached out to Matthew Powell-Palm, a thermodynamicist and assistant professor of mechanical engineering at Texas A&M in College Station, on what would become the cenotectic concept after reading a paper about the behavior of pure liquid water under confined conditions at low temperature and high pressure.
“He emailed me and said, ‘hey, there's this really interesting problem you might be intrigued by in icy worlds,’” Powell-Palm said.
Journaux and Powell-Palm described their discovery in “On the equilibrium limit of liquid stability in pressurized aqueous systems,” which was recently published in Nature Communications and funded by NASA.
The term cenotectic was coined by the researchers and draws from an old Greek term eutectic, which means “easy melt.” Cenotectic translates to “universal melt,” Powell-Palm explained.
“To understand anything about the habitability of these planets, we need to understand the state, the amount, and the content of the liquid water on that planet. We can’t do that without understanding how this liquid water behaves at the low temperature, high pressure conditions relative to those planets,” Powell-Palm said.
The thermodynamics concept also considers the salts on icy worlds, which play a role in liquid water reaching a freezing point.
“To understand anything about the habitability of these planets, we need to understand the state, the amount, and the content of the liquid water on that planet. We can't do that without understanding how this liquid water behaves at the low temperature, high pressure conditions relative to those planets.”
—Matthew Powell-Palm, assistant professor of mechanical engineering at Texas A&M University
The isochoric freezing chamber a simple device used to measure the cenotectic pressure-temperature coordinates. Photo: Texas A&M
NASA and the European Space Agency are spending billions on missions to seek out moons and planets with the largest known sources of water in the solar system. The agencies hope to find signs that might be compatible with the emergence of life.
These missions include NASA’s recent launch of the Europa Clipper, an unmanned probe that will reach and investigate Jupiter’s icy moon Europa in 2030 for signs of habitable conditions.
“We have six years to learn as much about the thermodynamic and chemical signatures of the things we might see as possible,” Powell-Palm said. “Once we start getting that data from Europa Clipper, we’re going to have a much better ability to apply these thermodynamic insights to evaluating how much liquid water there is, what its chemical and physical state is, and so on.”
The researchers started by examining the salts that are common to icy worlds, which include potassium chloride, sodium chloride, sodium carbonate, sodium sulfate, magnesium chloride, and urea.
Each salt was tested in isolation, and the researchers discovered that water could remain in liquid form at even lower temperatures—about 22 Kelvin lower—when higher pressures were applied. They were able to formulate a baseline for the cenotectic, or universal melting point, while accounting for both pressure and salts.
Conceptual long term evolution of extraterrestrial oceans toward the “end-game” cenotectic ocean. Image: Baptiste Journaux
But things got interesting in systems with brines that included sodium chloride and magnesium chloride, revealing new ways in which liquid water could behave. The researchers found that liquid water formed new crystalline phases the scientists had never seen before.
“That opened our eyes to the notion that this low temperature, high-pressure parameter space is so unexplored that we don't even know the materials that might exist down here,” Powell-Palm said. “That’s really essential to know if we’re going to accurately interpret any of the data coming from these missions going to these icy worlds.”
Giant oceans also have exterior shells of ice—something commonly found in freezers. Sometimes these icy shells work with gravity on a planet or moon to stabilize liquid water under the surface at lower temperatures than typically possible. A number of variables factor into reaching a cenotectic point, Powell-Palm said.
The researchers used a customized aluminum freezing and melting chamber along with pressure transducers from Ellison Sensors to measure phase changes. Calibration grade circulating coolers from Fluke were used to control temperature in the chamber, Powell-Palm said. MATLAB software helped the team analyze data.
Mechanical engineering skills can easily cross over to planetary sciences, especially temperature-pressure control, high-pressure cells, and cryogenic systems. “By bringing these to bear on the problems of other fields, we can collaboratively reach insights that radically advance the state of knowledge,” Powell-Palm said.
Agam Shah is a business and technology writer in Phoenix and an adjunct faculty member at the Walter Cronkite School of Journalism at Arizona State University.
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