Testing the waters: Analyzing different solid states of water on other planets and moons

 Aside from regular ice, water can exist in the form of peculiar solids called clathrate hydrates, which trap small gaseous molecules. They play a large role in the evolution of atmospheres, but predicting their presence in cryogenic temperatures is difficult. In a recent study, scientists from Okayama University developed statistical mechanics theory to determine their presence in Pluto and some of Jupiter's and Saturn's satellites, providing valuable information to revise existing interpretations.

Just like on Earth, water on other planets, satellites, and even comets comes in a variety of forms depending on multiple factors such as pressure and temperature. Aside from the gaseous, liquid, and solid states we are accustomed to, water can form a different type of crystalline solid called clathrate hydrate. Although they look similar to ice, clathrate hydrates have actually small water-based cages in which smaller molecules are trapped. These trapped "guest" molecules are essential for preserving the crystalline structure of clathrate hydrates, which would otherwise "collapse" into regular ice or water.

Clathrate hydrates play a crucial role in the evolution of a planet or satellite's atmosphere; volatile gases such as methane are stored in these crystals and released slowly over geological timescales. Because of the enormous amounts of time required for clathrate hydrates to form and dissociate at cryogenic temperatures, it has proven very difficult to conduct experiments on Earth to predict their presence in other celestial bodies.

In a recent study published in The Planetary Science Journal, a team of scientists tackled this issue with a combination of both theory and experimental data. Lead scientist, Professor Hideki Tanaka from Okayama University, Japan, explains: "For many years, we have been developing rigorous statistical mechanics theory to estimate and predict the behavior of clathrate hydrates. In this particular study, we focused on extending this theory to the cryogenic temperature range -- down to the 0 K limit."

A notable challenge was theoretically establishing the conditions for the formation and dissociation of clathrate hydrates under thermodynamic equilibrium at extremely low temperatures. This was necessary to use the renowned model of water/hydrate/guest coexistence in clathrate hydrates proposed by van der Waals and Platteeuw in 1959. Tanaka, Yagasaki, and Matsumoto revised this theory to fit the cryogenic conditions that would be found outside Earth and corroborated its validity based on thermodynamic data gathered by space probes.

Then, the scientists used this new theory to analyze the states of water on Saturn's moon Titan, Jupiter's moons Europa and Ganymede, and Pluto. According to their model, there is a remarkable contrast in the stable forms of water found on these celestial bodies. Whereas Europa and Ganymede contain only regular ice in contact with the thin atmosphere, all the water on the surface of Titan, and possibly Pluto, is in the form of clathrate hydrates. "It is remarkable," says Tanaka, "that one specific state of water appears exclusively in different satellite and planetary surfaces depending on temperature and pressure. In particular, the water in Titan seems to be completely in the form of methane-containing clathrate hydrates all the way up to the surface from the top of its subsurface ocean."

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