What do extreme icy moons in the Solar System and unruly water behavior have to do with each other? That's what scientists at University of Sheffield in England wanted to know. So, they simulated conditions at Europa and Enceladus in the lab. Europa orbits Jupiter and Enceladus circles Saturn. Both have frozen surfaces and internal oceans of salty water. That water plays a huge role in resurfacing and reshaping these icy moons. That process is called cryovolcanism and it shows that water behaves much differently "out there" than it does here on Earth, where it freezes below 0 C and boils above 100 C.
Cryovolcanism Affects Icy Worlds
Here on Earth, we see volcanism as the flow (or distribution) of molten lava (and its associated clouds of gas and rock) across (and under) the surface. It turns out water ice can do similar actions, under the right conditions. That's called cryovolcanism and planetary scientist Paul Geissler defined it as, "The eruption of liquid or vapor phases (with or without entrained solids) of water or other volatiles that would be frozen solid at the normal temperature of the icy satellite's surface."
Essentially cryovolcanism "redistributes" water, ammonia, and methane ices from the inside of an icy world to its surface and beyond. Europa and Enceladus are two very good examples of cryovolcanism in the Outer Solar System. We also see it at other icy worlds such as Ganymede, Callisto, Triton, and Pluto. The process requires incredibly cold temperatures. Enceladus, for example, has a surface temperature of -193 C. Water geysers send water vapor and ice particles out to space from deep beneath the surface in an example of explosive cryovolcanism. At Europa, the surface temperatures range from -160 C to 220 C, which turns the surface ices as hard as rock.
In addition to explosive cryovolcanism, there's another form called effusive cryovolcanism. In that process, ice flows like lava, rather than being blasted out from under the surface. That's the form that the Sheffield team, led by Petr Broz, wanted to understand, since it appears that water freezes and boils at different temperatures, leading to the cryovolcanism we see. They wanted to know in particular how effusive volcanism could occur. So, they created a simulation of how water acts in a low-temperature, near-vacuum environment like those at the outer icy moons.
Building a Cryovolcanism Simulator
The team used a low-pressure chamber nicknamed "George" and tested the environment by filming huge amounts of water flowing through the device. They lowered the pressure to the point where the water began to bubble and boil, even under low temperature. That's the "unruly" behavior of water. You'd expect it to NOT boil or bubble, but it did. The boiling created water vapor, which continued to cool the water down. That's when the team saw pieces of ice start to form in the boiling water. Eventually, an icy surface formed and amazingly enough, the water underneath continued to boil, according to team member Frances Butcher.
“If the ice was stronger, it would likely seal off the liquid water below and prevent further boiling. But our experiments show that as the water boils, the gas that is released gets trapped under the icy crust," said Butcher. "Pressure builds, the ice cracks, the gas escapes, and liquid water can briefly seep through the cracks onto the surface of the ice—only to be exposed again to the low-pressure environment. As soon as new fractures appear, water begins to boil again, and the entire process repeats itself.”
The simulation with George showed that water's freezing process changes drastically at low pressure, Broz explained. “In such conditions, water rapidly boils even at low temperatures, as it is not stable under low pressure," he said. "Simultaneously, it evaporates and begins to freeze, driven by the intense cooling effect caused by the evaporation itself. The ice crust that forms is repeatedly disrupted by vapor bubbles, which lift and fracture the ice, significantly slowing down, complicating, and prolonging the freezing process.”
Applying George's Simulation to Outer Worlds
This set of experiments should help planetary scientists in the search for other cryovolcanic sites in the Outer Solar System. That's because the experiment showed that the rising bubbles also deformed the newly formed ice cap in the chamber. The result was an uneven ice crust with bumps and ridges, possibly similar to surfaces of distant icy moons and other objects in the Solar System.
According to Manish Patel, team member and professor of planetary science, the surface effects could be easily identifiable. “These topographic irregularities—caused by trapped vapor beneath the ice—may leave distinct signatures that could be detectable by orbiting spacecraft, for example, by those equipped with radars, offering a potential new way to identify ancient cryovolcanic activity," he said. "This could provide valuable clues for planning future missions to these remote worlds—and help us better understand the still mysterious process of cryovolcanism.”
For More Information
The Complexity of Water Freezing Under Reduced Atmospheric Pressure