Scientists have been able to recreate the extreme conditions found on icy moons in deep space - and revealed the unstable behaviour of water.
In the near-zero pressure environment of space, water reacts very differently from how it does on Earth. It simultaneously undergoes both boiling and freezing.
The icy moons are covered in an ice exterior with liquid oceans existing below the ice crust. Just as lava through volcanic activity reshapes the Earth’s surface, water reshapes icy moons through a process called cryovolcanism.
To understand how the altered behaviour of water might be driving geologic change on the icy moons, researchers from the University of Sheffield, the Open University and the Czech Academy of Sciences used a specially-constructed low-pressure chamber to create the near-vacuum like conditions found on Europa and Enceladus.
Europa is the icy moon that orbits Jupiter. Enceladus orbits Saturn.
Both the icy moons have a frozen exterior. On Enceladus the temperature at the equator is -193 degrees C. Astronomers have seen evidence of giant jets of water vapour and water particles being vented or ejected into space by a volcano-like process known as explosive cryovolcanism.
There is an allied process called effusive cryovolcanism, where liquid is released as a flow on the surface of the icy moons - akin to a lava flow found on Earth - although evidence for such activity is hard to detect.
The research team wanted to see if they could identify how effusive cryovolcanism happens by studying the behaviour of water in a near vacuum environment. The findings are published in the journal Earth and Planetary Sciences Letters.
They used a low-pressure chamber - ‘George’ , the Large Dirty Mars Chamber, housed at the Open University. For the first time, scientists were able to run experiments with relatively large volumes of water and through observation ports, filmed what was happening.
As pressure inside the chamber was lowered, the water began to bubble and boil, despite being cold. Boiling created vapour which transported heat away from the water, and the water cooled, reaching its freezing point - and floating pieces of ice formed. They continued to grow in size, with new ice forming around their edges.
Within a few minutes, most of the water was covered by thin ice.
Below the ice covering, the liquid water continued to boil, with bubbles breaking through or deforming the ice layer, allowing water to effuse or escape through cracks onto the ice surface. Earlier studies involving much smaller volumes of water suggested thick ice would form and rapidly seal off the water to prevent further boiling.
Dr Frances Butcher, Research Fellow in the School of Geography and Planning at the University of Sheffield and one of the study’s authors, said: “The ice layer that forms is weak and full of holes and bubbles.
“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. 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.”
On Earth, water follows well-known physical rules: it freezes below 0 °C and boils above 100 °C.
Dr Petr Broz, from the Institute of Geophysics at the Czech Academy of Sciences and lead author of the study, said: “We found that the freezing process of water under very low pressure is much more complex than previously thought.
“In such conditions, water rapidly boils even at low temperatures, as it is not stable under low pressure. 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 vapour bubbles, which lift and fracture the ice, significantly slowing down, complicating, and prolonging the freezing process.”
The researchers hope their investigation will help identify ancient signs of cryovolcanic activity not only on icy moons but across other celestial bodies in the Solar System.
The process the scientists observed of bubbles rising up and deforming the ice cap resulted in an uneven ice crust with bumps and depressions.
Manish Patel, Professor of Planetary Science at the Open University, who supervises the Mars simulation facility, said: “These topographic irregularities - caused by trapped vapour 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.
“This could provide valuable clues for planning future missions to these remote worlds—and help us better understand the still mysterious process of cryovolcanism.”
The research was funded by the Czech Science Foundation.
