On Earth, water freezes below 0C and boils above 100C. In deep space, however, it can boil and freeze at the same time. This dual process plays a key role in cryovolcanism - the icy moon equivalent of lava flow - which can reshape the surfaces of moons like Enceladus and Europa. Scientists have long observed explosive cryovolcanism on these moons, including geyser-like jets ejecting from Enceladus, but evidence of effusive cryovolcanism has remained elusive.
To better understand this, researchers employed the 'George' Large Dirty Mars Chamber at the Open University. This facility allowed them to work with relatively large amounts of water and monitor how it reacted under low pressure. As the pressure dropped, water began to boil despite being cold, producing vapor that cooled the remaining liquid and triggered ice formation. Floating ice chunks formed and expanded, while trapped gas beneath the thin ice layer created cracks and deformations.
Dr Frances Butcher of the University of Sheffield explained, "The ice layer that forms is weak and full of holes and bubbles... 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... As soon as new fractures appear, water begins to boil again, and the entire process repeats itself."
Dr Petr Broz of the Czech Academy of Sciences noted, "We found that the freezing process of water under very low pressure is much more complex than previously thought... 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."
These results help explain how effusive cryovolcanism might occur on icy moons. According to Professor Manish Patel from the Open University, surface irregularities formed by trapped vapour could be detectable by spacecraft using radar. This could offer new methods to identify cryovolcanic activity and plan future exploratory missions.
Research Report:The complexity of water freezing under reduced atmospheric pressure
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