The complexity of water freezing under reduced atmospheric pressure

The surfaces of many icy bodies in the Solar System have been resurfaced by cryovolcanism, during which liquid and vapour are released from the subsurface into cold, near-vacuum conditions. Water is one of the most commonly released liquids, but it is not stable at low pressure – boiling near the water surface causes rapid cooling and induces surface freezing. Despite previous theoretical works and laboratory experiments it remains unclear how the three coexisting phases interact. Here we expose large volumes of liquid water (17 and 5 litres) to low pressure to study how the phase transitions interact in the near-surface layer, and what controls the dynamics of the system. We observe that subsurface boiling and associated bubble formation significantly affects the rate and manner of freezing. Ascending vapour deforms the ice and causes it to crack, which releases subsurface pressure. Once the pressure is released, the underlying liquid water is again exposed to the reduced atmospheric pressure, triggering a new cycle of vigorous boiling, bubble formation, ice deformation, and subsequent cracking. Thereby, the period of boiling and freeze-over is prolonged. Additionally, we observe that fracturing and vapour accumulation beneath the ice layer create an uneven surface, characterized by bumps and depressions a few centimetres in height. This shows that ice solidification during effusive cryovolcanic eruptions is likely to be a highly complex process and could leave distinct, observable signatures on and within cryolava ponds and flows.