Layered bubble-rich structures on icy worlds: near-vacuum experiments with large volumes of low-salinity water

• Near-vacuum freezing of water results in a layered, highly porous ice. • Its thickness is limited by a balance between its weight and the equilibrium vapor pressure. • Regions of cryo-effusions should have high porosity and are dangerous for landing. • In low-gravity environment, the phyllo-cellular ice could be tens of meters thick. Icy bodies of the outer Solar System display evidence of remarkable surface activities, including explosive and potentially effusive cryovolcanism. When water is released into their extremely low-pressure surface environments, or when the surface ice melts in response to an impact, the liquid water has traditionally been expected to form solid ice with a porous outer layer. Here, we expose 40 kg of low-salinity water in a specialist chamber at The Open University, UK, and show that freezing under near-vacuum conditions is a complex, dynamic process during which vapor puffs through the growing ice sheets, building previously unobserved ice structures. Millimeter-thin, sheet-like ice layers form, separated by centimeter-thick, large-aspect-ratio pockets of vapor. The overall height of this layered, bubble-rich ice is controlled by a balance between its weight and the equilibrium vapor pressure. In the laboratory, the height reaches approximately ten centimeters, which could plausibly extend to tens of meters in the low-gravity environments of icy bodies. The high porosity of such ice has significant implications for the interpretation of remote sensing observations, and its fragile character makes terrains created by effusive cryovolcanism risky for landed spacecraft.