On Earth, surrounded by the atmosphere and relatively close to the Sun, water can exist stably in a liquid state anywhere on the surface of the planet. But in space, the conditions are very different:the temperature and the pressure undergo drastic variations. In this context and in such an environment, would the water freeze or boil?
Unlike the surface of the Earth, space is an extremely cold environment, especially when you are far from any star. In addition, the pressure there is extremely low. While standard Earth atmospheric pressure is about 6×10 22 hydrogen atoms exerting pressure on every square meter of the planet's surface, the pressure of space is several million, if not several billion times, less than this.
Beyond the Earth's atmosphere, the temperature and pressure therefore drop relatively sharply. Far from any source of stellar heat, the only temperature bathing space is that of the cosmic microwave background, i.e. -270.45°C. This temperature being well below the freezing point of water, the latter should logically freeze directly in such an environment.
However, temperature is not the only physical property that affects the state of matter. Pressure also plays a vital role. Indeed, if the pressure drops in an environment containing liquid water, it will start to boil. This is an observation that is easily made at high altitude, where water boils below 100°C, because the atmosphere is reduced there and therefore the pressure too. In the same way, liquid water placed in a vacuum chamber will boil quickly and intensely.
A video showing how water boils when placed in a vacuum chamber :
Related topic:Does cold water freeze faster than hot water?
In outer space, where the temperature and pressure are low, does water freeze or boil? The answer is:both. First it boils, and then it freezes.
This observation was made when astronauts in Extra-Vehicular Exit (EVA) around the International Space Station urinated, and this urine was ejected into space:the urine begins to boil violently, then the steam passes in a solid state (a process called reverse sublimation) to end up as a cloud of urine crystals.
The reason for this phenomenon is due to the heat capacity of the water. Indeed, water is a compound that retains heat extremely well. In addition, due to surface tension, water tends to adopt a spherical shape in space, thus minimizing the heat exchange surface with a negative temperature environment.
The freezing process is therefore very slow. It would be practically immediate if each molecule of water were individually exposed to the vacuum of space. In view of the extremely low surrounding pressure, the water therefore immediately begins to boil, changing from a liquid state to a gaseous state. But once transformed into gas, the molecules are much more distant from each other. This means that water vapor (at effectively zero pressure) cools very quickly.
Below -63.5°C, water changes from a gaseous state to a solid state (ice), regardless of the pressure. So in space, liquid water first begins to boil violently and then vaporizes, then rapidly cools until it solidifies, eventually giving rise to a network of fine ice crystals.
