Watching a pot of water boil is one of the most familiar as well as one of the most fundamental processes that occur on earth. It is so simple that individuals do not even have to think about how to do it or about what makes it happen. Almost every adult knows that water on earth boils at 100° C (212° F). However, this same process becomes more complex and confusing when seen in space. One would, after all, expect water to freeze instantly out there because of it is so cold. In fact, however, temperature is not the only factor that determines the state of matter for water, or any other substance, at a given moment.
It should be noted that physicists have a hard enough time understanding the way fluids behave even here on earth, but the basic physics of it run as follows. The reason water boils at a particular temperature on earth, or anywhere in the universe, is because of the atmospheric pressure as well as the temperature of the air. Space, by contrast, is essentially a large vacuum with no pressure. In a vacuum there is really no temperature because there are no gas molecules. Therefore, a liquid will respond differently in space than it does on earth. A liquid brought into space will instantaneously boil as soon as the lid of its container is removed. This is because there is no pressure to hold the water molecules in place.
However, it is interesting to note that no application of temperature is needed to make the water boil. With no pressure to hold the liquid in the container, the liquid will immediately boil and leave the container. As soon as it has left the container, it will instantly form a solid. This interesting phenomenon was not known until 1992 when NASA scientists performed this experiment in space and filmed it. What can be seen in the footage is one huge bubble in the liquid instead of the hundreds of tiny ones, which can be seen on earth when boiling a liquid. Liquids in space certainly react differently than liquids found on earth.
An aside: sublimation
Another noteworthy property of water that can be noted in space is sublimation, which occurs when a solid goes directly into the gas state without passing through the liquid phase at all. This is what happens to water on Mars and it also happens to gray arsenic (the stable form of that element), iodine and Dry Ice on earth. Sublimation takes place because the atmospheric pressure at the triple point of water (the temperature at which all three states of a given substance exist together in thermodynamic equilibrium) is too high to permit it to exist as a liquid.
Now let us go back to the main topic, the boiling of water in space, and go into greater detail about what happens.
The boiling point of any liquid decreases as the pressure decreases. Indeed, this phenomenon can be observed on our own planet itself: At sea level water boils at 212°, but way up high on the top of Mt. Everest it does so at temperatures closer to 158°. In a complete vacuum (of which, incidentally, there can really be no such thing, except in a black hole) it hardly makes sense to speak of “the boiling point of water” because water will boil at any temperature. Strictly speaking, there is no temperature to speak of in outer space.
On the other hand, you could get solid water at temperatures close to absolute zero in outer space such as exist on Pluto. In such conditions ice would sublime into water vapor as described in the previous section. It would then refreeze. The experiment in a microgravity environment referred to above, in which the phenomenon of water boiling in space was discovered, had to be performed in a spacecraft in which the pressure was kept at or near that found at sea level on earth since that otherwise the astronauts could not survive.
Beyond water: liquid hydrogen and helium and iron rains
The boiling of water in space is just one of the many exotic ways in which matter behaves under conditions that differ greatly from those with which we are familiar. Way down deep in the atmosphere of Jupiter, the pressure is so great that hydrogen becomes a liquid even though it is boiling hot down there. Farther down still, hydrogen actually behaves like a metal: The atoms are crushed so much by the pressure that the electrons are separated from their nuclei. This metallic hydrogen layer is responsible for carrying out the electric current that emanates from Jupiter and which was measured by the Voyager spacecraft.
On Saturn, it actually rains helium! The drops create friction with the atmosphere, which is released as heat (just as do ordinary raindrops on earth), which is how Saturn, far though it is from the sun, can radiate twice as much energy as it receives from the parent star. It, too, has a metallic hydrogen layer. Helium, too, is a gas that cannot be frozen by changing the temperature alone.
Astrophysicists can also use their knowledge of theoretical physics and chemistry to predict how known substances will behave under conditions that have never actually been observed although the existence of such conditions somewhere in the universe is conceivable. Stars called brown dwarfs are up to 75 times as massive as Jupiter and the pressures there are much, much greater. They are also extremely hot! Brown dwarfs have atmospheres made of gaseous iron which condenses at lower depths into liquid droplets that are continually raining down out of the upper atmosphere.