How can water (in a frozen state) exist in space if there is no oxygen in space?

Answer

What colleague Sintubin states is that there is no molecular oxygen (O2) in space. But the element oxygen is ubiquitous, even after hydrogen and helium it is the most abundant element in the universe.

In what forms does oxygen occur in the interstellar environment? In the solid phase it is in all types of ice and dust grains. Liquids are not present, the density (and thus the pressure) is too low for that. In the gas phase, it occurs in an ionized state (where one or more electrons have been expelled by radiation), as a neutral atomic gas, or in all kinds of molecules.

How interstellar chemistry works is a very interesting area. The chemistry is intrinsically the same as on Earth, but the conditions are completely different: the medium is very thin, there is a mixture of dust and gas, it is embedded in the UV radiation of stars, and the composition is dominated by hydrogen (72% by mass) and helium (26%).

Since helium is chemically inactive, it is clear that the most abundant molecule must be H2, and it is. But it is not clear how that molecule is made. Any reaction of the form A + B -> AB must satisfy two conservation laws, that of momentum (mass x velocity) and that of energy. Since there is only one end product, and impulse and energy are connected, this means two conditions for only an unknown person, and that does not happen just like that. The solution is to give off the excess energy to a neighboring particle, but that is not possible in the very thin interstellar center: if two particles collide, it is very unlikely that a third particle will be nearby. The solution is brought by the dust particles: it starts with an H atom sticking to one dust particle, a little later another one joins, and then both H atoms go together, giving off their excess energy to the dust particle.

Once we have H2, we can move on. But not via H2 + O -> H2O, because that is again of the form A + B -> AB. Well via, for example, H2 + O -> H + OH, and then H2 + OH -> H + H2O. That channel is very slow. It goes faster if we work via ions (O+ instead of O), and there are, because of that UV radiation that shoots electrons from the atoms; after all, a chemical reaction is electromagnetic in nature and proceeds more efficiently if the particles feel each other’s charge from afar.

There is a whole literature (and a lot of contemporary research) about all kinds of chemical networks in the interstellar center, to explain the occurrence of all kinds of molecules. After H2, CO and H2O are the most abundant molecules: water is a compound of the most abundant element with the most abundant reactive element. Those networks are also useful to determine the conditions of the chemistry: at different densities, temperatures, …, the end products will be different, and by studying those end products, we can then gauge the conditions in different places.

A very interesting problem in astrochemistry is the formation of O2. According to several chemical networks, a significant fraction of the oxygen should still exist in the form of O2, and that is clearly not the case.

So far we have only discussed the chemistry in the gas phase, but the dust is also involved. Where it’s cold, water vapor tends to settle on cold solids like ice (like on our car windows in winter), creating immense clumps of ice like comets. Indeed, it is quite possible that most of Earth’s water ended up here through comet impacts.