How stuff works: Is water eternal?
It's one of those "fun facts" you often hear about: "All the water around today was already around when dinosaurs lived on the earth! You might be drinking recycled dino pee!"
But is this actually true? Turns out, not so much, but it depends a bit on exactly what we consider "water".
On the most basic level, water is a substance with the chemical formula H2O - that is, each water molecule has 2 Hydrogen atoms and one Oxygen atom. These molecules are most commonly arranged in one of three forms: when it is cold, as a crystal (ice), when it is very hot as a gas (steam) and most of the time as a liquid. Even at moderate tempteratures, some of the water molecules will often enter a gaseous state (sort of) by evaporating, only to then bond together, form a liquid again and rain back down. None of this is particularly new. So there's an eternal cycle of water just changing from liquid to gasoues and back, and sometimes going solid when it's cold, but the total amount stays the same, right?
Well, no. Like almost all chemical substances, water can also be part of chemical reactions where some of the component atoms split apart and sometimes bond with other atoms. One of these is so common that almost all life on earth utterly depends on it - Photosynthesis. That's right - when plants create oxygen from sunlight, what they actually do is take carbon dioxide and water and turn it into oxygen and carbohydrates. When animals use those carbohydrates as food, they reverse the process - so water is constantly destroyed and created. Other chemical reactions that lead to water being destroyed or created also exist, of course, for example when the oxygen in water bonds to iron creating rust (there's more to it, but that's the Cliff notes version).
But at least the hydrogen atoms are around forever, right? They are the most important part, so that still counts, yes?
Well, no, not even them. Hydrogen is a particularly simple chemical element - a hdrogen atom usually consists of only one proton and one electron. Most other elements also have neutrons in the mix, which hydrogen usually doesn't (if it does, we get radioactive heavy water). And single protons, as it turns out, are sometimes produced by radioactive decay, and also quite common in space, for example in what is known as solar wind. Earth picks up some of these hydrogen atoms that float in from space, some some of that water you are drinking may have been a relatively recent arrival from the sun!
Published at NZDT