Has there been new water created since the world began?

Today, water makes up a huge 71% of the earth's surface but has all this water been around from the beginning? We put Declan's question to Naked Scientist Chris Smith...

克里斯-新水。第一个啊f all, where did all the water on Earth come from in the first place? Because the Earth is about 4.5 billion years old, the stuff that it was made from which was a ring of dust and gas which was forming around the early sun, we've got models of that environment and there's probably not very much water in that. So, all the water on Earth at the moment must have come here from somewhere. The most likely source is comets which are dirty ice balls and also asteroids. Now people have measured the biochemical and chemical makeup of the water on Earth and compared it to the water measurements from comets. The comets they've studied don't seem to have the same chemical makeup. So, we think asteroids, which do appear to have a much closer makeup in terms of the isotope ratio as it's known between the asteroid in the Earth does appear to match much better. So we think that probably, the water we've got, most of it came to the Earth from things hitting the Earth in the first billion years or so after the planet formed. But at the same time, there's a lot of hydrogen here, there's a lot of oxygen on the Earth. The two can react together to produce water which they do all the time. In your body for example and in anything that's alive and respiring, we can take an example. If you burn a molecule of sugar, glucose - C6H12O6 and you burn it with 6 molecules of oxygen, 6O2, the product is some energy. That's what keeps you going plus, 6 molecules of CO2 (carbon dioxide) that you breathe out, plus, 6 molecules of water. You pee those out and you also breathe them out because you breathe out maybe half a litre or so of water every day. So, everything that's alive is doing that process. So, there's lots of water being made every day. So, the water molecules that are here are not the same water molecules but the chemicals that they're made from, the Earth is pretty much a closed system. We're losing a bit of hydrogen off into space which is rearranging molecules to make new ones all the time.