 Yeah, it's a really interesting question and it's one that in fact I think the thinking about has changed a bit in the last couple of years, in recent years. So the theory used to be that when the planet first formed it was too hot for there to be water. So what happened was the water arrived after the planet was, after Earth was created, fairly soon after its creation. There was a kind of bombardment of comets and meteorites which had water in them.