Origins of Oceans | National Geographic

Three quarters of Earth is covered in water. It's why it's called the Blue Planet. But imagine Earth without its oceans. And instead, with all the water in the world gathered together in a ball. There's not that much, really. But this water is what makes Earth different from every other planet in the solar system. The oceans have been here for almost four billion years, carving the coastline, driving the climate and controlling the destiny of life itself. But four and a half billion years ago, when our planet was born, it was just a molten inferno. There was no place for any liquid water at all. Yet the ingredients needed to create water were there, locked away deep inside the Earth when the planet formed. As the first volcanoes erupted, one of the gases that billowed out was steam. And as the planet cooled, it formed clouds. It rained for thousands of years. As the first rivers formed, they began to fill up the lower-lying areas on our young planet, possibly creating the first oceans. But it's reckoned that even this relentless downpour only delivered around half the water that makes up our oceans today. The rest came from outer space. This is a comet filmed in 2005. It's about four miles across, and like all comets, it's made of rock and water in the form of ice. To discover just how much water comets contain, scientists deliberately crashed a space probe straight into it. After it ploughed into the surface, more than 250,000 tonnes of water burst into space. In Earth's formative years, it was hit by thousands of comets like this. By the time the bombardment finished, it's thought they've delivered up to half the water in our oceans.