But that's what the European Space Agency (ESA) says about the unmanned Rosetta probe orbiting comet 67P/Churyumov–Gerasimenko. Though 67P is making its first visit to the inner Solar System and won't come closer to the Earth than hundreds of millions of miles, it is throwing new light on one of the fundamental questions in Earth's history: Where did the oceans come from?
According to current theories, the Earth was formed as the disk of dust and gas that surrounded the primordial Sun coalesced into planets. The water in the oceans could have come from that, but the formation of the newborn Earth 4.6 billion years ago left it an incandescent molten mass that would have boiled away any water into space before the crust even had a chance to form. And yet, today three quarters of the Earth is covered by water. So, what was the source?
One idea is that the water came from comets. It's a logical source. Comets are "dirty snowballs" made up largely of ice, so a steady rain of them during the early history of the Earth when the planet was being bombarded by objects from space could have been the origin of the oceans. However, proof needs more than plausibility; it needs evidence.