Some of the oldest known meteorites can only explain how water came to earth. Gizmodo reports that scientists have discovered evidence that carbonaceous chondrites, a group of meteorites from the formation of the solar system (about 4.5 billion years in the past), held liquid water until a few hundred thousand years ago. They then possibly sowed the earth with water when it was still very young.
The researchers dated the water flow in the rocks using a uranium-thorium combination. Since uranium is very mobile in liquid while thorium is relatively static, scientists could see when water last flowed through the meteorites. As the signature disappears over time, it becomes clear that the water has been active relatively recently.