Some of Earth's water may predate the Sun, formed in ancient interstellar clouds

Long before the Sun ignited or the first stone of Earth was laid, water was already forming in the cold darkness of interstellar space. A 2014 study in Science proposes that between 30 and 50 percent of the water in our solar system was not born here but inherited — preserved as ancient ice on dust grains in the molecular cloud that eventually collapsed into our star and its planets. The evidence lies in a subtle chemical fingerprint, the ratio of heavy to ordinary hydrogen, which carries a memory of conditions too cold and too old to have existed in the young solar nebula alone. In this light