To find the origin of the oceans, look in outer space

EARTHCC—the quintessential blue planet—has not always been covered by water. Around 4.6bn years ago, in the solar system’s early years, the energetic young sun’s radiation meant the zone immediately surrounding it was hot and dry. Earth, then coalescing from dust and gas in this region, thus began as a desiccated rock. How it subsequently acquired its oceans has long puzzled planetary scientists.One possible source of Earth’s water is carbonaceous (-type) asteroids, the most common variety. But it cannot be the sole source, because water in chunks of these that have landed as meteorites does not match the isotopic fingerprint of terrestrial water. This fingerprint is the ratio of normal water (HO, made from hydrogen and oxygen) to heavy water (DO and HDO, which both include deuterium, an isotope of hydrogen that has a neutron in its nucleus alongside the proton characteristic of every hydrogen atom). Water from -type asteroids has more deuterium in it than does terrestrial water.