The water in Earth's lakes and seas did not come from space but was formed here by chemical reactions researchers find.
Most scientists think they came from water-rich asteroids and comets raining down on the planet in its youth. Just after the Earth formed, it was very hot and dry. Prevailing theory suggests that millions of water-rich comets and asteroids bombarded our planet around 3.8 billion years ago, neatly explaining why oceans later appeared.
Also, the ratio of deuterium—or “heavy hydrogen” because it contains a neutron in addition to a proton—to hydrogen in our sea water matches the value found in water-rich asteroids, suggesting a common origin.
But planetary scientists from the Tokyo Institute of Technology suggest the oceans were actually created by Earth itself—they may have formed because the young Earth had a thick blanket of hydrogen, which reacted with oxides in the Earth’s mantle to form lakes and seas.
If the water on Earth did form from a thick hydrogen atmosphere, however, it should have originally had a far lower value of the deuterium-to-hydrogen ratio than we see in sea water today. But Genda and Ikoma have got around this problem. Their calculations show that the ratio would have naturally drifted upwards over time.
Several effects would have contributed to this rise, including leakage of hydrogen into space. Energy from the sun would have made most of the hydrogen escape, but the heavier deuterium would have escaped less easily, so it would have become more concentrated.