Liquid water covers most of Earth's surface, yet the newly formed Earth may have been too hot to hold water. Two scientists debate the source of most of that water. Both use the deuterium-to-hydrogen (D/H) ratio of water—a chemical "fingerprint," since deuterium is a heavy form of hydrogen and water from different bodies can carry different D/H ratios.
Liquid water covers most of Earth's surface, but the newly formed Earth may have been too hot to hold on to water. Scientists ask where most of that water came from. A useful clue is the deuterium-to-hydrogen (D/H) ratio of water. Deuterium (D) is a heavier form of hydrogen, and water from different places in the solar system can carry different D/H ratios, so comparing the D/H ratio of Earth's ocean water with that of other bodies can point to a likely source. Table 1 lists D/H ratios (in units of 10⁻⁴) for Earth's ocean water, for water-rich asteroids, and for three comets.
Scientist 1 Earth formed close to the young Sun, where it was so hot that water could neither condense nor stay trapped in the rocky material that built the planet; the early Earth was therefore essentially dry. Most of Earth's water arrived later, after the surface had cooled, delivered by impacts of water-rich bodies from the colder outer solar system. Water-rich asteroids are known to exist, and their D/H ratio is close to that of Earth's ocean water; some comets match it as well. Because these bodies carry water with the right fingerprint, repeated impacts of comets and water-rich asteroids could have supplied most of the water now in Earth's oceans.
Scientist 2 Earth's water was present from the beginning, held as hydrogen and water bound inside the minerals of the rocky grains that clumped together to build the planet. As Earth's interior heated and its rock partly melted, this water was released and carried to the surface by volcanic outgassing, the escape of gases from erupting magma. Measurements of minerals from deep inside Earth suggest the mantle still stores a very large amount of water today, supporting the idea that Earth was built wet. Moreover, several comets have D/H ratios two to three times that of Earth's ocean water (Table 1), so comet impacts cannot have delivered most of the ocean—the match would be far too poor.
Table 1: Deuterium-to-hydrogen (D/H) ratio of water measured in different bodies
| Body | D/H ratio (×10⁻⁴) |
|---|
| Earth's ocean water | 1.6 |
| Water-rich asteroids (typical) | 1.4 |
| Comet A | 1.6 |
| Comet B | 3.1 |
| Comet C | 5.3 |
