The origin of Earth’s water is a bit of a mystery. While water is common in our solar system, it’s much more common in the outer solar system, such as Jupiter’s moon Europa or Saturn’s moon Enceladus. Most of the worlds of the inner solar system are fairly dry. So how did Earth come to have large oceans on its surface?
There are two main ideas on the origin of Earth’s water. One is that Earth’s water was locked up in the original rocks and dust that formed our planet. As the material collapsed under its own gravitational weight, water was released and eventually formed the oceans we have now. The other idea is that any water in the original material escaped early on, and the current water of Earth came to our planet through the bombardment by asteroids and comets. Evidence that Venus and Mars were also wet in their early history points toward a formation origin of water, but there has generally been more evidence to support the bombardment model.
Three isotopes of atomic hydrogen. Credit: Dirk Hünniger, CC BY-SA 3.0.
This evidence comes through what’s known as the deuterium/hydrogen (D/H) ratio of Earth’s water. Deuterium is an isotope of hydrogen that has a nucleus of a proton and neutron, rather than the single proton of regular hydrogen. Chemically it reacts in the same way as hydrogen, but since it is heavier than regular hydrogen there are slight differences. For example, when a deuterium atom is part of a water molecule, the extra mass means it doesn’t evaporate as readily as regular water. Deuterium water is more likely to form in space than in the gravitational field of a planet, so the D/H ratio of water tells us about the origin of that water.
The D/H ratio for Earth’s oceans is about 150 parts per million, which is similar to that of chondrite asteroids. This would seem to support the bombardment model. But a new paper argues that such a conclusion is too simplistic. Our oceans cycle between the surface and interior of Earth, which could affect the D/H ratio. In this paper the team looked at rocks from Earth’s mantle, and they found that the water contained within these rocks has a much lower D/H ratio than that of our oceans. This suggests that mantle water formed locally rather than through astroid bombardment.
There’s still a number of unanswered questions. This latest work doesn’t disprove the bombardment model, and it’s possible that our water came from a number of sources. Further study on both fronts is needed to resolve this mystery.
Paper: Lydia J. Hallis, et al. Evidence for primordial water in Earth’s deep mantle. Science, Vol. 350 no. 6262 pp. 795-797 (2015) DOI: 10.1126/science.aac4834
