Ancient Seas

For an inner planet, Earth is bountiful with water. The origin of that water has been a matter of some debate. One idea is that a combination of Earth’s strong magnetic field and distance from the Sun allowed Earth to retain much of the water emitted from rocks as the planet cooled. Another is that water came to Earth through cometary or asteroid bombardment. But now it seems the origin of Earth’s water is more complex and more interesting that we’ve thought.

Last month an article in Science showed that much of Earth’s water existed before the formation of the solar system.¹ The authors demonstrated this by looking at levels of deuterium in terrestrial water. Deuterium is an isotope of hydrogen that has a proton and neutron in its nucleus, rather than just a proton. As a result, it’s almost twice as heavy as regular hydrogen, and this means the way it chemically reacts is slightly different from regular hydrogen.

Deuterium isn’t very common compared to hydrogen, and exists at about 26 parts per million. When the team measured levels of deuterium in the water of Earth and other solar system bodies, they found the water contained deuterium at about 150 parts per million. This is interesting, because deuterium water is more likely to form in interstellar space. Water formed in the heat of a young solar system isn’t likely to produce much deuterium water. Given measured deuterium levels, the authors calculate that about half of Earth’s water was produced in the depths of space, before the solar system was formed.

This month another paper in Science found that water arrived on Earth earlier than expected.² In this paper the team compared chondrite minerals on Earth with chondrite asteroids, specifically ones that likely originated from Vesta. Chondrite asteroids have a high quantity of water chemically bound to them, and one idea is that they could have been the source of Earth’s water. When they looked at the chemical makeup of terrestrial chondrites, they found them to be remarkably similar. This likely means terrestrial chondrites were themselves the source of Earth’s water. If that’s the case, then Earth was likely a water world a hundred million years earlier than the bombardment model predicts.

So it seems that Earth’s seas are more ancient both in origin and composition than we once thought.