Early Oceans From Meteorites
Earth’s vast oceans may owe their genesis to water-bearing meteorites that bombarded the planet in its infancy, according to a new study published in Science. The research team found a link between hydrogen samples from the waters of Earth, ancient meteorites and an asteroid, a finding that indicates the very early Earth was already a blue planet.
Earth is believed to have formed about 4.6 billion years ago as an accretion of matter within a solar nebula. The process involved frequent collisions with asteroids, meteoroids and other space debris, drawn by the force of gravity to impact with each other and with the infant planet.
Scientists have long argued that this process would not have been kind to any water molecules involved, either evaporating them or sending them careening away into space. Following this scenario, Earth gained its waters hundreds of millions of years later, thanks to collisions with comets and a special type of icy asteroids.
It was precisely this question about where the water of Earth’s oceans and atmosphere came from that lead author Adam Sarafian, of the Woods Hole Oceanographic Institution (WHOI), was prepared to test with his coauthors.
Water molecules are made of one oxygen atom and two hydrogen atoms, but not all hydrogen atoms are the same. While most hydrogen atoms are made of one proton and one electron, some hydrogen atoms also have a neutron, giving them about twice the mass of a regular hydrogen atom.
Known as deuterium, these heavy hydrogen atoms occur at very low but variable rates throughout the solar system. Different parts of the solar system are characterized by different ratios of deuterium to regular hydrogen, which can allow scientists to determine where a sample of water came from.
Sarafian and his team first looked at water from the oldest known type of meteorites, known as carbonaceous chondrites. Carbonaceous chondrites were being formed even before the planets, and their chemical makeup represents the early solar system.
Because of their own high water content, carbonaceous chondrites have been suggested as the source of Earth’s oceans and other bodies of water. However, the only way to really test whether carbonaceous chondrites brought water to the early Earth would be to analyze a sample from the same part of the solar system and the same period.
The scientists analyzed samples of basaltic rock from 4-Vesta, an asteroid known to have formed in the same part of the solar system as Earth. These samples have a particular deuterium-to-hydrogen ratio known to be one of the oldest in the solar system.
The researchers discovered a match between the ratios of deuterium to hydrogen in the 4-Vesta samples, the carbonaceous chondrites, and the waters of Earth. Since 4-Vesta formed about 14 million years after the very beginning of the solar system, the samples were of an appropriate age to provide a point of comparison.
While the new findings do not rule out the possibility that later impacts by comets and icy asteroids brought additional water to Earth’s oceans, they do provide evidence for a new understanding of the early Earth as a wet planet rather than a dry one. From the great depths of the oceans to the mantling rainclouds, the waters so characteristic of Earth were primarily brought by early meteorites.
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