Where Did Earth's Water Come From?

A new study suggests it wasn't comets and asteroids. NASA

Researchers from the University of Hawaii believe they have discovered the origin of Earth’s water. For many years, scientists were uncertain whether water was present when our planet formed or if it was carried by comets and asteroids at a later time.

By analyzing rocks from Baffin Island in Canada, the researchers were able to produce the most convincing piece of evidence in favor of the native water hypothesis. The rocks come directly from the mantle, and they have not been affected by material from the crust. In them, the researchers found glass crystals that have trapped small droplets of water. That water has the same composition of the water that is now present on our planet.

Water is made of oxygen and hydrogen, and hydrogen is often found in three forms, called isotopes: normal hydrogen, deuterium, and tritium. Water that is formed by oxygen and deuterium is called heavy water.

By studying the composition of different bodies in the Solar System, the researchers discovered that they tend to have a very distinct ratio between normal water and heavy water. Comets have shown a significantly higher ratio of heavy water to normal water.

The researchers are not discounting the comet and asteroid theory for water, but say it is not a necessity to explain how we got our oceans.

Scanning electron microscope image of a Baffin Island picrite (type of basaltic rock). The mineral olivine, shown as abundant mid-gray color cracked grains (A), hosts glassy melt inclusions (B) containing tiny amounts of water sourced from Earth's deep mantle. Lydia J. Hallis. 

“We cannot rule out the addition of water to Earth's surface after its formation (i.e. via comets and asteroids), but our data suggests that Earth had water from the very beginning of its formation, so a large amount of water addition later was not necessarily needed to produce our oceans.” Dr. Lydia Hallis, lead author of the study, told IFLScience.

“We can say that the water we measured from the deep mantle is highly unlikely to have been added in this way, because cometary and asteroidal impacts would not have been large enough or powerful enough to affect the deep mantle thousands of kilometers beneath the surface, and previously reported geochemical data suggests the source regions for our rocks have not been disturbed in about 4.5 billion years.”

The study, published in Science, provides important clues on the widespread presence of water in the Solar System. “Our research suggests that rocky bodies form with their water, and so we would expect to find that many of these bodies are water rich (as indeed we are finding with new images and data from Mars, the Moon, and asteroids),” added Dr. Hallis. “This research shows how vital sample return missions to asteroids and comets really are, to allow us to study the chemistry of these bodies and determine how and when they formed.” 


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