Earth's water present before impact formed moon, study finds

Based on an extensive collection of lunar and terrestrial samples, researchers have determined that most of the water on Earth was already present at the time of the impact that created the moon.

Scientists from the United States, Britain and France studied moon rocks brought back to Earth by astronauts on the six Apollo missions and volcanic rocks retrieved from the ocean floor by Earth-bound scientists. They published their findings Thursday in the journal Science Advances.

"The research discovered only a small difference in oxygen composition between the lunar and terrestrial rocks," Dr. Richard Greenwood, a research fellow at The New University in England and lead author of the new study, said in a press release. "This demonstrates how well mixed all of the pieces of rock created from the catastrophic impact were, and places strict limits on the types of material that could have been added to the Earth afterwards."

Oxygen isotopes in the rocks were examined by researchers to determine their origin. Material from different distinct origins such as asteroids, planets or comets tend to have their own unique composition signature.

The researchers found that oxygen isotopes from both surfaces are similar, carrying a three- to four-ppm difference. They also found no significant difference between the lunar samples and terrestrial olivine, a common mineral in Earth's subsurface.

The researchers suggest the difference can be explained by a "late veneer," or input of stony meteorite material to Earth, subsequent to the impact of the moon-forming object.

They believe much of the Earth's water was present earlier than the giant impact, and no more than 5 percent to 30 percent of water contributed to Earth from the late veneer process.

The researchers say their evidence bolsters a theory that a Mars-sized protoplanet slammed into protoplanetary Earth and the coalesced material formed the moon, but disputes a theory water came to Earth via asteroids and comets.

If the water came from space, the researchers say isotopes in rocks from the ocean would have differed from those in rocks from the moon. Based on what they found, they theorize the water likely survived the impact.

"Because water is such a vital ingredient for life, we rightly see it as precious. Our research shows that even an event as catastrophic as two planets colliding doesn't cause all of the water to be dispersed into space. The water stayed in the mix of debris, which gave birth to our planet and its only moon," Greenwood said.

"What's even more fascinating is that, because this worked for the Earth and Moon, it must also work for planets beyond our solar system. Exoplanets with water on their surfaces may be much more common than we previously thought. And where there is water, there could also be life!"

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