Earth had water even before the collision that made the moon

Earth 28 March 2018
Earth's oceans may have withstood the smash-up that made the moon
Earth’s oceans may have withstood the smash-up that made the moon

Todd Gipstein/National Geographic/Getty

Earth may have had water for far longer than we thought. The planet may have had lakes and oceans even before the giant impact that created the moon, and it could have survived that colossal collision.

Previously it was thought that most, or even all, of Earth’s ocean water was carried to the planet on comets and asteroids after the moon was formed. But Richard Greenwood at the Open University in Milton Keynes and his colleagues found that it might have already been here.

Greenwood and his colleagues compared the oxygen composition of moon rocks brought back to Earth by Apollo astronauts with that of volcanic rocks from the ocean floor.


The presence of liquid water alters the amounts of different oxygen isotopes in the rock. So, if most of the water on Earth had arrived after the giant impact, the rocks should have distinctly different oxygen compositions. The moon rocks should show little sign of being altered by water.

But Greenwood and his team detected only a small difference between the lunar and terrestrial rocks.

Early water

This suggests that liquid water must have existed on Earth before its moon-forming collision with a body the size of Mars, which is thought to have occurred about 100 million years after the solar system formed. The researchers found that most of the water we have now may have already been here, and then between 5 and 30 per cent of it was brought later by asteroids and comets.

Other work has shown that many distant worlds in other star systems underwent similar collisions early in their evolution, events which we thought meant they could not have liquid water unless it was delivered later on.

The fact that liquid water can survive catastrophic impacts by planet-sized bodies means it should be abundant on worlds in other star systems, Greenwood says. Life as we know it requires water, so this could make those exoplanets more hospitable.

“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,” says Greenwood.

Journal reference: Science Advances, DOI: 10.1126/sciadv.aao5928

Read more: The moon may have formed in a vaporised, doughnut-shaped Earth

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