Earth water survived 'giant impact': implications for alien life

A discovery about the origin of the Earth's oceans makes it more likely that life exists beyond the solar system.

Scientists now know that liquid surface water was present on Earth before the planet-scale "giant impact" that created the moon.

Previously it was thought that most if not all the ocean water was carried to the Earth in later asteroid and comet bombardments.

The fact that liquid water can survive catastrophic impacts by planet-sized bodies means it should be abundant on worlds in other star systems.

And the presence of liquid surface water in the form of lakes and oceans greatly increases the chances of finding life, experts believe.

Lead researcher Dr Richard Greenwood, from the Open University, said: "Because water is such a vital ingredient for life, we rightly see it as precious. Our research shows that water is also extremely resilient and can survive an event as catastrophic as two planets colliding."

The scientists compared the oxygen composition of moon rocks brought back to Earth by Apollo astronauts with that of volcanic rocks from the ocean floor.

They detected only a small difference between the lunar and terrestrial rocks. If most of the water on Earth had arrived after the giant impact, the rocks should have distinctly different oxygen compositions.

The findings, reported in the journal Science Advances,  suggests that liquid water must have existed on Earth before its moon-forming collision with a body the size of Mars.

The giant impact is thought to have occurred about 100 million years after the solar system formed out of a swirling cloud of dust and gas.

Scientists know that many exoplanets in other star systems experienced similar high energy collisions early in their evolution.

Dr Greenwood added: "Because this worked for the Earth and the 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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