Where did Earth get its water?
Did comets really fill our planet's oceans, or has Earth's water been here all along? Belinda Smith reports.
The abundance of water on Earth is an enigma. As the planet formed it became so hot all its surface water would have evaporated – prompting theories our oceans were slowly filled by collisions with icy comets and asteroids after it cooled. Now, a new study by planetary scientist Lydia Hallis of the University of Glasgow and colleagues, published in Science in November, suggests that some of the early Earth’s water was locked away in molten rock and later resurfaced.
If water does turn out to be a natural part of planetary birth, rather than due to a few lucky comet strikes, that increases the odds of life occurring elsewhere in the Universe, says Jonti Horner, an astrobiologist at the University of Southern Queensland in Toowoomba.
To establish conditions on a newborn Earth, Hallis and her team studied rocks from Iceland and Baffin Island in the Canadian Arctic where lava that originated deep within the mantle – where it had been kept pristine since the planet formed 4.5 billion years ago – spews to the surface. These plumes of molten rock offer a glimpse of the Earth’s original geological makeup.
The heavy water on comets is a strike against the theory that Earth’s
wetness is due to random comet collisions.
Hallis found water traces in the material then analysed the water’s hydrogen signature by measuring the ratio between two types of hydrogen. Most water on Earth has the familiar two hydrogen atoms and one oxygen (H2O). But in some water molecules, this “normal” hydrogen is replaced by its bigger sibling, deuterium, forming “heavy water”.
Earth’s oceans today have a characteristic deuterium-to-hydrogen (D/H) ratio. This is much lower than the D/H ratio of all but one of the comets that scientists have been able to investigate. The heavy water on comets is a strike against the theory that Earth’s wetness is due to random comet collisions.
Hallis found that the water in the primordial lava had a D/H ratio much lower than in today’s oceans. The result is what you would expect if the newborn Earth had retained some of its water, she says. This is because when the Solar System was a spinning disk of dust, gas and water swirling around the young Sun, all the heavy water was flung to the outer reaches.
As the spinning disk of dust gradually coalesced to form the Earth, conditions here were superhot. Hallis argues that the Earth’s surface water evaporated, but that the Earth was big enough to retain water in a reservoir locked into rock deep under the surface. We know water exists in the Earth's mantle today, but researchers had no idea it had been there since the Earth’s birth.
As the teenage Earth’s crust cooled and hardened, volcanic vents released steam from the reservoir into the atmosphere, Hallis says – just as they do in lava fields in Iceland today. Once the Earth’s atmosphere became moist enough, this water fell as rain. The transition from dry to a wet surface “wouldn’t have taken too long, geologically speaking”, she says.
The Earth’s D/H ratio increased to its present levels as it aged. Part of this deuterium enrichment probably came via heavy water-rich comets and asteroids. But the solar wind – a stream of particles from the Sun that can whisk away molecules from our upper atmosphere – has been a significant influence, Hallis says. Water vapour made from ordinary hydrogen is easier to blow away than heavy water.
Hallis is the first to admit that adding the flesh to the bones of her theory is going to be tricky. No one knows how much water can lodge in the hot, high-pressure environment of the Earth’s lower mantle – those conditions are difficult to replicate the lab. Confirming how the D/H ratio changed will also be hard because it is unclear how many comets and asteroids have hit the Earth, and how much water the Earth carried in the beginning.
Meanwhile, the next piece of the puzzle may well come from space. The Japanese Hayabusa 2 mission is en route to asteroid 1999 JU3, which orbits the Sun between Earth and Mars, and is due to return with samples in 2020.
If it brings water chock full of deuterium, it could swing the pendulum further towards the idea that Earth’s water has been here all along.
“They’re really pushing back the boundaries of what we know,” Horner says. “It’s good for astrobiologists.”
Also in Cosmos: What Rosetta found on Comet 67P