Earth has vast oceans today, but our planet was a dry rock when it first formed — and water was a late addition, rained down in asteroids from the icy outer solar system.
That's what the textbooks say, but new research published today in the journal Science, adds weight to a competing idea that Earth was actually born 'wet'.
Key points:
- Water is abundant in space but it was thought that Earth was dry when it formed
- A new study shows that meteorite rocks of the type that built the Earth contain the building blocks of water
- This means that water formed on Earth from day one rather than being a later addition
Water is abundant in space and is made up of hydrogen created in the Big Bang and oxygen released from dying stars.
The planets of our solar system were created around 4.6 billion years ago from clumps of rocks spinning around the Sun.
Earth was moulded from rocks that came from the inner solar system where the fierce heat of the Sun would have boiled away any water.
So, according to the textbooks, water must have come later.
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But cosmochemist Laurette Piani from the Université de Lorraine argues that the ingredients to form water were bound up in the rocks that formed Earth.
Dr Piani and colleagues analysed 13 rare meteorites that come from remnants of rocks that orbited the inner solar system when it was very young — before planets formed.
These meteorites (called enstatite chondrites), are made of the kind of rock believed to have formed Earth.
"At least three times the amount of water in the Earth's oceans can be provided by these enstatite chondrites," said Dr Piani.
She and colleagues didn't actually analyse water itself in the rocks, but measured one of its building blocks — hydrogen, bound up in the minerals — as a proxy instead.
"If you have hydrogen, it will combine with the oxygen to create water very easily on Earth," Dr Piani said.
The researchers found the hydrogen signature in the meteorites matched that of rocks found in a layer of the Earth called the mantle.
Like the meteorites, rocks in the mantle also contain a lot of oxygen bound up with minerals, which can be liberated under certain circ*mstances, and combine with the hydrogen to form actual water — H20.
This happens in magma, molten rock containing dissolved water that rises from the mantle to the surface in volcanoes.
As the pressure falls, the water vaporises and explodes into the atmosphere as steam, and later condenses, falling back to Earth to fill our rivers and oceans.
In fact, according to the theory supported by Dr Piani and colleagues, this is exactly the process that would have produced Earth's oceans from the precursors of water hidden in the planet's building blocks.
"It looks like what people thought was really dry rock has enough water — if you accumulate a lot of it — to explain Earth's water," noted NASA geochemist Anne Peslier, who wrote an accompanying editorial in Science.
Long debate about origins of Earth's water
A growing number of scientists have been challenging the long-standing textbook theory about how Earth got its oceans.
They include planetary scientist Trevor Ireland from the Australian National University, who is researching water in planetary bodies.
"The Piani article effectively nails anyone saying that the inner solar system is completely dry," he said.
Even though Dr Piani's team measured hydrogen rather than water directly, Professor Ireland was convinced the findings were valid.
"I'm happy that it does represent actual water," he said.
Meteorite researchers at Curtin University also welcomed the study.
"I am very excited about the outcomes of this work and where it will lead the debate on this topic," said Ellie Sansom, project manager of the Desert Fireball Network.
"I would certainly say that this study has incredible implications on our knowledge and understanding of where Earth's life-giving water came from."
Planetary geochemist Lucy Forman agreed.
"The origin of Earth's oceans is a major question within planetary science and beyond, and this research provides meaningful, impactful, and vital data needed to better understand this conundrum," Dr Forman said.
But not all the water in Earth's oceans formed here according to Dr Piani's analysis.
"To explain the water in the oceans we still need a bit of water from the outer solar system," she said.
"We estimated we need about 5 per cent from this process."
Professor Ireland said you can't exclude water arriving later.
"It's not a 1/0 type of thing," he said.
In fact, you really need the later visitors from the outer solar system for other important ingredients for life.
The most likely candidates from this far out are 'carbonaceous chondrites' — like the famous Murchison meteorite that landed in Australia — that contained not only water but carbon and amino acids.
What could this tell us about finding life on other planets?
In the past, there have been some very complicated theories developed to explain how enough water rained down on Earth from the outer solar system.
According to one contentious idea called the NICE theory, asteroids could have been slung into the inner solar system by a disruptive rearrangement of the planets.
But this kind of "special coincidence" that delivers water to planets orbiting close to their stars is unlikely to happen everywhere, said Josh Calcino, who has studied protoplanetary discs of the kind that our solar system was formed from.
The theory put forward by the new study bodes well for finding evidence of life on alien planets that orbit close to their stars, said Mr Calcino, a PhD candidate at the University of Queensland.
"If such planets are born 'wet' that means that many more of them are going to have the potential to form life," he said.
"You don't have to invoke these extravagant mechanisms if you can just have the water there from the get go."
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