Comets didn’t bring Earth’s water as it ‘was here from the start’, study finds

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Earth’s supply of water didn’t arrive with a bang from a comet or asteroid, researchers claim – it was likely here at the time our world first formed.

A new study by the Centre de Recherches Petrographiques et Geochimiques, France, examined a meteorite called an enstatite chondrite for signs of hydrogen.

Researchers discovered the space rock – composed of material found in the inner solar system – contained sufficient hydrogen to deliver at least three times the amount of water found in the Earth’s oceans and probably considerably more.

As this rock is made of the same material as the Earth, it suggests that when it formed 4.5 billion years ago it did so with the materials for water already present. 

This goes against a leading theory that far-reaching space rocks came from the outer solar system and collided with the Earth to deliver our planet’s water.  

Researchers discovered that the meteorite called an enstatite chondrite - composed of material found in the inner solar system - contained sufficient hydrogen to deliver at least three times the amount of water found in the Earth's oceans and probably considerably more

Researchers discovered that the meteorite called an enstatite chondrite – composed of material found in the inner solar system – contained sufficient hydrogen to deliver at least three times the amount of water found in the Earth’s oceans and probably considerably more

‘Our discovery shows that the Earth’s building blocks might have significantly contributed to the Earth’s water,’ said lead author Laurette Piani.

‘Hydrogen-bearing material was present in the inner solar system at the time of the rocky planet formation, even though the temperatures were too high for water to condense.’

The Earth’s building blocks are often presumed to be dry – which is why leading theories have suggested an external source for water. 

‘The most interesting part of the discovery for me is that enstatite chondrites, which were believed to be almost ‘dry,’ contain an unexpectedly high abundance of water,’ said Lionel Vacher from Washington University.

Vacher prepared some of the enstatite chondrites in this study for water analysis and is now working on understanding the composition of water in other meteorites.

Enstatite chondrites are rare, making up only about two per cent of known meteorites in collections currently on the Earth.

But their isotopic similarity to Earth make them particularly compelling to people studying the origin of our planet and the early solar system. 

Enstatite chondrites have similar oxygen, titanium and calcium isotopes as Earth, and this showed that their hydrogen and nitrogen isotopes are similar too. 

In the study of extraterrestrial materials, the abundances of an element’s isotopes are used as a distinctive signature to identify where that element originated.

Vacher added: ‘If enstatite chondrites were effectively the building blocks of our planet – as strongly suggested by their similar isotopic compositions – this result implies that these types of chondrites supplied enough water to Earth to explain the origin of Earth’s water, which is amazing!’

The findings from this study are surprising because the Earth's building blocks are often presumed to be dry - which is why leading theories suggest an external source for the planet's water supply

The findings from this study are surprising because the Earth’s building blocks are often presumed to be dry – which is why leading theories suggest an external source for the planet’s water supply

The paper proposes that a large amount of the atmospheric nitrogen could have come from the enstatite chondrites.

‘Only a few pristine enstatite chondrites exist: ones that were not altered on their asteroid nor on Earth,’ Piani said. 

‘In our study we have carefully selected the enstatite chondrite meteorites and applied a special analytical procedure to avoid being biased by the input of terrestrial water.’

Coupling two analytical techniques – conventional mass spectrometry and secondary ion mass spectrometry (SIMS) – allowed researchers to precisely measure the content and composition of the small amounts of water in the meteorites.

Prior to this study, ‘it was commonly assumed that these chondrites formed close to the sun,’ Piani said. 

‘Enstatite chondrites were thus commonly considered ‘dry,’ and this frequently reasserted assumption has probably prevented any exhaustive analyses to be done for hydrogen.’

The findings have been published in the journal Science.

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