Did comets open the tap on Earth or not?

It’s a long held belief – or a disputed idea if you like – that comets and  carbonaceous chondrites, a type of primitive meteorites – were the bearers of Earth’s abundant water. That is: the elements that make up water, as well as a lot of organic molecules. It seems this is not the case, and it’s more likely that asteroids and their meteorites much closer to Earth – from the asteroid belt – are responsible for the delivery.

The idea has been so far that comets and  carbonaceous chondrites with their composition was the likely source of water. During the early days of the Earth a bombardment of (micro)comets and chondrites (meteorites) took care of the development of the water reservoir surrounding our planet.
The amount of deuterium in the comets and meteorites seems to be telling this tale. Objects that formed in the outer reaches of out solar system, like comets, have larger amounts of deuterium isotopes. Comparing the amount of deuterium in a space rock thus gives an indication of the place of birth. It was thought to be the same for carbonaceous chondrites, forming in the same region of solar system space as comets.

Now research of 85 carbonaceous chondrites shows that this is likely not the case. The carbonaceous chondrites (say that 100 times to have fun) have a much lower amount of deuterium isotopes than comets, indicating that they formed in the inner regions of the solar system, most likely the asteroid belt.
The research team also suggests that these chondrites, not comets, are the primary source. The research and findings are important for the models of how the solar system formed and developed. Earlier it has been suggested that there are also comets in the asteroid belt responsible for Earth’s water. It would mean that Earth wasn’t supplied from the outer reaches of the solare system, but by its neighborhood. Come to think of it, that might be a lot more plausible.

For more on this research visit the Carnegie Institute for Science.

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