How did water end up on Earth?

In GCSE science, students are taught about the formation of the atmosphere as we know it.  They are expected to know that volcanic activity caused carbon dioxide, water vapour, methane and ammonia to be released into the early atmosphere.
BUT… it is unlikely that volcanic activity can explain all of the water that we have on earth (about 1,260,000,000,000,000,000,000 liters!).   For some time it has been thought that the remaining water was brought to earth on comets but New Scientist has reported this week that it was more likely to have come in on asteroids.

What’s the difference?

It’s all about what they’re made up of.  Asteroids tend to be made up of rock and metal, whilst comets are made of ice (water and frozen gasses like methane), dust and rock.  It seems more intuitive that water should come from comets then.

But studies have recently been carried out on the amount of deuterium in meteorites called “carbonaceous chondrites” which are the type believed to bring water to Earth.  Deuterium is an isotope of hydrogen (a version with an extra neutron) and is found in larger amounts when you get further out in the solar system.  The carbonaceous chondrites do not contain as much deuterium as we would find in comets, so they must come from asteroids.

Why is this of any importance?

We needed water for life on earth.  Understanding the origins of the water means we’re one step closer to understanding how life came to be on Earth.

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