How Earth got its ocean
Why do we have oceans? Where did all the water come from in the first place?
Well, there is plenty of water out in space. According to Nicolas Cowan, an astronomer and astrobiologist at the University of Washington, “Water is ridiculously common, one of the most common molecules in the universe.”
Back in 2011 the largest reservoir of water so far was found in a cloud around a black hole 12 billion light years away. How much water? 140 trillion times the amount on Earth!!!
Of course, in space water either vaporises if it’s too hot, or freezes if it’s too cold. It’s liquid water that is hard to find – or at least this was the thought before it was found all throughout the solar system…
Up until fairly recently, there were two generally accepted theories as to the presence of water on Earth:
• planetary degassing
• comet impact
According to the theory of planetary degassing, Earth’s water came from the release of water vapour from the molten rocks and volcanoes as the planet cooled. This process continues today, with water vapour being expelled from volcanoes and in some other places, such as the Craters of the Moon just out of Taupo, New Zealand (shown in the photo).
According to the comet impact theory, Earth was bombarded by water-rich comets about 3.8 billion years ago. This is known as the late heavy bombardment period.
The correct explanation is most likely a combination of the two.
Both planetary degassing and comet impact resulted in water vapour being put into the atmosphere. After Earth’s surface had cooled to a temperature below the boiling point of water, rain began to fall, and continued to fall for thousands of years. As the water drained into the hollows and depressions in the Earth’s surface, the primeval ocean came into existence.
In 2010 a new theory was hypothesised by Nora de Leeuw and her team at University College London (a research university in, funnily enough, London). The new theory suggests that water has been present since the Earth formed. As the dust grains in the planetary disc surrounding the newly formed Sun coalesced to form the Earth, they were able to hold onto water molecules despite the high temperatures generated in the formation of the planet.
Computer models were made and calculations were performed to see what would happen when water molecules attached themselves to these dust grains. It turns out that the dust grains were able to hold onto the water molecules up to a temperature of 630 °C. This temperature is high enough for water molecules to be retained during the formation of the Earth.
So, is this where the Earth’s water came from???
Deuterium is an isotope of hydrogen – it is hydrogen with the addition of a neutron, and as it has more mass it is also called heavy hydrogen. By looking at the hydrogen/deuterium ratio of seawater and comparing it to the ratio from comets and meteorites, it has been found that meteorites have less deuterium, whilst comets have more. So meteorites and comets don't seem to explain the presence of water on Earth. At least, not all of it. Some asteroids are also known to be rich in water, and would have crashed into the Earth during the early days of the solar system.
It has been estimated that water being present on the dust grains that coalesced to form the Earth could have supplied about 22 % of the water that the Earth has now, although this number could be much higher.
So where does that leave us?
The best answer at the moment is, once again, a combination. Earth got its ocean from dust grains as the planet formed, and from planetary degassing, and from comet impacts.