More than 70% of the Earth is covered by water and 96.5% of this is contained in our oceans, which equates to around 1.35 billion km³. Water in its liquid form is so plentiful on our planet but so scarce in the rest of the solar system that it leaves many people wondering where all that water came from. To answer that question, you'll first need a brief history of how our planet was formed.
In the early days of our solar system, the newly-formed Sun began to pull objects into orbit around it. Gas molecules and dust particles fused to form larger objects and, by around 4.5 billion years ago, roughly 65% of planet Earth had been assembled.
But the process was not without problems. Around 4.53 billion years ago, Earth was struck by an object the size of Mars. The energy from this collision melted the planet's upper layers, eventually melting it right to the core.
And the violence continued. The Earth was repeatedly struck during a period known as the Late Heavy Bombardment between 4.1 and 3.8 billion years ago. One possible explanation for the presence of water on Earth is that it arrived during this time, brought by icy comets and asteroids from the outer solar system.
However, for oceans, ponds, and other bodies of water to have formed at this time, there would need to be a solid surface for them to sit on, suggesting the Earth's crust was already in place. Also, for there to be liquid water, there would have to be a thick atmosphere in order to prevent the water from boiling off in the heat.
Recent evidence contradicts the comet theory of the oceans' origin. Ancient volcanic basalt rocks from Baffin Island, in the Canadian Arctic, contain inclusions (material trapped inside a mineral during its formation) formed in the Earth's mantle around 4.5 billion years ago. These inclusions contain hydrogen molecules from water of the same age.
The amount of deuterium, a hydrogen isotope, in these inclusions is low, which could rule out meteorites as the source of the water since these usually have high amounts of deuterium. This suggests that water initially came from the cloud from which the Sun and planets originally condensed. In this scenario, water would have clung tightly to the coalescing dust particles during Earth's formation.
Our planet also has more water than we would expect beneath its surface. It's possible that some of this water migrated upwards to the surface to form the oceans. It is estimated that there is three times the volume of all the oceans put together way down in the mantle, in the internal reservoir. And there could be more reservoirs of water even deeper.
So, there's no definitive answer for where the oceans came from but, instead, there are several theories, each with supporting evidence.
Regardless of how they were formed, there is no doubt that our oceans are important. They are able to regulate the planet's temperature, storing heat on a large scale. They also absorb carbon and CO2 in large amounts, removing it from the atmosphere and locking it away; particularly important in the fight against climate change.
So, as you can see, healthier oceans and seas will lead to an overall healthier planet. It's probably for the best that we look after them.
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