We still do not fully understand where all the water on this planet, including that in the ocean, originally came from. At a fundamental level we know how water in the universe comes into existence: when stars reach the end of their lives in a violent explosion called a super­nova, there is enough heat and pressure to bring oxygen and hydrogen together to form water. When The Solar System was forming, 9 billion years after the big bang, there is no doubt that water would have been an ingredient in the dust cloud that went on to form The Sun and its planets. But scientists think that Earth would have been a lot hotter than it is today; and with no atmosphere any water hanging around would have simply evaporated into space. That means that The Earth must have gained its water sometime after it was formed.

The first major source of water probably came from within the Earth itself. Water trapped inside rocks and the Earth’s mantle would have been liberated through volcanic activity, eventually condensing in the atmosphere and falling as rain. However, scientists have their doubts whether this would have supplied enough water, and for this reason think that a “top-up” was probably required. The most likely source of this “top-up” is from outer space. Comets, asteroids and meteorites all contain water in the form of ice. As the Earth was maturing, collisions with these objects would have been very common. Scientists have looked at the chemical composition of the water in a number of comets, asteroids and meteorites. The closest match comes from a type of meteorite called a carbonaceous chondrite.

Not all scientists agree that the water on this planet came from space. Some say that the water released from volcanic activity would have been enough to fill the oceans on its own, while others have proposed more outlandish theories: a team in Japan believes that a thick layer of hydrogen may have once covered the Earth’s surface, eventually interacting with oxides in the Earth’s crust to form our planet’s oceans. Personally, I like the idea that every time I drink a glass of water, I am drinking defrosted ice from a meteorite that collided with the Earth billions of years ago.

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This post is an excerpt from “Do Fish Sleep?: and 38 other ocean mysteries”, by David Aldridge. It is available to buy on kindle here for the reduced  price of $1.46 (99p) until the end of January. To read it, you do not need to own a Kindle device, just the Kindle App which is a free download for smartphones or tablets. It can also be read with“Kindle Cloud Reader” on a PC or Laptop.