Where did Earth get its water from?

Water, water everywhere. This memorable phrase from Samuel Taylor Coleridge's Rime of the Ancient Mariner may be the desperate musings of a thirsty sailor stranded at sea, but it could also be considered a concise description of planet Earth itself.

About 71% of Earth's surface is called water: it's not for nothing that our home is known as the 'blue planet'.

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Human beings are mostly made of water. We require regular intake of water for our body to perform its necessary functions; to keep us alive.

We may have originally come from water, before evolving into the land-based mammals that dominate the planet today.

We see water throughout the Solar System, too: beneath the icy crusts of moons like Enceladus and Europa, or in the geology of Mars, which tells us that the Red Planet was once a much wetter, more hospitable place than it is today.

Without water, we would be nothing. But where did Earth's water come from?

What are the origins of water across the Universe, within our Solar System, and how did water find its way to our planet?

These are tricky questions to answer, but planetary science has come a long way to providing solutions as to where exactly water came from.

How water is formed

Water molecules form in interstellar space by chemical reactions between hydrogen molecules and oxygen-bearing molecules such as carbon monoxide.

The Solar System inherited its water from ice-coated interstellar grains in the dust cloud from which the Sun and planets formed 4.6 billion years ago.

In the Solar System this water became locked in two principal forms.

Far from the Sun, where temperatures are low, water formed icy objects such as comets, while closer to the Sun water reacted with rocky materials to form hydrated minerals.

It's thought that the mostly likely way that planet Earth inherited its water was from asteroids and comets crashing into it.

Much of this water would have initially been added to the Earth’s growing mantle as it formed and was released from the interior by subsequent volcanic activity.

The next time you pour yourself a cold drink of water on a hot day, consider the cosmological history contained within that single glass of clear liquid, and how vital it has been in the evolution of the Solar System.

This article originally appeared in the July 2008 issue of BBC Sky at Night Magazine.