Where did Earth’s water come from?

William Reville: The discovery of vast reservoirs of water deep within the Earth is an example of the power of science to excite wonder

Handlers bathing a horse in the Caribbean Sea near Bridgetown, Barbados. We must now extend the range of the water cycle to include the mantle of the Earth. Photograph: Reuters/Adrees Latif

Some 70 per cent of the surface of the Earth is covered by water, but where all this water came from is the subject of lively debate.

The most popular hypothesis has been that water was carried to the early Earth by icy asteroids and comets.

A rival hypothesis is that water was present on Earth right from the start. This second hypothesis received a great fillip recently with the amazing discovery of compelling evidence for a water reservoir three times the volume of all the oceans deep beneath the Earth's surface. This discovery supports the idea that the Earth had water from the start, and the oceans were formed when water "oozed out" of the interior of the young Earth.

The earth consists of four concentric layers: inner core, outer core, mantle and crust. The inner solid core at the centre of the earth is the hottest layer with a temperature of up to 5,500 degrees Celsius, the heat generated by radioactive decay. The outer liquid core surrounding the inner core is also very hot. The mantle, the widest layer, is around 2,900km thick, and made of semi-molten rock called magma. The outer crust of Earth is 0-60km thick, and is broken into several large pieces called plates.

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The inner core is the engine room of the Earth. Heat rises from the inner core causing convection currents to well upwards and downwards in the mantle, and these currents move the tectonic plates – plate tectonics.

Earth is the only planet known to have plate tectonics. Some adjacent plates collide with each other, some slide past each other and other adjacent plates move away from each other.

Plates collide

Plate tectonics causes earthquakes and volcanoes. When adjacent plates move apart, magna wells up from the mantle to the surface and cools to form solid igneous rock. When adjacent plates collide, one plate is forced down beneath the other into the mantle and melts to form magma. When a plate under an ocean is forced beneath a plate under a continent, water also enters the mantle.

There is scientific evidence, based on the type of hydrogen found in water, that asteroids carried water to the early Earth, although this evidence is contested.

The recent discovery of the huge reservoir of water 700km-1,000km deep in the Earth's mantle – one third of the way to the Earth's core – was made by Steve Jacobsen, Northwestern University in the US (reported by Andy Coghlan, New Scientist, November). In Jacobsen's words, "we should be grateful for this deep reservoir. If it wasn't there it would be on the surface of the Earth and mountain tops would be the only land poking out."

The presence of water on Earth from the beginning would solve the riddle of how life appeared on Earth so quickly after a solid surface formed on the planet – the water essential for life was already present and the origin of life did not have to wait until asteroids later delivered water to Earth.

This hidden water inside the Earth could also work as a buffer for the oceans on the surface of the Earth, thereby explaining why the oceans have remained the same size for millions of years.

Water cycle

The new discoveries also mean that we must expand our understanding of the water cycle, ie the continuous movement of water on, above and below the surface of the Earth.

Common understanding is that water evaporates into the atmosphere from oceans, rivers and so on, condenses into clouds and returns to Earth as precipitation (such as rain and snow), evaporates again, returns again as precipitation, and so on and on.

We must now extend the range of this cycle to include the mantle of the Earth – water can move from the mantle to the ocean, to the atmosphere, return to Earth as precipitation and return again to the mantle.

The discovery of vast reservoirs of water deep within the Earth is an example of the power of science to excite wonder and awe at the majesty of the natural world. It is just as powerful as literature – remember the oceans deep within the Earth in Jules Verne's Journey to the Centre of the Earth?

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