How did water originate on Earth?

22nd October 2009

Asked by: Stephen Asprey, Wolverhampton

Water’s origins are somewhat controversial. Most of it is believed to have originated in ice locked up in the dust grains that orbited the Sun during the earliest days of the Solar System, around 4.6 billion years ago.

Gravity pulled these grains together into ‘planetesimals’, which collided to form the Earth and other planets. The resulting heat led the ice to vaporise, forming a steam-filled atmosphere, which the Earth’s gravity was able to retain until it cooled and condensed into liquid water.

But some (perhaps even most) of this primordial water may still be trapped inside the Earth. This bizarre possibility received a boost in August when a team from Oregon State University claimed to have found evidence for vast amounts of water deep below the surface.

Meanwhile, recent observations of comets have revealed the presence of water with a chemical and isotopic composition similar to that of terrestrial water. Thus some of the liquid we drink each day may have been dumped here by comet impacts.