Theories for the Origin of Water on Earth

Water Delivery Theories

Comets, made of ice and dust, are often viewed as the most likely source of Earth’s water.
Comets are basically ‘dirty snowballs’ that originate from the Kuiper belt and the Oort cloud at the outer edges of the solar system.
They would have bombarded Earth during its early formation, depositing water and other volatile compounds.
Recent measurements of comet water have shown that it is significantly different from Earth’s ocean water in terms of deuterium/hydrogen (D/H) ratio, which presents a challenge to the comet theory. However, the Rosetta mission has found at least one comet with a D/H ratio similar to Earth’s oceans, supporting the possibility.

Asteroids, primitive solar system bodies that orbit the sun in a region between Mars and Jupiter known as the asteroid belt, are another potential source.
Unlike larger planets, asteroids didn’t get hot enough to lose the water they formed with.
Some studies suggest the chemical fingerprint of Earth’s water is more similar to water found in some meteorites from asteroids, rather than comets.
As asteroids also contain rocky materials, they could explain not only the origin of water but also that of Earth’s other volatile components.

Volcanoes release water vapour and other gases trapped in Earth’s mantle and crust, a process known as volcanic degassing, or just outgassing.
At the time of Earth’s formation, water and volatile compounds could have been trapped within dust grains which eventually formed the Earth.
Over a period of hundreds of thousands of years, these materials could have been released through volcanic activity.
This could potentially account for some of the water in Earth’s oceans, but it’s unclear if it could account for all of it.

Regardless of how water first arrived, it’s equally important that Earth was able to retain it, which is largely due to our planet’s magnetic field.
The magnetic field shields Earth from the solar wind, which can strip away a planet’s atmosphere and oceans over time, as believed to have happened with Mars.
It’s also helped that Earth orbits in the habitable or “Goldilocks” zone, the region around a star where conditions allow liquid water to exist on the surface.