11.13 - Understand the main theories for the origin of water on Earth

Water is needed for life to exist. It is considered to have been crucial to forming life due to its properties as a solvent allowing various chemicals to form.

Water is formed by star formation. Hydrogen and Oxygen are common throughout the Universe and these group to form water.

There are 2 main theories about where our water came from:


This theory says that water was part of Earth’s body when it formed. As the Earth cooled, water leaked from rocks and the atmosphere condensed the water into liquid form.

Delivery by Comets

It is thought that Earth was heavily bombarded by comets and other bodies early in its evolution and the Moon bears evidence of this. Comets contain large amounts of ice and this would have fed our oceans on impact. This theory is controversial as it would have taken an enormous amount of water to cover 71% of the surface and most scientists have difficulty with the theory. The Rosetta probe was launched by the European Space Agency in 2004 to understand the origin and evolution of the Solar System. Rosetta and its lander Philae detected water-rich carbons in the comet and took numerous water detecting measurements of the comet 67P/Churyumov–Gerasimenko.

Where Water is...

So far water ice has been detected on the Moon, Mars, Titan, Europa, Saturn's rings, Enceladus, Pluto and Charon. Traces of water have been found in the clouds of Venus and Jupiter. Water also exists in the exosphere (the outmost part of its extremely faint atmosphere) of Mercury.


  • Why is liquid water important for life?
  • Why does Earth have water and the ability to keep it in liquid form?
  • How might scientists use data from comets to determine if they brought water to Earth?