- Demonstrate an understanding that the presence of liquid water is probably an essential requirement for life
- Describe the present theories about the origin of water on Earth
- Describe methods used by astronomers to determine the origin of water on Earth (for example analysis of water on a comet by the Rosetta probe)

Water is needed for life to exist. It is considered to have been crucial to forming life due to its properities 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. Scientists are engaged in exploring this theory.

The Rosetta probe was launched by the European Space Agency in 2004 to understand the origin and evolution of the Solar System. As well as flybys of Earth and Mars it has (and will) encounter asteroids and comets until 2015. It will launch a Lander to measure isotopes of chemicals to understand how much water it may have carried. If the balance between isotopes is similar to that of our oceans then it may support the theory of our water coming from space. 

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?