Water contains atoms of oxygen and hydrogen, and life on Earth would not have evolved without it. Fruit and vegetables are made of 80% water, and our bodies are 60–70% water. (We lose about 2.5 litres on average every day but regain this through our food and drink.)

Water is the signature feature of our ‘blue planet’ – 70% of the Earth’s surface is covered in water – and it is something that many people in New Zealand take for granted. We live in a country where we can turn on the tap and have a glass of water. We are surrounded by oceans we can swim in, and often it seems like we have an over-abundance of rain. But where is all this water coming from? And how much water is there?

The water you used for your shower this morning may have come from a local reservoir, which might be from groundwater or could be part of a river system collecting melting snow from the tops of mountains. But this still doesn’t really answer the question of where water comes from.

So where did all the water come from?

This question is very hard to answer, but scientists have a number of theories. One theory suggests that the very early Earth formed with very little or no water at all. Astrophysicists believe that the Earth’s water came from comets and asteroids from the edges of our solar system. It is thought that approximately 4 billion years ago these comets and asteroids heavily bombarded the young Earth.

What is known is that the Earth has had oceans since very early times. There has been a constant exchange of materials between the interior of the Earth and the surface, where magma (liquid rock) rises to the surface and cools, forming the Earth’s crust and releasing water.

How much water is there?

The amount of water on Earth is finite, which means the total amount of water does not vary. Some areas on Earth experience changes in the amount of available water, but the total amount of water in the Earth’s system doesn’t change. This is because water is constantly being cycled between the geosphere (Earth), hydrosphere (oceans and so on), biosphere (living things) and atmosphere (gases) – this is the water cycle in action.

There are a number of estimates given for just how much water there is on Earth, but the amount is probably between 1–2 billion cubic kilometres. Most of this water is in the form of salt water in our oceans – just a tiny amount is actually freshwater.

Water resource

Water volume (km3)

Percent of total water

Oceans

1,320,000,000

97.24%

Icecaps, glaciers

29,000,000

2.14%

Groundwater

8,000,000

0.61%

Freshwater lakes

125,000

0.009%

Inland seas

104,000

0.008%

Soil moisture

67,000

0.005%

Atmosphere

13,000

0.001%

Rivers

1,000

0.0001%

Most of our water is in a saline form (saltwater in the oceans). Of the small amount that is available as freshwater, only a fraction is readily available for our everyday use (such as drinking water). This water distribution explains why water is such a precious resource, even though it seems like we have such an abundance of it.

Nature of science

Science knowledge is subject to change in the light of new evidence or new interpretation of existing evidence. The current theories about how Earth’s water originated are tentative rather than the absolute truth.

    Published 2 June 2009