Earth is often called the water planet because about 70 per cent of it is covered by water. Pictures from space show it to be a beautiful blue jewel, veiled in trailing clouds. It is a lovely home for living beings.

 Not all of this water is really available for our use, however. 97.3 % of the water is in the oceans, and the oceans are salty. Over billions of years rain has washed salts and other minerals into the seas. The water has evaporated, leaving the minerals behind. Our bodies, and the land plants around us, cannot make use of salty water for life processes.

     The oceans do make our planet habitable, however. The water that evaporates from the ocean surface forms clouds that water the land area with rain and snow. The huge volume of water tends to stabilize temperatures on the planet.

     The ocean is also rich in fisheries and other sources of food.

     As land dwellers, however, let us look at the water situation more closely now.  

     Beside the huge cube that represents the ocean water, we see the tiny portion of the water that is fresh. However, this is not all available to us.

 
When we see how little of the earth's water sustains life on the land, we cannot help but be concerned about preserving that water. Right now our water supply is being contaminated with biological and industrial wastes and poisons. Once groundwater is contaminated, it is very difficult (perhaps impossible) to restore its purity. We are also pumping water out of the deep aquifers more quickly than rainfall can refill them. Our children may come to regret our thoughtless misuse of the precious water on our world.

     As you design your planetary system, consider the presence of salt among the minerals on your planet. If you have very little land, your oceans may not be as saline as the oceans on earth. If you have much less water, you can expect more salty seas. We have some small, salty seas on earth: the Dead Sea and the Great Salt Lake, for instance. When water is saturated with dissolved salt, the excess salt precipitates out as crystals.