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A World of Limited Water: Earth Image of the Week June 1, 2012
Satellite Image of Strait of Gibraltar
About 97 percent of Earth's water is salty and contained in the oceans, salt lakes and seas. The rest is in rivers, lakes, ponds and streams.
Peering down on Earth from space, or even looking at a map of the planet, can give the impression that is composed mostly of water.

But very little of Earth’s total mass is actually made up of that key ingredient of life.

While water does cover about 70 percent of the world’s surface, the vast oceans are very shallow compared to Earth’s radius.

To illustrate just how limited water resources are on our home planet, Jack Cook of the Woods Hole Oceanographic Institution created the graphic to the right. It simulates just how small the world’s total water supply would be if captured in a single sphere.

The radius of such a ball would be about 435 miles across, capable of covering only parts of the American West.

That’s also only about half the radius of the moon and slightly larger than Saturn’s moon Rhea, which is mostly composed of water ice.

Howard Perlman, of the U.S. Geological Survey, points out that how this much water came to be on Earth is still a topic of heated debate and research.

The same is true whether there are any significant amounts of water trapped far beneath the planet’s surface or not.

Image: Jack Cook - Woods Hole Oceanographic Institution