WATER: The Liquid of Life

It's the most amazing substance in the universe. Without it, there would be no life - no plants, no animais, no you. Somebody should bottle this stuff.

From space, astronauts see Earth as it really is-a big, blue ball, a water planet. Oceans cover more than two-thirds of Earth's surface. The continents are mere islands in this vast global sea. The Pacific Ocean alone covers an area larger than all the continents put together.

Shimmering like a blue jewel in the darkness of space, Earth is an oasis in our solar system. Our planet formed at just the right distance from the sun. We are neither too hot nor too cold: Earth's moderate temperatures make liquid water possible. Other planets have ice or water vapor. Some bodies in our solar system have liquid water trapped inside them. But only Earth has liquid water on its surface. And liquid water means life.

Did It Come from Outer Space?

Nobody knows how liquid water formed on Earth, but there are several theories. One is that the gas (mostly hydrogen) and particles that formed Earth and other planets contained water vapor. The region of the solar system where Earth formed was cool enough for the water vapor to become liquid water, and for the liquid water to then stick to the Earth's rocky surface.

Another theory is that asteroids brought water when they collided with the young Earth. Some of these asteroids were the size of the moon or Mars, or even bigger, and they contained a lot of liquid water inside them. Just a few collisions would have supplied all of Earth's water.

We may never know where Earth's water came from, but, whatever its origin, there is evidence that it has existed here for more than 4 billion of our planet's 4.5 billion years. In other words, water was present soon after Earth and our solar system formed.

Water Born

Water not only makes life possible, it is where life on Earth began. Scientists suspect that life first came to be in shallow tidal pools where the chemistry was just right for living cells to form. The oldest fossils that have been found are reef-building organisms called cyanobacteria that date back 3.5 billion years.

Cyanobacteria are still among the most important bacteria on the planet because they help fill our atmosphere with oxygen we need to breathe. They do this as they make food for themselves from sunlight, water, and carbon dioxide. This amazing process, called photosynthesis, gives off oxygen as a waste product. Cyanobacteria have been pumping oxygen into the atmosphere for millions of years.

Once life got started, it evolved from simple one-celled bacteria to more complex organisms. Eventually, a wondrous variety of animals and plants inhabited land and sea. Today, 9 out of 10 organisms on the planet live in the oceans.

Water Off a Dinosaur's Back

Amazingly, nearly every drop of water present when the seas formed remains on Earth today. The water coming out of your faucet may have quenched a dinosaur's thirst or glistened as dewdrops on the petals of an ancient flower. And, new water does not form. The water here 4 billion years ago is the water we have today.

Water is always in motion, cycling from oceans to clouds to rain and snow (sometimes stopping for a while as ice) and back to the ocean. This is called the water cycle, and it connects all life on the planet.

Even as all that water recycles around and around, there is surprisingly little that's suitable for us to drink. Wherever water flows, it picks up chemicals, minerals, and nutrients and deposits them into the oceans. For millions of years, rivers and streams have washed over rocks, picking up sodium chloride, which we know as salt, and carrying it to the sea. As the water in oceans evaporates, the salt remains. This means that seawater becomes more and more salty-and undrinkable. Only about 3 percent of the water on Earth is salt free. But more than half of that is locked in glaciers and the polar ice caps. Of the remaining fresh water, found in lakes, rivers, and below ground, we can reach less than half. All told, humans can use only about 1/300th of all the water on Earth to drink and water crops.

Wonderfully Weird

Water is really weird. It doesn't act like anything, else on Earth. As you know, water can be liquid, ice, or vapor (in the form of clouds, mist, or fog-or steam, when water boils). It is the only substance that naturally exists on Earth in all three of these forms-liquid, solid, and gas. It can even exist in all three forms in the same place at the same time-imagine clouds over a lake that's frozen solid on top, with liquid water below.

If water acted like other substances, its solid form, ice, would sink. Almost every other substance becomes heavier and smaller as it changes from a liquid to a solid. But water expands and becomes lighter as it freezes. So ice floats. It's a good thing for us that it does. If ice sank, the lakes, rivers, and oceans would be frozen solid. Earth would be unlivable.

Water's freezing and boiling points are also strange. Because of the water molecule's small size, physicists say it should freeze at around -148 degrees Fahrenheit and boil at -112 degrees Fahrenheit. But, again, water shows its freakish nature: it freezes at 32 degrees Fahrenheit, and boils at 212 degrees Fahrenheit. This gives water the highest freezing and boiling points of almost any liquid or solid.