From space, Earth looks like the watery planet that it is. Water covers more than 70% of Earth's surface. Pure liquid water is one of nature's most important gifts. Odorless, tasteless, and clear, liquid water is essential for life as we know it. Without it plants, animals, and people would die.

Water's properties make it unique. Each water molecule is made of one oxygen atom and two hydrogen atoms. The arrangement of the one oxygen atom to two hydrogen atoms creates a polar molecule , which means that the hydrogen ends of the molecule have a positive charge. The oxygen end of the molecule is negatively charged. These charges cause water molecules to be attracted to other water molecules, making water kind of "sticky." As water molecules attract other water molecules, they tend to clump together and create a water drop. Without Earth's gravity pulling on a water drop, the shape of that drop would be a perfect sphere. When a substance is attracted to itself, it is called cohesion.

Water's polarity also attracts other materials. When two different substances are attracted to each other, the force is called adhesion. Adhesion is at work when children build sand castles. Dry sand particles won't stick together, but a little water helps you shape and mold the sand. Another clear example of adhesion is how a paper towel attracts water.

Liquid water is also an excellent solvent. In fact, it's called the "universal solvent" because more materials dissolve in water than in any other liquid. The problem though, is that wherever water flows, it picks up valuable chemicals, minerals, and nutrients. Traveling over and under ground, or through your bodies, water picks up and transports materials. While water has the ability to dissolve more substances than any other liquid, luckily for us it cannot dissolve lipids and other fatty acids. These types of molecules make up cell membranes. If cell membranes dissolved in water, we would end up as goo.

Liquid water helps human beings digest and absorb foods. It also helps the body eliminate waste materials. It maintains good muscle tone and helps supply oxygen and nutrients to cells. Water, as sweat, cools our bodies. To maintain good health, experts recommend that we drink at least eight glasses of clean water every day. In fact, by some counts, up to 60% of the human body's weight is water. Your brain is approximately 70% water, your blood is about 82% water, and your lungs are nearly 90% water.

Human beings aren't the only water-heavy living things. By some accounts, most living things are, by weight, 50% to 90% water.

Just as liquid water is critical for life on Earth, NASA believes that it is an essential ingredient for life on other planets. Scientists can find traces of past liquid water activity on Mars in the rocks, minerals, and geologic landforms, particularly in those that can only form in the presence of water. The Mars Exploration Rovers explore how past liquid on Mars has influenced the red planet's environment. Rovers are specially equipped with tools to study a diverse collection of rocks and surface soils that may hold clues to Mars's watery past.

Currently, water on Mars exists in two states: gas and solid. Liquid water does not currently exist on the surface of Mars, but evidence from the Mars Global Surveyor, Odyssey, and Exploration Rover missions suggest that water once flowed in canyons and persisted in shallow lakes billions of years ago.

The Phoenix Mars Lander, scheduled for launch in August 2007, will dig for water and complex organic molecules in the artic plains of Mars. Phoenix will probe the history of liquid water that may have existed in the Martian arctic region as recently as 100,000 years ago. Scientists will better understand the history of the Martian arctic after analyzing the chemistry and mineralogy of the soil and ice.

Because liquid water has played an important role in the origin and development of life on Earth, it may be a key player in understanding any possible signs of life on Mars.