"Water is key because almost everywhere we find water on Earth, we find life. If Mars once had liquid water, or still does today, it's compelling to ask whether any microscopic life forms could have developed on its surface."
Water is not evidence of life but it's such a neat little molecule that it means that if water was around, it may have helped make life possible. This is because of some of water's unique properties. For instance, water remains liquid at relatively low temperatures and relatively high ones, a very unusual characteristic for such a large molecule. In addition, water decreases in density when it freezes. Most things become more dense. However, ice floats. Imagine how different our Earth would be if it weren't for that one little fact. For one thing, if Earth cooled a little, water on the surface would freeze and sink, pushing the cold water to the surface, where it would also freeze and sink, until all the water on Earth was frozen. Water is the only substance found naturally in three forms: ice, liquid, and gas (vapor).
Our Earth is made of 70% water and our bodies are composed of 75% water. 75%! Water is essential in order for all the trillions of cells in our bodies to stay hydrated, excrete wastes, and absorb nutrients. It's essential for chemical reactions, oxygen delivery to the cells in our bodies, and basically every process needed for our bodies to run. That's why it's important to stay properly hydrated. Drink lots of water (about 8 glasses a day)! Did you know? It takes a human up to a few weeks to starve to death but only a few days to die of dehydration. In addition, all life on Earth is dependent upon a clean, reliable supply of water to survive. This is why water conservation efforts are so important.
NASA/JPL-Caltech/Univ. of Ariz.
Water on Mars
Water and the Body
Why is Water Important