Over the last decade and a half, NASA's sent seven successful probes to Mars: a lander, three rovers, and three orbiters. Their orders were simple: Follow the water. And they've followed those orders quite well, finding that there's water just about everywhere you look.

Water is a necessary ingredient for life as we know it, so finding water on Mars enhances the prospects that we'll find life there -- either life today, or evidence of life in the past. Water is also a great resource for future explorers: Not only can they drink it, they can also use it to produce oxygen and rocket fuel.

Even before the new round of exploration, there was plenty of evidence that Mars was once a warm, wet world. Riverbeds and outflow channels cross the planet, and there's evidence of ancient coastlines.

The recent probes have found that the Martian ice caps consist mainly of frozen water, not carbon dioxide as previously thought. They've found clouds made of ice crystals floating through the sky, and evidence of occasional snowfall.

The Phoenix lander found solid ice just an inch or two below the surface. And the orbiters have found large deposits of sub-surface ice across much of the planet.

So the Mars explorers have done their job -- they followed the water. The next Mars mission -- a sophisticated chemical laboratory -- will extend that work to learn more about the Red Planet's suitability for life.

We'll talk about a Martian moon tomorrow.

Script by Damond Benningfield, Copyright 2011