As NASA struggles to refocus its Mars program on the heels of two failed missions, one supremely logical mantra has emerged to guide the search for Martian life: Follow the water. Great advice. But hard to heed on such a dry and dusty planet. No matter how much water might once have flowed on Mars, the place now looks downright dead. Liquid water, if it exists, might be a mile deep or more, scientists say. But there are two places where water is known to exist in mass quantities, right at the surface: The polar caps.