- If Mars once-upon-a-time had liquid water on the surface (under a thicker atmosphere), then maybe it had life then, too.
- If Mars has water today (as subsurface ice or maybe even liquid water deep underground), that has implications for human exploration. The less you have to import from earth, the easier it is to support a long-duration human presence on the planet.
So far, we know that Mars has CO2 (in the atmosphere), excess oxygen (in rocks containing perchlorate salts), the trace minerals (like iron and phosphorus) that living things need, and some sunlight. Add water. All you need to grow food is a structure to hold an atmosphere (think dome) and maybe some artificial heat (think nuclear reactor).