There’s now more evidence that water did in fact flow on Mars. The NASA Exploration Rover Opportunity, which has been exploring the Martian surface for more than 7 years, found a “mineral vein” made up of gypsum.
According to NASA:
“This tells a slam-dunk story that water flowed through underground fractures in the rock … This stuff is a fairly pure chemical deposit that formed in place right where we see it. That can’t be said for other gypsum seen on Mars or for other water-related minerals Opportunity has found. It’s not uncommon on Earth, but on Mars, it’s the kind of thing that makes geologists jump out of their chairs.”
Read more about the possibility of water on other planets, and even some moons, here.