Ethan Siegel in Forbes:
When we think about the ingredients necessary for life on Earth, liquid water is always right at the top of the list. Without it, there’s simply no good way to for transport of ions and molecules to occur. Sure, some other liquid might — in principle — be able to substitute for water, but nitrogen, carbon dioxide and methane don’t have that awesome polar structure that water has, which allows for such a wide variety of molecules to dissolve and be moved from one location to another.
For a long time, it was known that Mars had a wet, watery past, likely for over a billion years in the early Solar System. Things like dried up riverbeds, sedimentary rock formations, round spherules known as Martian blueberries and other patterns of erosion teach us that — at one point long ago — Mars likely had oceans covering its surface more than a mile deep.
But all that ended long ago. As Mars is much smaller than Earth, its core cooled more quickly, meaning it lost its protective magnetic field. Without it, there was insufficient protection against the solar wind, which over just a few million years stripped that thick atmosphere away, making stable, liquid water on the surface impossible. What was left either had to exist as water vapor (the gaseous phase) or ice (the solid phase), something we’ve found copious amounts of evidence for. When we look at the Martian surface today, we find polar icecaps, clouds, sub-surface ice (via digging) that sublimates as soon as it’s exposed, and even seasonal frozen lakes.