A lot of intriguing evidence has been collected over the years in support of the idea that life once existed on Mars. No definitive proof has been found, in the form of indisputable fossil evidence, for example. But it is known that Mars was once a watery world, and the existence of liquid water on the surface of a planet is the primary requirement for life to evolve.
Ample evidence has revealed that water once ran across the Martian landscape freely and in multiple locations. Liquid water can be found even now on Mars, at significant depths below the planet’s surface. Frozen water is located closer to the surface, and in impressive amounts. Much of this is no doubt the remnants of water that once formed Martian lakes or rivers.
The existence of liquid water on Mars, alone, wouldn’t guarantee it was inhabited. In fact, for a long time it was believed that surface water on Mars would have been highly acidic and therefore incapable of supporting living creatures.
However, a 2013 discovery by NASA’s Mars Rover Opportunity proved that at least some of the water that ran across the surface of Mars in ancient times carried a neutral pH, which means it was benign and could be consumed. This was an extremely significant finding by the Opportunity rover, since it proved that water quality on Mars in the distant past would not have been a barrier to the evolution and development of living species.