Life could exist anywhere, really. Sure, you can base the existence of life in other planets based on how it is on Earth (water, oxygen, and all other factors) but that's how we evolved-- we evolved in these circumstances.

If there will be life in other planets where we humans cannot possibly live in because of biological factors, it is possible too because the life there would be existing and evolving based on the circumstances of that planet's nature and composition.

Regarding Mars, however, we cannot say for sure until we have explored every space in that planet.