Alien life on Mars is a very real possibility. It's not green men, martians, or anything like that, but the organic basis of life, in my opinion, and the opinions of some others, is that there's a big possiblity that life exists on Mars in a very primitive form.
Many conditions on Mars prohibit life from evolving, but not from existing. Many phenomenon that occur on Mars are, if we observe them on this planet, of an organic nature. And there are plenty of inorganic ways that these can be achieved, but my hypthesis is that if their are multiple processes occuring that appear to be of an organic nature, and you can explain each one individually in an inorganic way, doesn't the likelihood of life increase with each seemingly organic process?
For a more detailed post definitely check out my previous post about why I think theres life on Mars.