It is possible that a region of 1% or less may have been hospitable enough to support primitive life. I presume that we will not be able to rule out past life on Mars ever, even if all of Mars has been studied and documented. If indeed evidence of life is found, it would surely alter the prospects of life elsewhere throughout the universe. The debate will then be is life very possible throughout the galaxy or is the solar system (as opposed to earth alone) just unique.