Even though many scientists would say that there is a possibility that environments hospitable to life exist, there are dissenting views about whether it would make any difference if microbes from Earth are introduced to them.
In a 2013 paper in Nature Geoscience, "The Overprotection of Mars", Alberto Fairén and Dirk Schulze-Makuch suggested that we no longer need to protect Mars, arguing that if life can get to Mars on our spacecraft, then Earth life has probably already reached it on meteorites [REF].