Microbial life are the microorganisms found in things. These can be cells and on Earth microbial life lives almost everywhere. Since water has been found on Mars, there is a belief that there is microbial life on this planet. However, there are some people who do not believe this just because water was found there.
Plus, years ago a mission went to Mars in the mid-1970s and coupled with what they found it is believed that life existed on Mars, so microbial life most likely existed on Mars as well. It is difficult to try to explore the other planets other than Venus and Mars, due to the temperature on that planet since it is so close to the sun.