Probability of microbial life on Mars

From Why Not Mars by Maciej Cegłowski

"At this point, it is hard to not find life on Earth. Microbes have been discovered living in cloud tops[28], inside nuclear reactor cores[29], and in aerosols high in the stratosphere[30]. Bacteria not only stay viable for years on the space station hull, but sometimes do better out there[31] than inside the spacecraft. Environments long thought to be sterile, like anoxic brines at the bottom of the Mediterranean sea[32], are in fact as rich in microbial life as a gas station hot dog. Even microbes trapped for millions of years in salt crystals[33] or Antarctic ice[34] have shown they can wake up and get back to metabolizing[35] without so much as a cup of coffee."

As Ceglowski points out, this should make us pretty confident that microbial life already exists on Mars, if only from a stray asteroid.