from Scientific American:
NASA scientists announced today the best evidence yet that Mars, once thought dry, sterile and dead, may yet have life in it: Liquid water still flows on at least some parts of the Red Planet, seeping from slopes to accumulate in what might be life-nurturing pools at the bases of equatorial hills and craters. These remarkable sites on Mars may be the best locations in the solar system to search for extant extraterrestrial life-- but doing so will be far from easy.
Examining potentially habitable regions of Mars for signs of life is arguably the primary scientific justification for sending humans there-- but according to a new joint review from the National Academy of Sciences and the European Science Foundation, we are not presently prepared to do so.
The problem is not exploding rockets, shrinking budgets, political gamesmanship or fickle public support—all the usual explanations spaceflight advocates offer for the generations- spanning lapse in human voyages anywhere beyond low Earth orbit. Rather, the problem is life itself-- specifically, the tenacity of Earthly microbes, and the potential fragility of Martian ones. The easiest way to find life on Mars, it turns out, may be to import bacteria from Cape Canaveral-- contamination that could sabotage the search for native Martians. The need to protect any possible Martian biosphere from Earthly contamination, the review's authors wrote, could "prevent humans from landing in or entering areas" where Martian life might thrive. Although this sentiment is not new, its frank, formal acknowledgement in such an authoritative study is rare indeed. NASA is planning to send humans to Mars as soon as the 2030s; that such missions may unavoidably pose extreme contamination risks is understandably not something the agency is eager to highlight, even as it actively researches possible solutions to the problem.
The details can be read here.