The whole purpose of manned space programs is to eventually land astronauts on Mars. The Red Planet is the ultimate destination in any effort to explore our neighborhood of the solar system. Mars is the most Earth-like planet that we know, and as such it can tell us a great deal about the history of our own planet, including how life arose and evolved.
A trip to Mars is not only a complex operation, it is quite likely the most complex undertaking in all of human history. By travelling farther than anyone before them, Mars-bound astronauts will be subject to an environment and a sequence of events unlike anything ever experienced by a human being. In addition to the technological problems that will need to be solved to make such a trip possible (hardware, software, propulsion, life support, communication with mission control on Earth, etc.), astronauts could experience potentially serious physical conditions derived from long-duration weightlessness, such as bone decalcification, muscle atrophy or loss of visual acuity. Mission planners will need to provide astronauts with contingency plans in case radiation levels become dangerously high, due to an increase in solar activity or another astrophysical source.
The drive for exploration will undoubtedly overcome these and many other challenges on the way to Mars. Despite currently uncertain political and economic times, a large number of exciting projects are already being undertaken to prepare for an eventual Martian mission. For example, the Mars Society, a non-profit organization that promotes human colonization of Mars, runs a project called Mars Desert Research Station, in which volunteers conduct geological and biological field research in the Utah desert. The volunteers (students and professional scientists) work in conditions similar to those that future astronauts might encounter on the surface of the Red Planet. Different techniques and operational scenarios are tested for human habitation, sample collection, and exploration strategies.
We know that Mars was once wet. Data from space probes that were sent to orbit Mars have provided unequivocal evidence of ancient flows of liquid water on its surface. Life as we know it requires water in the liquid state to thrive, so there is every reason to believe that Martian life could have developed billions of years ago. Perhaps there are fossil remnants of micro-organisms under the surface, or maybe even scattered among rocks and boulders such as those photographed by the Mars exploration rovers, Spirit and Opportunity. Even more exciting would be the possibility that Martian micro-organisms are alive today, in which case a mission that includes astronauts would require additional considerations.
In the case that live Martian microbes exist, there could be a health risk to the crew if adequate measures are not taken. Given that it is probably too early to know the pathology associated with extraterrestrial micro-organisms, different courses of action will need to be devised during the stage of mission design, to prevent crew members from falling ill due to Mars microbes. Likewise, it will be of the utmost importance to protect the Martian environment from imported terrestrial micro-organisms. Even though it is possible to avoid contamination by robotic probes through strict sterilization requirements, once astronauts arrive on the Martian surface the transfer of human bacteria to the Mars environment may be almost inevitable.
One could think that the conditions on Mars are too harsh for terrestrial micro-organisms to survive, and that might very well be the case. The surface of Mars is subject to a very low atmospheric pressure (less than 1% of the mean pressure on Earth), it has an average temperature of -65 degrees Celsius (-85 Fahrenheit), and is showered with energetic cosmic and solar UV radiation. We don’t know of any life-forms that could survive in such an environment. However, there could be niches on Mars where these hostile conditions are attenuated. The conditions in these niches could be similar to those found in extreme Earth environments. For example, we know that terrestrial arid environments contain several life-forms that have adapted to conditions of extreme dryness. Cryptoendolithic microbial communities (that is, organisms that hide inside the pores of rocks) are known to flourish in Antarctica, a true cold desert . Extreme cryophilic (cold-loving) microbes can grow at temperatures of -20 degrees Celsius . Some experiments have shown that microbial species can survive high doses of radiation, even higher than those that can be received on the surface of Mars .
Evidently, mission planners will need to take all of these considerations into account to protect both the crew and the planet itself. They will also have to prepare for the possibility of hypothetical Martian organisms “hitching a ride” to Earth on the spaceship, and so adequate screening procedures will need to be followed. But the important point would be, if microscopic life is found on Mars, that we would have absolute certainty that we are not alone in the universe.
 de la Torre, J. R. et al. 2003, Applied Environmental Microbiology, 69, 3858.
 Rivkina, E. M. et al. 2000, Applied and Environmental Microbiology, 66, 3230.
 Horneck, G. 2008, Acta Astronautica, 63, 1015.