The surface of Mars, as far as we can determine, is inhospitable to Life as we know it. Could we willfully change the conditions on that planet to make it able to host living things from Earth? People included, maybe?
Courtesy: Don Dixon
The concept of changing another planets environment to make it more hospitable has been dubbed "terraforming", meaning "making similar to Earth". Is it worth thinking about?
The main problem is that there is virtually no atmosphere, barely 7 millibars, or 0.07% (seven hundredths of one percent!) the average pressure at the surface of Earth. We should need a pressure suit to go for a walk in this desert. What little atmosphere there is consists of carbon dioxide (95%), which is poisonous to us oxygen breathers. Although plants can use it, they also need oxygen for respiration. The rest of the atmosphere is mainly nitrogen (2.7%) and argon (1.6%), which we are accustomed to. There is very little oxygen (0.15%) and only a trace of water vapor.
How then would a Martian environmental engineer proceed to make this planet habitable?
To increase the atmospheric pressure one could heat the planet and evaporate carbon dioxide out of the existing polar caps (giving as a generous estimate 300 millibars). Also, we might derive much gas from a hypothetical reservoir in the churned-up Martian surface layer, the regolith. (See: NASA Astrobiology Institute for an artificial greenhouse gas technique by Chris McKay and student Margarita Marnova.) These techniques would provide a fairly thick but breathable atmosphere.
We could next introduce certain bacteria brought from Earth, with the aim to have them modify the Martian atmosphere, as their ancestors did for Earth. The primitive cyanobacterium Chroococcidiopsis would be a good candidate. Farming this hardy extremophile could convert some of the carbon dioxide to useful oxygen and sequester the carbon.
There are other problems that are likely to arise. One is that the surface of Mars appears to be covered by an extremely reactive hyperoxide layer. The constitution, origin and depth of this layer are currently unknown, but it would rapidly oxidize (burn up) any organic materials with which it came in contact (including our favorite bacteria). We are not familiar with any organisms which deal with such environments here on Earth, but perhaps there is some such adaptation. After all, we have found a bacterium which tolerates a radiation dosage 1000 times that lethal to humans. Bioengineering an organism that can survive high oxygenation potential may not be out of the question. Acid-loving bacteria might have good candidates.
One problem, however, seems especially difficult to handle. In the long term, Mars is simply too small a planet to hold onto an atmosphere for any great length of time.