Scientists are using the forested slopes of a Mexican volcano as a test bed to see if trees could grow on Mars, part of an experiment of making the chilly and barren planet habitable for humans. Scientists at NASA and Mexican universities believe if they can warm Mars using heat-trapping gases, raise the air pressure and start photosynthesis, they could create an atmosphere that would support oxygen-breathing life forms.? Getting trees growing would be a crucial step. The scientists’ quest has taken them to the snow-capped Pico de Orizaba, a dormant volcano and Mexico’s tallest mountain to examine trees growing at a higher altitude than anywhere else on Earth.? We have experienced warming on Earth with greenhouse gases, but on Mars scientists could do it faster with more powerful gases.? The first human mission to Mars is seen 10 to 15 years away, and the warming-up process could start 50 years later.? By pumping in highly insulating gases like methane or nitrous oxide, the scientists think they could heat Mars to 41 degrees Fahrenheit from minus 67 F now. That would match temperatures where trees grow at 13,780 feet on Pico de Orizaba.? Having trees on Mars, as opposed to only simple plant forms like algae or lichens, would open the possibility of humans one day being able to breathe Martian air.