MEXICO CITY (Reuters) - Scientists are using the pine-forested slopes of a Mexican volcano as a test bed to see if trees could grow on a heated-up Mars, part of a vision of making the chilly and barren red planet habitable for humans one day.

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.

"It sounds like science fiction, but we think it's feasible," said research professor Rafael Navarro-Gonzalez, who has spent nine years examining Pico de Orizaba's pine forests.

"We have experienced warming our planet with greenhouse gases, but on Mars we could do it faster with more powerful gases," he said in his lab at Mexico City's UNAM university.

The first human mission to Mars is seen 10 to 15 years away, and the warming-up process could start 50 years later, NASA scientist Chris McKay said. There will also be ethical issues to overcome.

"It's playing gardener more than playing God, but the ethical questions are important," McKay said.

By pumping in highly insulating gases like methane or nitrous oxide, the scientists think they could heat Mars to 41 degrees Fahrenheit (5 degrees Celsius) from minus 67 F (minus 55 C) 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.

The scientists are studying what makes trees refuse to grow above a certain point, where temperatures drop and the air becomes thinner, to see how easily they could grow on Mars.

"Things don't really start cooking from a biological point of view until trees start growing. Trees are the engines of the biosphere," McKay said.

"It's possible Mars could have trees in 100 years. (But first) we need to understand what sets the tree line on Earth," McKay said by telephone from NASA's Ames center in California.

NO CALLS TO EARTHLINGS

Despite Mars' lifeless rocky surface, burning ultra-violet radiation and its extremely thin, carbon dioxide-loaded air, humans have for long been obsessed with finding life there.

Scientists believe Mars has ice at its polar caps that could melt into seas and that its subsoil contains the key elements needed for life.

Even though none will live to see the fruit of their work, the scientists on the Pico de Orizaba project believe it would be fairly straightforward to pump greenhouse gases into Mars' atmosphere, introduce bacteria to start photosynthesis and finally send up tree seeds with a human mission.

"Nothing that we know rules it out. There's still a lot of uncertainty, but nothing that's a showstopper," McKay said.

The project would be called off if life was found to already exist on Mars.

"The idea is to explore the possibility of colonizing Mars. If there is life, we have no right to destroy it. But if Mars is barren we could take life from Earth to Mars," said Navarro-Gonzalez, spinning a Mars globe that shows ravines 6 miles deep and dizzying 10 mile high mountains.

His "before" and "after" images show the arid planet transformed into a new world of lush green plains, lakes and mineral-rich mountains that could one day supply earth.

Still, that vision is centuries away. For now, anyone braving the six-month flight to Mars would have to live in a pressurized dome, suffer violent dust storms and be cut off from earthlings too far away to easily speak to.

In the long term, Mars's low gravity could also have odd effects on would-be settlers, causing people to grow alarmingly tall, and cosmic radiation could cause cancers and mutations.

McKay ruled out anything more permanent than short-term research bases for the next century. "I don't have this vision of people moving to Mars the way people settled the New World, setting up homes and bringing their families."

COMMENTS: YUPPPPIIIIII!!!!!.....If you learn how life got to earth and how at one point....You see  you have to go to the beggining when the atmosphere on earth was not breathable and how after that began life evolution...One thing i never did learn is how plants/trees got here on Earth...I think you will have to do some genetically ingeenered trees, trees that can breathe whatever gas they got over there in mars (i forgot what kind of gas it is) and convert it to Oxygen, they will have to do some serious biochemistry genetically engineer research & experimentation to accomplish that.. ....You see years of evolution can't be easily design and i think it is easier than trying to change the atmosphere of mars...Unless they invent like some wheather balloons (like how i saw on Discovery/TLC channel) that will go up in the air stay in the air and convert That particular gas into oxygen..I believe they can do it...I saw it on the discovery channel but the problem was that the machine could only manufacture very little CO2 (i think it was) into oxygen...But before they do this THEY GOT to find the FOUNTAIN OF YOUTH...Something that will make us live to be 300 YEARS OLD and find the cure for CANCER...You have to believe me..

to be continued...