The best way to make Mars habitable would be to inject synthetic greenhouse gases into its atmosphere, researchers said Thursday.

The stuff could be shipped to Mars or manufactured there.

Scientists and science-fiction authors have long pondered terraforming Mars, melting the vast stores of ice in its polar caps to create an environment suitable for humans. The topic is highly controversial.

Some think earthlings have no right to mess with the climate of another planet. Others see Mars as a refuge for people who might need to flee this world as conditions deteriorate. Another argument holds that Mars was likely warmer and wetter in its distant past, and it might have harbored life, so bringing it back to a previous state makes sense.

Among the ideas for how to warm Mars: sprinkling stuff near the poles that would absorb more sunlight; or placing large mirrors in orbit around the planet to reflect more sunlight onto it.

The new research suggests that forcing global warming by injecting greenhouse gases may be the best way to terraform, should governments decide to do so. The conditions warming Earth could be harnessed to transform Mars, the scientists determined.

Jumpstarting global warming in a planet-sized laboratory would be a boon to science in some respects.

"Bringing life to Mars and studying its growth would contribute to our understanding of evolution, and the ability of life to adapt and proliferate on other worlds," says Margarita Marinova, at the NASA Ames Research Center when the study was done. "Since warming Mars effectively reverts it to its past, more habitable state, this would give any possibly dormant life on Mars the chance to be revived and develop further."

The research is presented in the February issue of the Journal of Geophysical Research-Planets, published by the American Geophysical Union.

With a very thin atmosphere and being farther from the Sun, Mars is much colder than Earth. There is no evidence for any liquid water presently on the surface. Liquid water is considered essential to life as we know it.

The polar regions contain vast stores of water ice and carbon dioxide, or dry ice. Theorists have said in the past that melting the poles might thicken the atmosphere, which like a blanket would insulate the surface and eventually create a more Earth-like climate.

Studies suggest Mars had surface water and bouts of rain in its early history.

The new research modeled how manmade greenhouse gases would affect Martian temperature and melt water ice and carbon dioxide ice at the poles.

Artificially created gases could be 10,000 times more effective than carbon dioxide in warming up the Red Planet, the study determined. The gases that would work the best are flourines and could be made from elements readily available on Mars, Marinova and her colleagues found.

Adding 300 parts per million of the gas mixture into the Martian air would trigger a runaway greenhouse effect, according to the models. The polar ice sheets that would slowly evaporate. The newly released carbon dioxide would cause further warming and melting. Atmospheric pressure would rise.

The process would take hundreds or thousands of years to complete, the scientists report.

• Terraforming Mars: Experts Debate How, Why and Whether