Artificial greenhouse gases that are bad news on Earth could provide the means to make Mars a more comfortable place for humans to live.

by Dr Tony Philips

To say that Mars is a chilly place would be an understatement. The Red Planet's mean annual temperature is 55 degrees C below zero - that's about the same as the temperature of Earth's south pole during winter.

If humans ever build communities on Mars, they might want to find a way to turn up the global thermostat. At a NASA-sponsored conference, "The Physics and Biology of Making Mars Habitable", scientists discussed ways that future colonists might make the frigid planet a little more comfortable.

One solution might be to pump enough greenhouse gases into the Martian atmosphere to create a runaway greenhouse effect. Here on Earth, the idea of a runaway greenhouse sets off alarm bells. But on Mars it could be a plus. Scientists at the conference speculated how it might be possible to warm Mars just enough to evaporate the planet's available carbon dioxide (CO2 trapped in ices and frost) into the atmosphere, where such gases could contribute to keeping the planet warm.

But there are two problems. First, even if all of Mars's available CO2 were coaxed into the atmosphere, it wouldn't necessarily warm the planet enough to make it a comfortable place for humans, because no one knows just how much CO2 is there. Second, the best way to get Mars to release its CO2 spontaneously is, well... to warm it up. It's a "Catch-22" situation!

PFCs have several advantages. First, they are super-greenhouse gases. A little bit does a lot of warming. Second, PFCs have a very long lifetime. This causes serious problems on Earth, but their longevity would be a positive factor on Mars. Third, they do not have any negative effects on living organisms.

Finally, unlike their chemical cousins, chlorofluorocarbons (CFCs), PFCs don't deplete ozone. Ozone in Earth's atmosphere provides protection against ultraviolet (UV) radiation, which is harmful to life. On Mars, building up an ozone layer in the atmosphere would be an important goal of terraformers. "You don't want to destroy ozone," says Marinova, "because it's a UV protector." 

The sunlight that hits a planet's surface arrives primarily as visible and ultraviolet light. The planet absorbs this solar energy, and then re-radiates warming infrared energy back out into the atmosphere. Greenhouse gases in the atmosphere work as a global layer of insulation, trapping that infrared radiation and preventing it from escaping into space.

CO2 and water are good at trapping some of this infrared energy, but not all of it. On Earth, there's so much CO2 and water in the atmosphere that it doesn't matter if some infrared radiation escapes back into space. 

But on Mars, terraformers will want to trap every bit of heat they can. A carefully chosen combination of PFCs could do the job quite handily. 

"When we first start warming Mars," explains Marinova, "we'll want to cover the whole spectrum" of thermal infrared radiation. "Once CO2 is released, it will take over" part of the job, and PFCs will only need to be used to plug the gaps. 

An artist's concept of a full-fledged Martian greenhouse, an essential part of any future human colony on the Red Planet.

And how fast can Mars be heated up? 

"That depends," says Marinova, "on how fast we make the gases." According to rough calculations, "if you had 100 factories, each having the energy of a nuclear reactor, working for 100 years, you could warm Mars six to eight degrees." At that rate, to increase the average Martian temperature to the melting point of water -- it's about minus 55 degrees Celsius now -- would take about eight centuries. Actually, it wouldn't take quite that long, Marinova points out, because her calculation doesn't include the feedback effect of the CO2 that would be released as Mars got steadily warmer. "Devising more efficient artificial super-greenhouse gases will also make it faster," Marinova adds.

Human habitation of Mars is a long way off. NASA's current plan for exploring the Red Planet, which spans the next two decades, does not include even a pioneering human mission to Mars. By the time a permanent settlement is established there -- one that might begin the task of terraforming the planet -- technological advances may make it possible to warm its atmosphere far more efficiently than is possible using the techniques being studied today by scientists like Marinova.