Greenhouse gases might one day be used to warm the cold surface of Mars, and make the planet habitable for humans.
To say that Mars is a chilly place is a bit of an understatement. The mean annual temperature on the red planet is minus 55 degrees Celsius. It is far too cold for human habitation. Which explains why those who believe that humanity one day will establish colonies on Mars take very seriously the problem of how to warm the planet up.
Humans won't be building thriving communities on Mars any time soon. But that hasn't stopped scientists from trying to figure out how the task of turning up the Martian thermostat might be accomplished. At a recent NASA-sponsored conference
Mars, the topic was given a good deal of attention.
One solution is to pump enough greenhouse gases into the Martian atmosphere to create a runaway greenhouse effect. On Earth, the idea of a runaway greenhouse sets off alarm bells. But on Mars, say those who set their sights on terraforming, it's a plus. It simply means warming up Mars enough that all of the planet's available carbon dioxide (CO2
) evaporates into the atmosphere, where it can contribute to keeping the planet warm.
But there are two problems. First, even if all Mars's available CO2 were coaxed into the atmosphere, it still 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 kind of a vicious cycle.
Margarita Marinova, an undergraduate student at MIT, believes she has an answer to both problems: use artificially created perfluorocarbons (PFCs) to initiate the planetary warming process. Marinova has been studying the warming effects of PFCs, in collaboration with Chris McKay, a member of the NASA Astrobiology Institute at NASA Ames Research Center. McKay was one of the organizers of the terraforming conference at which Marinova presented the results of her research.
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 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 - there is only a miniscule amount at present - 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 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 out 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.
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 at present - 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 on Mars - one that can 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.