Human Life on Mars

Now that we've discussed the volatile resources of Mars and its atmospheric evolution in some detail, we can begin to look at the prospects of complex life, human or otherwise on Mars. The following table, adapted from McKay et al. [1991] shows the upper and lower limits of various parameters for the survival of plant and human life, as well as the current values for Earth and Mars.

Parameter	Earth	Mars	Lower limit for plants	Lower limit for humans	Upper limit for plants	Upper limit for humans
Surface Temperature	288 K	208 K	273 K	273 K	303 K	303 K
Atmospheric Pressure	1.01 bar	7 mbar	10 mbar	0.5 bar	-	5 bar
O2 concentration	0.2 bar	0.9 mbar	1 mbar	0.13 bar	0.3 bar	-
N2 concentration	80 mbar	2 mbar	1 - 10 mbar	0.3 bar	-	-
CO2 concentration	0.37 mbar	6.7 mbar	0.15 mbar	-	-	10 mbar

It is critical to factor in the requirements for plant life as well as for humans when considering terraforming. Although we may be able to withstand higher oxygen concentrations than plants, we cannot survive a prolonged period without them. They replenish our oxygen supply and provide most of our food. In fact, it wouldn't be terraforming without plants. The most striking thing one notices when looking at our planet (aside from the liquid water) is that most of the land is completely covered in plant life.

Oddly enough, nitrogen is the limiting gas species for plant life on Mars. There is more than enough CO₂ for photosynthesis, nearly enough oxygen for respiration and the atmospheric pressure is close to the lower limit. In fact in some regions, the atmospheric pressure reaches 9 or 10 millibars and some hardy terrestrial plants could be found to survive such conditions. However, there is too little N₂ for nitrogen fixation for some plants by a factor of 5. Nitrogen must also be found in the soil for plants to thrive. Currently, the N₂ levels in the Martian soil are unknown. If these are later found to be low, Robert Zubrin [Zubrin & Wagner, 1996] suggests we may be able to make fertilizer out of the atmosphere.

Nitrogen is also important for humans and other animals. We require more than 100 times martian levels of nitrogen, not for our chemistry like plants, but because we need some non-reactive buffer gas to bring the total atmospheric pressure to acceptable levels.

While the chemistry and atmospheric pressure may be acceptable to certain species of plant life, the temperatures certainly are not. All life known on earth requires liquid water, and Mars is too cold by 65 K for this to occur. Occasionally temperatures in the tropics rise above freezing, but the low atmospheric pressure causes it to evaporate. The gamma ray neutron spectrometer on the Mars Odyssey found little water in the equatorial regions, while it was plentiful as permafrost in higher latitudes. Surface temperatures and atmospheric pressures need to be raised significantly if Mars is to be terraformed.

Studies by Zubrin and McKay [1996] show that if the temperature at the South Polar region were raised by just 4 K, a "runaway" greenhouse effect would begin. By runaway, I simply mean a positive feedback cycle, not the same severity of runaway greenhouse that happened to Venus. The cycle on Mars would eventually be limited by the availability of volatiles. With this temperature increase, the CO₂ frozen in the southern polar cap would begin to sublimate, releasing the gas into the atmosphere, raising the pressure somewhat. Zubrin and McKay claim that 50 to 100 millibars of CO₂ could be added into the atmosphere in this way in a matter of ten years or so. This added greenhouse gas would then raise polar temperatures on the order of 40 K. At these temperatures, the CO₂ reserves locked in the regolith may start to diffuse into the atmosphere. Up to 400 millibars could be released in this way, although it is unlikely that the entire CO₂ inventory of the ground would be released into the atmosphere. As the temperatures warm, the ground will pull some of the gas back in. Nevertheless, the resulting greenhouse effect has the potential to raise average tropical temperatures on Mars to about 265 K. To diffuse all of this CO₂ out of the regolith, the higher temperatures must penetrate deeper and deeper into the regolith. A timescale of hundreds of years would be required for this stage. We can see that it is not out of the question for Mars to become a place suitable for plants, although such a process would require many generations. What remains to be determined is how to initiate the 4 K polar temperature increase, and how to raise the temperature the final ~10 K to get liquid water. Below are shown graphs of the temperature and pressure increases due to this estimate influx of CO₂.

The above graphs come from Zubrin and McKay's page on Technological Requirements for Terraforming Mars"

Images of southern polar cap formations show the ice cap is receding. Image taken from Enterprise Mission page on Martian Warming

Click on the above image for an animation showing snow accumulation and ablation over a Martian year. Taken from MOLA page

Recent studies [Malin et al., 2001; Paige, 2001] indicate the initial temperature rise may be occuring now, without our help. The image to the left shows evidence that the upper layers of the permanent southern cap are disappearing. Smith et al. [2001] have commented on the variations in snow depth, and an unexpected period of ablation. The image on the right links to a movie which shows these variations in snow depth. Malin et al. predict that this CO₂ should be precipitated on the northern cap, but no evidence is seen for this. Therefore, they infer that it is being added to the atmosphere. However, these changes are taking place on a timescale much greater than the decade predicted by Zubrin. The martian atmosphere is expected to thicken by about 1% every decade and to double every few centuries.

Zubrin and McKay propose a method of quickening this process by using kilometer scale orbital mirrors to focus sunlight onto the pole. This plan may also be used to heat the greenhoused Mars the final few Kelvins required for water to melt and flow on the surface. Once this happens, water vapor can enter the atmosphere boosting the greenhouse to a level capable of sustaining the temperatures without the need for the mirrors. These mirrors will be far too large to assemble on Earth and launch into space. Perhaps they might be built on one of the Martian moons.

Once the challenge is met of changing Mars such that liquid water can flow on the surface, plants may thrive there. It is a common misconception that the White Mars model does not allow this. Although liquid water played no part in shaping White Mars, there is abundant water trapped as clathrates in the regolith. In fact, in this model, there is a larger pool of volatiles to use, since the water was trapped in the ground, not lost to space. Given time the plants will oxygenate Mars as they did on Earth billions of years ago. However, this will be a long process, requiring thousands of years at the least. During this time, humans may live on Mars in oxygenated enclosed cities. The temperature and surface pressure of outside Mars will be acceptable to humans, and they will require only breathing gear when venturing outside the domes.

While we've seen now that terraforming Mars may be possible (if lengthy), we must consider whether such an activity is right. Just because we can do a thing doesn't necessarily mean we should, and there are matters of Planetary Protection to consider.

These pages designed by James Roberts
Last updated:  05 May 2002

http://anquetil.colorado.edu/~jhr/terraform/habitability-nf.html