Super Greenhouse Gases

So far, we have only considered the warming effects from an increase in the CO2 abundance in the Martian atmosphere. Other greenhouse gases, however, may be employed to raise the planet's temperature still higher. Zubrin and Mckay, for example, have noted that cometary nuclei and possibly some asteroids contain ammonia (NH3) ice, which in its gaseous phase is a strong greenhouse gas, and they suggest that such cometary nuclei and asteroids might have their orbits altered in order to impact upon the surface of Mars (option 10).

Ammonia ice is generally estimated to make up something like 1% of the mass of a cometary nucleus, and to produce a partial pressure of 0.1 Pa (1 microbar) of order 4 x 1012 kg of ammonia gas would need to be imported to Mars.5 This amount of ammonia

280 260

V 240

3 220

I 200

0 200 400 600 800 1000

NH3 pressure (microbars)

Figure 6.16. Greenhouse-heating effect due to the addition of ammonia into the Martian atmosphere. A CO2 partial pressure of 6 millibars has been assumed in the calculations, and note that the partial pressure of ammonia is given in microbars. The horizontal dashed line indicates a temperature of 273 K (See the Appendix in this book for calculation details).

might be delivered by a single impact from a cometary nucleus with a diameter of about 10 km across.

Figure 6.16 shows the greenhouse-heating effect that will result due to increasing the ammonia content of the Martian atmosphere. An additional 10° of heating over that provided by CO2 is realized when the partial pressure of ammonia is increased to 0.5 Pa (5 microbars). The equatorial temperature exceeds the freezing point of water once the partial pressure of ammonia is greater than 250 microbars.

Provided that the technological infrastructure can be put in place, and there are no physical reasons why they cannot be, there is a vast reserve of water and ammonia ice within the KBO and the Oort Cloud (see Figure 4.3) that might be utilized in option 10. Not only will the atmospheric and ground disruption of cometary nuclei in the early stages of terraforming Mars provided ammonia, but such actions will also provide additional water vapor, itself a strong greenhouse agent, additional CO2, and, important for eventual human habitability, atmospheric nitrogen, and oxygen.

In principle, strong greenhouse gases such as ammonia and methane6 might be mined from the atmospheres of the Jovian planets. This option, however, is (from all appearances) likely to be dependent upon technologies that won't be available for many centuries beyond the present, a time frame beyond which the

0 200 400 600 800 1000

NH3 pressure (microbars)

Figure 6.16. Greenhouse-heating effect due to the addition of ammonia into the Martian atmosphere. A CO2 partial pressure of 6 millibars has been assumed in the calculations, and note that the partial pressure of ammonia is given in microbars. The horizontal dashed line indicates a temperature of 273 K (See the Appendix in this book for calculation details).

terraforming of Mars is likely to begin. Robotic spacecraft with the capability of altering the paths of cometary nuclei, on the other hand, are not only being contemplated at this very time, but there are no specific reasons (other than political and funding intransigence) to believe that multiple spacecraft missions couldn't be in place to begin the terraforming of Mars within the next several centuries.

Although the ability to extract large quantities of basic greenhouse gases from the atmospheres of the Jovian planets will probably not be in place by the time Martian terraforming begins, large-volume mining may well play a role in the terraforming of Venus.7 It has also been suggested that atmospheric mining might play an important role in the final stages of producing a breathable Martian atmosphere. In this latter case, it is the importation of nitrogen that will need to be performed, and at least one published paper suggests that the nitrogen might be extracted from the atmosphere of Titan, Saturn's largest moon (see Figure 3.6). This latter possibility reminds us of the fact that the Solar System is literally full of resources and that there are no reasons to suppose that terraforming can't proceed for the want of basic raw materials and chemical components. The problem for humanity, of course, is exactly how to perform the large-scale extraction and transportation of the resources from one location in the Solar System to another.

Staggeringly large amounts of ammonia would need to be added to the Martian atmosphere in order to push its equatorial temperature above the freezing point of water. Indeed, a partial pressure of some 250 microbars (= 25 Pa) would be required (Figure 6.16), corresponding to an imported mass of some 1015 kg of ammonia (see Note 5). Rather than use gases such as ammonia, therefore, James Lovelock and Michael Allaby realized in the early 1980 s that it would make much more sense to utilize super-greenhouse gases such as the chlorofluorocarbons (CFCs) and perfluorocarbons (PFCs) to warm Mars. In the case of CFCs, a partial pressure of order 0.25 microbars, corresponding to an atmospheric mass of about 1012 kg, would produce an equatorial temperature above the freezing point of water.8 During the peak production time, in the mid-1980s, worldwide production of CFCs amounted to some 1 billion kilograms per year. With a relatively modest increase in this production rate it would be possible, therefore, to substantially warm Mars via CFC greenhouse gases alone within a few hundred years.

As briefly noted in Chapter 5, there are several serious side effects associated with the introduction of large quantities of CFC gases into a planetary atmosphere. The most important issue (on the Earth currently) concerns ozone depletion through CFC chemistry. Although there is no ozone in the Martian atmosphere at the present time, it is certainly a gas that will warrant eventual accumulation. The key point about the Earth's ozone layer (and eventually that surrounding the Martian surface) is that it is highly efficient at absorbing the potentially deadly solar UV radiation. In contrast to the CFCs, the various PFC gases do not destroy ozone, since they lack the chlorine (and bromine) that catalytically destroys the O3, and their atmospheric lifetime against destruction is much longer than those of the CFCs. The fluorine-based gas C3F8 is one atmospheric-heating agent that shows particular promise with respect to the warming of Mars, since it is a strong absorber over a large fraction of the infrared spectrum (see Figure B.1). A 1 microbar partial pressure of C3F8 alone will produce a 12° temperature increase in the equatorial temperature on Mars, while a 10-microbar pressure would push the equatorial temperature to 6° above the freezing point of water.

Was this article helpful?

0 0

Post a comment