The initial warming of Mars and volatilizing its polar caps remains a major problem. Investigators have suggested engineering solutions such as a large reflecting solar sail over the Martian north pole (see reviews in refs. 1 and 2).
Relative to the difficulty of increasing the volatiles on the Martian surface, the task of maintaining them at an Earth-like temperature would seem easy. Our work should be refined with a multilayered radiative-transfer model, and specific absorption line-spacings and -strengths used. If we have overestimated bandwidths, the strategy of choice would not be to increase column amounts but to include another gas with a strong vibrational fundamental near 10.25 µm (the clearest spectral region in the presence of a terrestrial atmosphere and "our" gas mixture).
Although the lifetimes in Table 3 look long, they imply that the greenhouse gases considered here would require replenishment at a rate of nearly 400 kilotons per year to offset photolysis. Because the lifetimes in Table 3 were modeled at 1 astronomical unit from the sun, it would be plausible to multiply those lifetimes by 2.3, reducing the annual requirement to 170 kilotons. In either case, the rate compares very favorably with the 3-teraton annual rate of CFC production mentioned in ref. 1, which was based on a recognition that CFCs would destroy any ozone layer.
Fluorine on Mars would have to be mined locally. For comparison, South African export of acid-grade fluorspar was 470 kilotons in 1980 but then receded somewhat because of weak commodity markets (20). It takes 2.2 tons of acid-grade fluorspar to produce a ton of HF, and the majority of the weight of the gases we are discussing is the fluorine weight. Even though the bulk composition of Mars may be richer in fluorine than that of Earth's (8), whether the element can be found there in sufficient concentrations is unknown.
Considering the likely 3% rms error in band-center frequencies, our solution is presented as an example, not a prescription. During the course of our own calculations, we had selected some other gases at various times as members of our optimal subset (CF3SCF3, CF3OCF2OCF3, CF3SCF2SCF3, CF3OCF2NFCF3). With our final set, a judicious shift of just two strong band centers by 30 cm1 in opposite directions increases the manufacturing requirement by almost 15%. Here again, the best response to an unexpected window in the true spectrum of the gases, when all have been synthesized, would be to identify an additional gas to fill the window.
A secondary implication of our findings is a reminder that the current terrestrial warming scare or controversy may be too fixated on the likelihood of CO2 doubling, when the greater danger may be from new trace gases with strong absorption bands in the window such as SF5CF3, which is observed in the Earth's atmosphere (14).
A more speculative corollary is that advanced extrasolar civilizations, if they exist, already may have engineered planetary environments in zones we would consider inhospitably cold. Therefore, searches for extraterrestrial intelligence, which now mainly seek radio waves, should perhaps include looking for spectra of manufacturable molecules such as those mentioned here.
Speculations on planetary environmental engineering have considered most often materials found in nature or materials already engineered for some terrestrial purpose. We have taken the next step by including consideration of some materials not found in nature and not previously manufactured. Our methods of investigating these materials are not in themselves novel; nor would we want them to be, preferring to build our argument on accepted foundations.
We thank J. Blamont, A. Haldemann, and K. Nealson for stimulating discussions on climate modifications by using greenhouse gases and G. Blake for a conversation on bandwidths. We thank M. Marinova for a thoughtful reading and for pointing out a reference we had neglected. We thank the National Academy of Science's Member Editor and two anonymous referees for their comments. This research is supported in part by the National Aeronautics and Space Administration Grant NAG5-4022 and National Science Foundation Grant AST-9816409.
CFC, chlorofluorocarbons; B3LYP, Lee-Yang-Parr method.
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This paper was submitted directly (Track II) to the PNAS office.