The Runaway CO₂ Greenhouse

    Sagan (1973) and others were the first to attempt the modelling of terraformation processes. These early models were based on the now-defunct “Long Winter Model” wherein it was assumed that large quantities of relatively accessible CO₂ ice were deposited in the Martian polar caps. While such large reservoirs are known now not to be present, McKay (1982) suggested that CO₂ might still be found in sufficient quantities adsorbed in the regolith to induce a runaway greenhouse effect. Computer modelling by McKay et al. (1991) has shown that, for a 1-bar regolith reservoir, an initial warming of 5-20 K could be enough to trigger such an effect, yielding a final steady state of ~ 800 mbar, and surface temperature of 250 K.

    Several methods have been suggested for this initial heating. One is the introduction of relatively small amounts of CFC gasses in order to create an “artificial” greenhouse effect. CFCs are > 10,000 times more greenhouse efficient than CO₂, are non-toxic, and have long (10s or 100s of years) lifetimes. Unfortunately, dissociation by the comparatively intense UV on Mars is enough to reduce this lifetime to mere hours. An alternative could be to use perflourocarbons instead, though their relevant absorption bands are largely unknown, so this remains speculative.

    Another, perhaps no less feasible, option would be to incorporate large orbiting mirrors ("solettas") which would reflect a large amount of solar energy onto the Martian surface. Zubrin and McKay (1993) found that a 125 km diameter mirror may be sufficient to raise the polar cap temperature by enough to cause evaporation and subsequent regolith degassing. Fogg (1998) points out that such a mirror would constitute only 5 days worth of the Earth’s aluminum production, so should not be dismissed outright due to its size.

       Large orbiting mirrors could provide substantial heating.                  Proposed mirrors as seen from the Martian surface

    More fundamental problems with regolith outgassing remain, however. An especially distressing scenerio would be if the primary reservoirs of Martian CO₂ were found to be, not adsorbed in the regolith (in which case it is fairly accessible) but rather chemically bound in the form of carbonates. Freeing the necessary quantities would then require vastly greater energies, perhaps even the use of buried nuclear weapons or asteroids deflected toward the planet. One would hope that such extreme measures would not be necessary for terraformation.