Altered States The Means of Terraforming Mars

Earlier we discussed the essential changes in surface pressure and temperature that must be brought about to make Mars habitable. Two specific initial issues dominate:

1. The atmospheric pressure must be increased by at least a factor of 100.

2. The mean global temperature must be increased by at least 60 K. Additional, longer-term requirements for habitability will also require the following:

3. The establishment of standing liquid-water reserves on the Martian surface.

4. A change in the chemical composition of the Martian atmosphere.

5. A reduction in the surface UV flux.

Exactly how these basic goals might be achieved is still unclear, but what is known at the present time is that there are a number of options open to the future terraformers of Mars. A list of possibilities follows. Some of these ideas are certainly more exotic than others, but the truly exciting point is that there are options:

1. Change the orbital eccentricity of Mars's orbit about the Sun.

2. Change the obliquity of Mars's spin axis.

3. Change the Martian precession cycle.

4. Spread dark, heat-absorbing dust grains over the polar ice caps.

5. Degas the carbon dioxide within the Martian regolith.

6. Add super-greenhouse gases to the Martian atmosphere.

7. Seed the Martian atmosphere with heat-absorbing, cloud-forming particles.

8. Devolatize carbonates within the Martian crust.

9. Heat the polar ice caps by large statite (solar sail) mirrors.

10. Channel volatile-rich cometary nuclei into the Martian atmosphere.

11. Induce large-scale drainage of Martian aquifers.

12. Introduce bioengineered microbes to alter the atmospheric composition.

13. Introduce bioengineered plants to change the planet's surface albedo.

Options 1, 2, and 3 are the most drastic and perhaps the least desirable methods by which Mars might be terraformed, but there is no reason to suppose that in the deep future such terraforming tools won't become available, as Burns and Harwit argued in 1973. Option 1 essentially aims to reduce the perihelion distance of Mars (see Figure 6.12) while keeping the aphelion distance at about its current value. The reduced perihelion distance will result in periodic bouts of more intensive solar heating [the distance D term in Equation (5.1) is being periodically reduced in this process], and this, over time, will result in the degassing of the polar ice caps and the Martian regolith, producing a denser and warmer atmosphere. The aphelion distance must be kept near its current value, since if Mars moves much further away from the Sun, then it will begin to strongly interact with the inner regions of the asteroid belt, a process that will likely result in an increased number of asteroid impacts on all of the inner Solar System planets.

Option 2 is also rather drastic, but in this case the idea is to tip the Martian spin axis over by perhaps as much as 65° so that in the extreme case it lies along the orbital plane, similar to the spin-axis orientation of Uranus. In this state, each polar cap will be directly heated for half of each orbit of Mars around the Sun. This extended seasonal heating might then result in the degassing of the polar ices, producing, eventually, the desired denser, warmer atmosphere. Sagan and co-workers first described this scenario in 1973, and later investigations found that an additional tilt of just 6° (to an obliquity of 31 °) would suffice to devolatize the polar caps and trigger a climatic runaway to higher global temperatures.

Both orbit change and the tilt adjustment of the Martian spin axis can be achieved through the application of controlled close-gravitational encounters. In this process, the orbit of a large main-belt asteroid, or Kuiper Belt object (KBO), is altered in such a manner that a close flyby or, more likely, multiple close flybys, with Mars is achieved. Such encounters can be repeated until the desired change occurs.

Certainly the process of altering the orbit of a large asteroid or KBO are beyond our current technological capabilities, but the dynamics of the process and the means of calculating the required close encounter conditions are fully understood. There are no physical reasons to suppose, therefore, that options 1, 2, and 3 will never be utilized. Indeed, a variant of the Barns-Harwit maneuver, proposed as a means of effecting option 3, must ultimately be implemented, since the orbit of Phobos (Figure 6.13) is slowly decreasing.

Detailed model calculations indicate that Phobos will impact the Martian surface in about 100 million years, an epoch well beyond the stage when Mars will have been fully terraformed and inhabited. Either the orbit of Phobos will have to be changed so that it maintains a stable orbit around Mars, or it could be mined for mineral resources and then ejected from Mars orbit. The latter option might, in fact, be modified with Phobos being mined

Figure 6.13. Phobos, the largest and innermost of the two Martian moons discovered by Asaph Hall in 1877. It has an average radius of 11.1 km, weighs in at 1.07 x 1016 km, and has an orbital period of 7 h 39.2 min. The 10-km-wide crater Stickney can be seen to the middle-left of the view. Image courtesy of ESA/Mars Express.

Figure 6.13. Phobos, the largest and innermost of the two Martian moons discovered by Asaph Hall in 1877. It has an average radius of 11.1 km, weighs in at 1.07 x 1016 km, and has an orbital period of 7 h 39.2 min. The 10-km-wide crater Stickney can be seen to the middle-left of the view. Image courtesy of ESA/Mars Express.

very early on in the terraforming process and then, once exhausted of useful minerals, made to impact the Martian surface. This early prehuman colonization and destruction of Phobos option has the added advantage of supplementing terraforming options 4, 8, and 11.

Options 4, 7, and 13 are aimed at decreasing the Martian albedo. A reduced albedo, as indicated by Equation (5.1), will result in an enhanced heating of the planet. Sagan favored the application of option 4 and suggested that the polar caps might be devolatized by the deposition of 1-mm layer of carbon-rich material over just 6% of the exposed surface. Pulverizing a 300-m diameter asteroid could produce the material required to generate the thin carbon-rich coating. Option 13 is a Mars-specific variant of the Daisy World model described in Vignette D at the end of in this chapter. Detailed calculations suggest that darkening the polar caps from their present A = 0.77 to an albedo of about A = 0.73 (a 5% reduction in reflectivity) would be sufficient to initiate complete devolatization.

Options 5, 8, 9, and 11 are all concerned with the introduction of greater amounts of carbon dioxide or water vapor into the Martian atmosphere. Option 5 essentially aims to amplify the seasonal ice-dust jet effect illustrated in Figure 6.3, while option 11 aims to greatly enhance the natural water leakage effect recorded in Figure 6.4. Likewise, options 6, 10, and 12 aim to alter the Martian atmosphere in such a way that its greenhouse-heating effect is increased. The details and reasons behind the various options are discussed below. The atmosphere and greenhouse-heating effect model that we shall see in the sections below is described in the Appendix of this book, and readers with an interest in the mathematical details are directed there.

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