Pale Blue Dot - Carl Sagan [123]
As we know very well from Earth and Venus, carbon dioxide is a greenhouse gas. There are carbonate minerals found on Mars, and dry ice in one of the polar caps. They could be converted into CO2 gas. But to make enough of a greenhouse effect to generate comfortable temperatures on Mars would require the entire surface of the planet to be plowed up and processed to a depth of kilometers. Apart from the daunting obstacles in practical engineering that this represents—fusion power or no fusion power—and the inconvenience to whatever self-contained, closed ecological systems humans had already es tabished on the planet it would also constitute the irresponsible destruction of a unique scientific resource and database, the Martian surface.
What about other greenhouse gases? Alternatively, we might take chlorofluorocarbons (CFCs or HCFCs) to Mars after manufacturing them on Earth. These are artificial substances that, so far as we know, are found nowhere else in the Solar System. We can certainly imagine manufacturing enough CFCs on Earth to warm Mars, because by accident in a few decades with present technology on Earth we’ve managed to synthesize enough to contribute to global warming on our planet. Transportation to Mars would be expensive, though: Even using Saturn V- or Energiya-class boosters, it would require at least a launch a day for a century. But perhaps they could be manufactured from fluorine-containing minerals on Mars.
There is, in addition, a serious drawback: On Mars as on Earth, abundant CFCs would prevent formation of an ozone layer. CFCs might bring Martian temperatures into a clement range, but guarantee that the solar ultraviolet hazard would remain extremely serious. Perhaps the solar ultraviolet light could be absorbed by an atmospheric layer of pulverized asteroidal or surface debris injected in carefully titrated amounts above the CFCs. But now we’re in the troubling circumstance of having to deal with propagating side effects, each of which requires its own large-scale technological solution.
A third possible greenhouse gas for warming Mars is ammonia (NH3). Only a little ammonia would be enough to warm the Martian surface to above the freezing point of water. In principle, this might be done by specially engineered microorganisms that would convert Martian atmospheric N2 to NH3 as some microbes do on Earth, but do it under Martian conditions. Or the same conversion might be done in special factories. Alternatively, the nitrogen required could be carried to Mars from elsewhere in the Solar System. (N2 is the principal constituent in the atmospheres of both Earth and Titan.) Ultraviolet light would convert ammonia back into N2 in about 30 years, so there would have to be a continuous resupply of NH3.
A judicious combination of CO2, CFC, and NH3 greenhouse effects on Mars looks as if it might be able to bring surface temperatures close enough to the freezing point of water for the second phase of Martian terraforming to begin—temperatures rising due to the pressure of substantial water vapor in the air, widespread production of O2 by genetically engineered plants, and fine-tuning the surface environment. Microbes and larger plants and animals could be established on Mars before the overall environment was suitable for unprotected human settlers.
Terraforming Mars is plainly much easier than terraforming Venus. But it is still very expensive by present standards, and environmentally destructive. If there were sufficient justification, though, perhaps the terraforming of Mars could be under way by the twenty-second century.
THE MOONS OF JUPITER AND SATURN: Terraforming the satellites of the Jovian planets presents varying degrees of difficulty. Perhaps the easiest to contemplate is Titan. It already