We could use a giant artificial sunshade in orbit around Venus to cool the surface; but it would be enormously expensive, as well as having many of the deficiencies of the dust layer. However, if the temperatures could be lowered sufficiently, the CO2 in the atmosphere would rain out. There would be a transitional time of CO2 oceans on Venus. If those oceans could be covered over to prevent re-evaporation—for example, with water oceans made by melting a large, icy moon transported from the outer Solar System—then the CO2 might conceivably be sequestered away, and Venus converted into a water (or low-fizz seltzer) planet. Ways have also been suggested to convert the CO2 into carbonate rock.

Thus all proposals for terraforming Venus are still brute-force, inelegant, and absurdly expensive. The desired planetary metamorphosis may be beyond our reach for a very long time, even if we thought it was desirable and responsible. The Asian colonization of Venus that Jack Williamson imagined may have to be redirected somewhere else.

Mars: For Mars we have just the opposite problem. There’s not enough greenhouse effect. The planet is a frozen desert. But the fact that Mars seems to have had abundant rivers, lakes, and perhaps even oceans 4 billion years ago—at a time when the Sun was less bright than it is today—makes you wonder if there’s solve natural instability in the Martian climate, something on hair trigger that once released would all by itself return the planet to its ancient clement state. (Let’s note from the start that doing so would destroy Martian landforms that hold key data on the past—especially the laminated polar terrain.)

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 established 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 has an atmosphere, made mainly of N2 like the Earth’s, and is much closer to terrestrial atmospheric pressures than either Venus or Mars. Moreover, important greenhouse gases, such as NH3 and H20, are almost certainly frozen out on its surface. Manufacture of initial greenhouse gases that do not freeze out at present Titan temperatures plus direct warming of the surface by nuclear fusion could, it seems, be the key early steps to one day terraform Titan.

If there were a compelling reason for terraforming other worlds, this greatest of engineering projects might be feasible on the timescale we’ve been describing—certainly for asteroids, possibly for Mars, Titan, and other moons of the outer planets, and probably not for Venus. Pollack and I recognized that there are those who feel a powerful attraction to the idea of rendering other worlds in the Solar System suitable for human habitation—in establishing observatories, exploratory bases, communities, and homesteads there. Because of its pioneering history, this may be a particularly natural and attractive idea in the United States.

In any case, massive alteration of the environments of other worlds can be done competently and responsibly only when we have a much better understanding of those worlds than is available today. Advocates of terraforming must first become advocates of the long-term and thorough scientific exploration of other worlds.

Perhaps when we really understand the difficulties of terraforming, the costs or the environmental penalties will prove too steep, and we will lower our sights to domed or subsurface cities or other local, closed ecological systems, greatly improved versions of Biosphere II, on other worlds. Perhaps we will abandon the dream of converting the surfaces of other worlds to something approaching the Earth’s. Or perhaps there are much more elegant, cost-effective, and environmentally responsible ways of terraforming that we have not yet imagined.

But if we are seriously to pursue the matter, certain questions ought to be asked: Given that any terraforming scheme entails a balance of benefits against costs, how certain must we be that key scientific information will not thereby be destroyed before proceeding? How much understanding of the world in question do we need before planetary engineering can be relied upon to produce the desired end state? Can we guarantee a long-term human commitment to maintain and replenish an engineered world, when human political institutions are so short lived? If a world is even conceivably inhabited—perhaps only by microorganisms—do humans have a right to alter it? What is our responsibility to preserve the worlds of the Solar System in their present wilderness states for future generations-who may contemplate uses that today we are too ignorant to foresee? These questions may perhaps be encapsulated into a final question: Can we, who have made such a mess of this world, be trusted with others?