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ld make them all collide with Venus （this is the overkill approach to the impact hazard problem）； think what we would have lost。 Who knows what wonders； what practical knowledge they contain？ We would also obliterate much of Venus' gorgeous surface geology—which we've just begun to understand； and which may teach us much about the Earth。 This is an example of bruteforce terraforming。 I suggest we want to steer entirely clear of such methods； even if someday we'll be able to afford them （which I very much doubt）。 We want something more elegant； more subtle； more respectful of the environments of other worlds。 A microbial approach has some of those virtues； but does not do the trick； as we've just seen。
We can imagine pulverizing a dark asteroid and spreading the powder through the upper atmosphere of Venus； or carrying such dust up from the surface。 This would be the physical equivalent of nuclear winter or the Cretaceous…Tertiary post…impact climate。 If the sunlight reaching the ground is sufficiently attenuated； the surface temperature must fall。 But by its very nature； this option plunges Venus into deep gloom； with daytime light levels perhaps only as bright as on a moonlit night on Earth。 The oppressive； crushing 90…bar atmosphere would remain untouched。 Since the emplaced dust would sediment out every few years； the layer would have to be replenished in the same period of time。 Perhaps such an approach would be acceptable for short exploratory missions； but the environment generated seems very stark for a self…sustaining human munity on Venus。
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 fortable 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 bination 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 H2O； are almost certa