ted 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 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 PELLING 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； munities； 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 petently and responsibly only when we have a much better understanding of those worlds than is available today。 Advocates of terraforming must first bee 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 mitment 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？
It is just conceivable that some of the techniques that might eventually terraform other worlds might be applied to ameliorate the damage we have done to this one。 Considering the relative urgencies； a useful indication of when the human species is ready to consider terraforming seriously is when we have put our own world right。 We can consider it a test of the depth of our understanding and our mitment。 The first step in engineering the Solar System is to guarantee the habitability of the Earth。
Then we'll be ready to spread out to asteroids； ets； Mars； the moons of the outer Solar System； and beyond。 Jack Williamson 's prediction that this will begin to e about by the twenty…second century may not be far off the mark。
THE NOTION OF OUR DESCENDANTS living and working on other worlds； and even moving some of them around for their convenience； seems the most extravagant science fiction。 Be realistic； a voice inside my head counsels。 But this is realistic。 We're on the cusp of the technology； near the midpoint between impossible and routine。 It's easy to be conflicted about it。 If we don't do something awful to ourselves in the interim； in another century terraforming may seem no more impossible than a human…tended space station does today。
I think the experience of living on other worlds is bound to change us。 Our descendants； born and raised elsewhere； will naturally begin to owe primary loyalty to the worlds of their birth； whatever affection they retain for the Earth。 Their physical needs； their methods of supplying those needs； their technologies； and their social structures will all have to be different。
A blade of grass is a monplace on Earth； it would be a miracle on Mars。 Our descendants on Mars will know the value of a patch of green。 And if a blade of grass is priceless； what is the value of a human being？ The American revolutionary Tom Paine； in describing his contemporaries； had thoughts along these lines：
The wants which necessarily acpany the cultivation of a wilderness produced among them a state of society which countries long harassed by the quarrels and intrigues of governments had neglected to cherish。 In such a situation man bees what he ought to be。 He sees his species 。 。 。 as kindred。
Having seen at first hand a procession of barren and desolate worlds； it will be natural for our spacefaring descendants to cherish life。 Having learned something from the tenure of our species on Earth； they may wish to apply those lessons to other worlds—to spare generations to e the avoidable suffering that their ancestors were obliged to endure； and to draw upon our experience and our mistakes as we begin our open…ended evolution into space。
CHAPTER 20 DARKNESS
Far away； hidden from the eyes of daylight； there are watchers in the skies。
—EURIPIDES； THE BACCHAE （CA。 406 B。C。）
As children； we fear the dark。 Anything might be out。 here。 The unknown troubles us。 Ironically； it is our fate to live in the dark。 This unexpected finding of science is only about three centuries old。 Head out from the Earth in any direction you choose； and—after an initial flash of blue and a longer wait while the Sun fades—you are surrounded by blackness； punctuated only here and there by the faint and distant stars。
Even after we are grown； the darkness retains its power to frighten us。 And so there are those who say we should not inquire too closely into who else might be living in that darkness。 Better not to know； they say。
There are 400 billion stars in the Milky Way Galaxy。 Of this immense multitude； could it be that our humdrum Sun is the only one with an inhabited planet？ Maybe。 Maybe the origin of life or intelligence is exceedingly improbable。 Or maybe civilizations arise all the time； but wipe themselves out as soon as they are able。
Or； here and there； peppered across space； orbiting other suns； maybe there are worlds something like our own； on which other beings gaze up and wonder as we do about who else lives in the dark。 Could the Milky Way be rippling with life and intelligence—worlds calling out to worlds—while we on Earth are alive at the critical moment when we first decide to listen？
Our species has discovered a way to municate through the dark； to transcend immense distances。 No means of munication is faster or cheaper or reaches out farther。 It's called r