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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　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