pale blue dot -carl sagan-第34部分

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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　radio。

After　billions　of　years　of　biological　evolution—on　their　planet　and　ours—an　alien　civilization　cannot　be　in　technological　lockstep　with　us。　There　have　been　humans　for　more　than　twenty　thousand　centuries；　but　we've　had　radio　only　for　about　one　century。　If　alien　civilizations　are　behind　us；　they're　likely　to　be　too　far　behind　to　have　radio。　And　if　they're　ahead　of　us；　they're　likely　to　be　far　ahead　of　us。　Think　of　the　technical　advances　on　our　world　over　just　the　last　few　centuries。　What　is　for　us　technologically　difficult　or　impossible；　what　might　seem　to　us　like　magic；　might　for　them　be　trivially　easy。　They　might　use　other；　very　advanced　means　to　municate　with　their　peers；　but　they　would　know　about　radio　as　an　approach　to　newly　emerging　civilizations。　Even　with　no　more　than　our　level　of　technology　at　the　transmitting　and　receiving　ends；　we　could　municate　today　across　much　of　the　Galaxy。　They　should　be　able　to　do　much　better。

If　they　exist。

But　our　fear　of　the　dark　rebels。　The　idea　of　alien　beings　troubles　us。　We　conjure　up　objections：

〃It's　too　expensive。〃　But；　in　its　fullest　modern　technological　expression；　it　costs　less　than　one　attack　helicopter　a　year。

〃We'll　never　understand　what　they're　saying。〃　But；　because　the　message　…is　transmitted　by　radio；　we　and　they　must　have　radio　physics；　radio　astronomy；　and　radio　technology　in　mon。　The　laws　of　Nature　are　the　same　everywhere；　so　science　itself　provides　a　means　and　language　of　munication　even　between　very　different　kinds　of　beings—provided　they　both　have　science。　Figuring　out　the　message；　if　we're　fortunate　enough　to　receive　one；　may　be　much　easier　than　acquiring　it。

〃It　would　be　demoralizing　to　learn　that　our　science　is　primitive。〃　But　by　the　standards　of　the　next　few　centuries；　at　least　some　of　our　present　science　will　be　considered　primitive；　extraterrestrials　or　no　extraterrestrials。　（So　will　some　of　our　present　politics；　ethics；　economics；　and　religion。）　To　go　beyond　present　science　is　one　of。　the　chief　goals　of　science。　Serious　students　are　not　monly　plunged　into　fits　of　despair　on　turning　the　pages　of　a　textbook　and　discovering　that　some　further　topic　is　known　to　the　author　but　not　yet　to　the　student。　Usually　the　students　struggle　a　little；　acquire　the　new　knowledge；　and；　following　an　ancient　human　tradition；　continue　to　turn　the　pages。

〃All　through　history　advanced　civilizations　have　ruined　civilizations　just　slightly　more　backward。〃　Certainly。　But　malevolent　aliens；　should　they　exist；　will　not　discover　our　existence　from　the　fact　that　we　listen。　The　search　programs　only　receive；　they　do　not　send。*

*　Surprisingly　many　people；　including　New　York　Times　editorialists；　are　concerned　that　once　extraterrestrials　know　where　we　are；　they　will　e　here　and　eat　us。　Put　aside　the　profound　biological　differences　that　must　exist　between　the　hypothetical　aliens　and　ourselves；　imagine　that　we　constitute　an　interstellar　gastronomic　delicacy。　Why　transport　large　numbers　of　us　to　alien　restaurants？　The　freightage　is　enormous。　Wouldn't　it　be　better　just　to　steal　a　few　humans；　sequence　our　amino　acids　or　whatever　else　is　the　source　of　our　delectability；　and　then　just　synthesize　the　identical　food　product　from　scratch？



THE　DEBATE　IS；　for　the　moment；　moot。　We　are　now；　on　an　unprecedented　scale；　listening　for　radio　signals　from　possible　other　civilizations　in　the　depths　of　space。　Alive　today　is　the　first　generation　of　scientists　to　interrogate　the　darkness。　Conceivably　it　might　also　be　the　last　generation　before　contact　is　made—and　this　the　last　moment　before　we　discover　that　someone　in　the　darkness　is　calling　out　to　us。

This　quest　is　called　the　Search　for　Extraterrestrial　Intelligence　（SETI）。　Let　me　describe　how　far　we've　e。

The　first　SETI　program　was　carried　out　by　Frank　Drake　at　the　National　Radio　Astronomy　Observatory　in　Greenbank；　West　Virginia；　in　1960。　He　listened　to　two　nearby　Sun…like　stars　for　two　weeks　at　one　particular　frequency。　（〃Nearby〃　is　a　relative　term：　The　nearest　was　12　light…years—70　trillion　miles—away。）

Almost　at　the　moment　Drake　pointed　the　radio　telescope　and　turned　the　system　on；　he　picked　up　a　very　strong　signal。　Was　it　a　message　from　alien　beings？　Then　it　went　away。　If　the　signal　disappears；　you　can't　scrutinize　it。　You　can't　see　if；　because　of　the　Earth's　rotation；　it　moves　with　the　sky。　If　it's　not　repeatable；　you've　learned　almost　nothing　from　it—it　might　be　terrestrial　radio　interference；　or　a　failure　of　your　amplifier　or　detector　。　。　。　or　an　alien　signal。　Unrepeatable　data；　no　matter　how　illustrious　the　scientist　reporting　them；　are　not　worth　much。

Weeks　later；　the　signal　was　detected　again。　It　turned　out　to　be　a　military　aircraft　broadcasting　on　an　unauthorized　frequency。　Drake　reported　negative　results。　But　in　science　a　negative　result　is　not　at　all　the　same　thing　as　a　failure。　His　great　achievement　was　to　show　that　modern　technology　is　fully　able　to　listen　for　signals　from　hypothetical　civilizations　on　the　planets　of　other　stars。

Since　then　there've　been　a　number　of　attempts；　often　on　time　borrowed　from　other　radio　telescope　observing　programs；　and　almost　never　for　longer　than　a　few　months。　There've　been　some　more　false　alarms；　at　Ohio　State；　in　Arecibo；　Puerto　Rico；　in　France；　Russia；　and　elsewhere；　but　nothing　that　could　pass　muster　with　the　world　scientific　munity。

Meanwhile；　the　technology　for　detection　has　been　getting　cheaper；　the　sensitivity　keeps　improving；　the　scientific　respectability　of　SETI　has　continued　to　grow；　and　even　NASA　and　Congress　have　bee　a　little　less　afraid　to　support　it。　Diverse；　plementary　search　strategies　are　possible　and　necessary。　It　was　clear　years　ago　that　if　the　trend　continued；　the　technology　for　a　prehensive　SETI　effort　would　eventually　fall　within　the　reach　even　of　private　organizations　（or　wealthy　individuals）；　and　sooner　or　later；　the　government　would　be　willing　to　support　a　major　program。　After　30　years　of　work；　for　some　of　us　it's　been　later　rather　than　sooner。　But　at　last　the　time　has　e。



THE　PLANETARY　SOCIETY…a　nonprofit　membership　organization　that　Bru