pale blue dot -carl sagan-第55部分

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　runaway　greenhouse　effect；　as　happened　on　Venus。　It　is　not　at　all　clear　that　our　knowledge　is　sufficient　to　this　purpose。

As　far　as　I　know；　the　first　suggestion　in　the　scientific　literature　about　terraforming　the　planets　was　made　in　a　1961　article　I　wrote　about　Venus。　I　was　pretty　sure　then　that　Venus　had　a　surface　temperature　well　above　the　normal　boiling　point　of　water；　produced　by　a　carbon　dioxide/water　vapor　greenhouse　effect。　I　imagined　seeding　its　high　clouds　with　genetically　engineered　microorganisms　that　would　take　CO2；　N2；　and　H2O　out　of　the　atmosphere　and　convert　them　into　organic　molecules。　The　more　CO2　removed；　the　smaller　the　greenhouse　effect　and　the　cooler　the　surface。　The　microbes　would　be　carried　down　through　the　atmosphere　toward　the　ground；　where　they　would　be　fried；　so　water　vapor　would　be　returned　to　the　atmosphere；　but　the　carbon　from　the　CO2　would　be　converted　irreversibly　by　the　high　temperatures　into　graphite　or　some　other　involatile　form　of　carbon。　Eventually；　the　temperatures　would　fall　below　the　boiling　point　and　the　surface　of　Venus　would　bee　habitable；　dotted　with　pools　and　lakes　of　warm　water。

The　idea　was　soon　taken　up　by　a　number　of　science　fiction　authors　in　the　continuing　dance　between　science　and　science　fiction—in　which　the　science　stimulates　the　fiction；　and　the　fiction　stimulates　a　new　generation　of　scientists；　a　process　benefiting　both　genres。　But　as　the　next　step　in　the　dance；　it　is　now　clear　that　seeding　Venus　with　special　photosynthetic　microorganisms　will　not　work。　Since　1961　we've　discovered　that　the　clouds　of　Venus　are　a　concentrated　solution　of　sulfuric　acid；　which　makes　the　genetic　engineering　rather　more　challenging。　But　that　in　itself　is　not　a　fatal　flaw。　（There　are　microorganisms　that　live　out　their　lives　in　concentrated　solutions　of　sulfuric　acid。）　Here's　the　fatal　flaw：　In　1961　I　thought　the　atmospheric　pressure　at　the　surface　of　Venus　N…as　a　few　〃bars；〃　a　few　times　the　surface　pressure　on　Earth。　We　now　know　it　to　be　90　bars；　so　that　if　the　scheme　worked；　the　result　would　be　a　surface　buried　in　hundreds　of　meters　of　fine　graphite；　and　an　atmosphere　made　of　65　bars　of　almost　pure　molecular　oxygen。　Whether　we　would　first　implode　under　the　atmospheric　pressure　or　spontaneously　burst　into　flames　in　all　that　oxygen　is　an　open　question。　However；　long　before　so　much　oxygen　could　build　up；　the　graphite　would　spontaneously　burn　back　into　CO2；　short…circuiting　the　process。　At　best；　such　a　scheme　can　carry　the　terraforming　of　Venus　only　partway。

Let's　assume　that　by　the　early　twenty…second　century　we　have　paratively　inexpensive　heavy…lift　vehicles；　so　we　can　carry　large　payloads　to　other　worlds；　abundant　and　powerful　fusion　reactors；　and　well…developed　genetic　engineering。　All　three　assumptions　are　likely；　given　current　trends。　Could　we　terraform　the　planets？*　James　Pollack　of　NASA's　Ames　Research　Center　and　I　surveyed　this　problem。　Here's　a　summary　of　what　we　found：

*　Williamson；　Professor　Emeritus　of　English　at　Eastern　New　Mexico　University　at　age　85　wrote　to　me　that　he　was　〃amazed　to　see　how　far　actual　science　has　e〃　since　he　first　suggested　terraforming　other　worlds。　We　are　accumulating　the　technology　that　will　one　day　permit　terraforming；　but　at　present　all　'V…a　have　are　suggestions　by　and　large　less　ground　breaking　than　Williamson's　original　ideas。

VENUS：　Clearly　the　problem　with　Venus　is　its　massive　greenhouse　effect。　If　we　could　reduce　the　greenhouse　effect　almost　to　zero；　the　climate　might　be　balmy。　But　a　90…bar　CO2　atmosphere　is　oppressively　thick。　Over　every　postage　stamp…sized　square　inch　of　surface；　the　air　weighs　as　much　as　six　professional　football　players；　piled　one　on　top　of　another。　Making　all　that　go　away　will　take　some　doing。

Imagine　bombarding　Venus　with　asteroids　and　ets。　Each　impact　would　blow　away　some　of　the　atmosphere。　To　blow　away　almost　all　of　it；　though；　would　require　using　up　more　big　asteroids　and　ets　than　there　are—at　least　in　the　planetary　part　of　the　Solar　System。　Even　if　that　many　potential　impactors　existed；　even　if　we　could　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　propa