pale blue dot -carl sagan-第29章

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The　planets；　in　their　various　stages　of　development；　are　subjected　to　the　same　formative　forces　that　operate　on　our　earth；　and　have；　therefore；　the　same　geologic　formation；　and　probably　life；　of　our　own　past；　and　perhaps　future；　but。　further　than　this；　these　forces　are　acting；　in　some　cases；　under　totally　different　conditions　from　those　under　which　they　operate　on　the　earth；　and　hence　must　evolve　forms　different　from　those　ever　known　to　man。　The　value　of　such　material　as　this　to　the　parative　sciences　is　too　obvious　to　need　discussion。

—ROBERT　H。　GODDARD；　NOTEBOOK　（1907）

For　the　first　time　in　my　life；　I　saw　the　horizon　as　a　curved　line。　It　was　accentuated　by　a　thin　seam　of　dark　blue　light…our　atmosphere。　Obviously；　this　was　not　the　〃ocean〃　of　air　I　had　been　told　it　was　so　many　times　in　my　life。　I　was　terrified　by　its　fragile　appearance。

—ULF　MERBOLD；　GERMAN　SPACE　SHUTTLE　ASTRONAUT　（1988）

When　you　look　down　at　the　Earth　from　orbital　altitudes；　you　see　a　lovely；　fragile　world　embedded　in　black　vacuum。　But　peering　at　a　piece　of　the　Earth　through　a　spacecraft　porthole　is　nothing　like　the　joy　of　seeing　it　entire　against　the　backdrop　of　black；　or—better—sweeping　across　your　field　of　view　as　you　float　in　space　unencumbered　by　a　spacecraft。　The　first　human　to　have　this　experience　was　Alexei　Leonov；　who　on　March　18；　1965；　left　Voskhod　2　in　the　original　space　〃walk〃：　〃I　looked　down　at　the　Earth；〃　he　recalls；　〃and　the　first　thought　that　crossed　my　mind　was　‘The　world　is　round；　after　all。'　In　one　glance　I　could　see　from　Gibraltar　to　the　Caspian　Sea　。　。　。　I　felt　like　a　bird—with　wings；　and　able　to　fly。〃

When　you　view　the　Earth　from　farther　away；　as　the　Apollo　astronauts　did；　it　shrinks　in　apparent　size；　until　nothing　but　a　little　geography　remains。　You're　struck　by　how　self…contained　it　is。　An　occasional　hydrogen　atom　leaves；　a　pitter…patter　of　etary　dust　arrives。　Sunlight；　generated　in　the　immense；　silent　thermonuclear　engine　deep　in　the　solar　interior；　pours　out　of　the　Sun　in　all　directions；　and　the　Earth　intercepts　enough　of　it　to　provide　a　little　illumination　and　enough　heat　for　our　modest　purposes。　Apart　from　that；　this　small　world　is　on　its　own。

From　the　surface　of　the　Moon　you　can　see　it；　perhaps　as　a　crescent；　even　its　continents　now　indistinct。　And　from　the　vantage　point　of　the　outermost　planet　it　is　a　mere　point　of　pale　light。

From　Earth　orbit；　you　are　struck　by　the　tender　blue　arc　of　the　horizon—the　Earth's　thin　atmosphere　seen　tangentially。　You　can　understand　why　there　is　no　longer　such　a　thing　as　a　local　environmental　problem。　Molecules　are　stupid。　Industrial　poisons；　greenhouse　gases；　and　substances　that　attack　the　protective　ozone　layer；　because　of　their　abysmal　ignorance；　do　not　respect　borders。　They　are　oblivious　of　the　notion　of　national　sovereignty。　And　so；　due　to　the　almost　mythic　powers　of　our　technology　（and　the　prevalence　of　short…term　thinking）；　we　are　beginning—on　Continental　and　on　planetary　scales—to　pose　a　danger　to　ourselves。　Plainly；　if　these　problems　are　to　be　solved；　it　will　require　many　nations　acting　in　concert　over　many　years。

I'm　struck　again　by　the　irony　that　spaceflight—conceived　in　tile　cauldron　of　nationalist　rivalries　and　hatreds—brings　with　it　a　stunning　transnational　vision。　You　spend　even　a　little　time　contemplating　the　Earth　from　orbit　and　the　most　deeply　engrained　nationalisms　begin　to　erode。　They　seem　the　squabbles　of　mites　on　a　plum。

If　we're　stuck　on　one　world；　we're　limited　to　a　single　case；　we　don't　know　what　else　is　possible。　Then—like　an　art　fancier　familiar　only　with　Fayoum　tomb　paintings；　a　dentist　who　knows　only　molars；　a　philosopher　trained　merely　in　NeoPlatonism；　a　linguist　who　has　studied　only　Chinese；　or　a　physicist　whose　knowledge　of　gravity　is　restricted　to　falling　bodies　on　Earth—our　perspective　is　foreshortened；　our　insights　narrow；　our　predictive　abilities　circumscribed。　By　contrast；　when　we　explore　other　worlds；　what　once　seemed　the　only　way　a　planet　could　be　turns　out　to　be　somewhere　in　the　middle　range　of　a　vast　spectrum　of　possibilities。　When　we　look　at　those　other　worlds；　we　begin　to　understand　what　happens　when　we　have　too　much　of　one　thing　or　too　little　of　another。　We　learn　how　a　planet　can　go　wrong。　We　gain　a　new　understanding；　foreseen　by　the　spaceflight　pioneer　Robert　Goddard；　called　parative　planetology。

The　exploration　of　other　worlds　has　opened　our　eyes　in　the　study　of　volcanos；　earthquakes；　and　weather。　It　may　one　day　have　profound　implications　for　biology；　because　all　life　on　Earth　is　built　on　a　mon　biochemical　master　plan。　The　discovery　of　a　single　extraterrestrial　organism—even　something　as　humble　as　a　bacterium—would　revolutionize　our　understanding　of　living　things。　But　the　connection　between　exploring　other　worlds　and　protecting　this　one　is　most　evident　in　the　study　of　Earth's　climate　and　the　burgeoning　threat　to　that　climate　that　our　technology　poses。　Other　worlds　provide　vital　insights　about　what　dumb　things　not　to　do　on　Earth。

Three　potential　environmental　catastrophes—all　operating　on　a　global　scale—have　recently　been　uncovered：　ozone　layer　depletion；　greenhouse　warming；　and　nuclear　winter。　All　three　discoveries；　it　turns　out；　have　strong　ties　to　the　exploration　of　the　planets。

（1）　It　was　disturbing　to　find　that　an　inert　material　with　all　sorts　of　practical　applications—it　serves　as　the　working　fluid　in　refrigerators　and　air　conditioners；　as　aerosol　propellant　for　deodorants　and　other　products；　as　lightweight　foamy　packaging　for　fast　foods；　and　as　a　cleaning　agent　in　microelectronics；　to　name　only　a　few…can　pose　a　danger　to　life　on　Earth。　Who　would　have　figured？

The　molecules　in　question　are　called　chlorofluorocarbons　（CFCs）。　Chemically；　they're　extremely　inert；　which　means　they're　invulnerable—until　they　find　themselves　up　in　the　ozone　layer；　where　they're　broken　apart　by　ultraviolet　light　from　the　Sun。　The　chlorine　atoms　thus　liberated　attack　and　break　down　the　protective　ozone；　letting　more　ultraviolet　light　reach　the　ground。　This　increased　ultraviolet　intensity　ushers　in　a　ghastly　procession　of　potential　consequences　involving　not　just　skin　cancer　and　cataracts；　but　weakening　of　the　human　immune　system　and；　most　dangerous　of　all；　possible　harm　to　agriculture　and　to　photosynthetic　organisms　at　the　base　of　the　food　chain　on　which　most　life　on　Earth　depends。

Who　discovered　that　CFCs　posed　a　threat　to　the　ozone　layer？　Was　it　the　principal　manufacturer；　the　DuPont　Corporation；　exercising　corporate　responsibility？　Was　it　the　Environmental　Protection　Agency　protecting　us？　Was　it　the　Department　of　Defense　defending　us？　No；　it　was　two　Ivory…tower；　white…coated　university　scientists　working　on　something　else—Sherwood　Rowland　and　Mario　Molina　of　the　University　of　California；　Irvine。　Not　even　an　Ivy　League　university。　No　one　instructed　them　to　look　for　dangers　to　the　environment。　They　were　pursuing　fundamental　research。　They　were　scientists　following　their　own　interests。　Their　names　should　be　known　to　every　schoolchild。

In　their　original　calculations；　Rowland　and　Molina　used　rate　constants　of　chemical　reactions　involving　chlorine　and　other　halogens　that　had　been　measured　in　part　with　NASA　support。　Why　NASA？　Because　Venus　has　chlorine　and　fluorine　molecules　in　its　atmosphere；　and　planetary　aeronomers　had　wanted　to　understand　what's　happening　there。

Confirming　theoretical　work　on　the　role　of　CFCs　in　ozone　depletion　was　soon　done　by　a　group　led　by　Michael　McElroy　at　Harvard　How　is　it　they　had　all　these　branching　networks　of　halogen　chemical　kinetics　in　their　puter　ready　to　go？　Because　they　were　working　on　the　chlorine　and　fluorine　chemistry　of　the　atmosphere　of　Venus。　Venus　helped　make　and　helped　confirm　the　discovery　that　the　Earth's　ozone　layer　is　in　danger。　An　entirely　unexpected　connection　was　found　between　the　atmospheric　photochemistries　of　the　two　planets。　A　result　of　importance　to　everyone　on　Earth　emerged　from　what　might　well　have　seemed　the　most　blue…sky；　abstract；　impractical　kind　of　work；　understanding　the　chemistry　of　minor　constituents　in　the　upper　atmosphere　of　another　world。

There's　also　a　Mars　connection。　With　Viking　we　found　the　surface　of　Mars　to　be　apparently　lifeless　and　remarkably　deficient　even　in　simple　organic　molecules。　But　simple　organic　molecules　ought　to　be　there；　because　of　the　impact　of　organic…rich　meteorites　from　the　nearby　asteroid　belt。　This　deficiency　is　widely　attributed　to　the　lack　of　ozone　on　Mars。　The　Viking　microbiology　experiments　found　that　organic　matter　carried　from　Earth　to　Mars　and　sprinkled　on　Martian　surface　dust　is　quickly　oxidized　arid　destroyed。　The　materials　in　the　dust　that　do　the　destruction　are　molecules　something　like　hydrogen　peroxide—which　we　use　as　an　antiseptic　because　it　kills　microbes　by　oxidizing　there；　Ultraviolet　light　from　the　Sun　strikes　the　surface　of　Mars　unimpeded　by　an　ozone　layer；　if　any　organic　matter　were　there；　it　would　be　quickly　destroyed　by　the　ultraviolet　light　itself　and　its　oxidation　products。　Thus　part　of　the　reason　the　topmost　layers　of　Martian　soil　are　antiseptic　is　that　Mars　has　an　ozone　hole　of　planetary　dimensions—by　itself　a　useful　cautionary　tale　for　us；　who　are　busily　thinning　and　puncturing　our　ozone　layer。

（2）　Global　warming　is　predicted　to　follow　from　the　increasing　greenhouse　effect　caused　largely　by　carbon　dioxide　generated　in　the　burning　of　fossil　fuels—but　also　from　the　buildup　of　other　infrared…absorbing　gases　（oxides　of　nitrogen；　methane；　those　same　CFCs；　and　other　molecules）。

Suppose　that　we　have　a　three…dimensional　general　circulation　puter　model　of　the　Earth's　climate。　Its　programmers　claim　it's　able　to　predict　what　the　Earth　will　be　like　if　there's　more　of　one　atmospheric　constituent　or　less　of　another。　The　model　does　very　well　at　〃predicting〃　the　present　climate。　But　there　is　a　nagging　worry：　The　model　has　been　〃tuned〃　so　it　will　e　out　right—that　is；　certain　adjustable　parameters　are　chosen；　not　from　first　principles　of　physics；　but　to　get　the　right　answer。　This　is　not　exactly　cheating；　but　if　we　apply　the　same　puter　model　to　rather　different　climatic　regimes—deep　global　warming；　for　instance—the　tuning　might　then　be　inappropriate。　The　model　might　be　valid　for　today's　climate；　but　not　extrapolatable　to　others。

One　way　to　test　this　program　is　to　apply　it　to　the　very　different　climates　of　other　planets。　Can　it　predict　the　structure　of　the　atmosphere　on　Mars　and　the　climate　there？　The　weather？　What　about　Venus？　If　it　were　to　fail　these　test　cases；　we　would　be　right　in　mistrusting　it　when　it　makes　predictions　for　our　own　planet。　In　fact；　climate　models　now　in　use　do　very　well　in　predicting　from　first　principles　of　physics　the　climates　on　Venus　and　Mars。

On　Earth；　huge　upwellings　of　molten　lava　are　known　and　attributed　to　superplumes　convecting　up　from　the　deep　mantle　and　generating　vast　plateaus　of　frozen　basalt。　A　spectacular　example　occurred　about　a　hundred　million　years　ago；　and　added　perhaps　ten　times　the　present　carbon　dioxide　content　to　the　atmosphere；　inducing　substantial　global　warming。　These　plumes；　it　is　thought；　occur　episodically　throughout　Earth's　history。　Similar　mantle　upwelling　seem　to　have　occurred　on　Mars　and　Venus。　There　are　sound　practical　reasons　for　us　to　want　to　unders