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respectively。　He　courageously　testified　before　Congress　in　the　face　of　a　politically　generated　order　from　the　White　House　Office　of　Management　and　Budget　（this　was　in　the　Reagan　years）　to　exaggerate　the　uncertainties　and　minimize　the　dangers。　His　calculation　on　the　explosion　of　the　Philippine　volcano　Mt。　Pinatubo　and　his　prediction　of　the　resulting　temporary　decline　in　the　Earth's　temperature　（about　half　a　degree　Celsius）　were　right　on　the　money。　He　has　been　a　force　in　convincing　governments　worldwide　that　global　warming　is　something　to　be　taken　seriously。

How　did　Hansen　get　interested　in　the　greenhouse　effect　in　the　first　place？　His　doctoral　thesis　（at　the　University　of　Iowa　in　1967）　was　about　Venus。　He　agreed　that　the　high　radio　brightness　of　Venus　is　due　to　a　very　hot　surface；　agreed　that　greenhouse　gases　keep　the　heat　in；　but　proposed　that　heat　from　the　interior　rather　than　sunlight　was　the　principal　energy　source。　The　Pioneer　12　mission　to　Venus　in　1978　dropped　entry　probes　into　the　atmosphere；　they　showed　directly　that　the　ordinary　greenhouse　effect—the　surface　heated　by　the　Sun　and　the　heat　retained　by　the　blanket　of　air—was　the　operative　cause。　But　it's　Venus　that　got　Hansen　thinking　about　the　greenhouse　effect。

Radio　astronomers；　you　note；　find　Venus　to　be　an　intense　source　of　radio　waves。　Other　explanations　of　the　radio　emission　fail。　You　conclude　that　the　surface　must　be　ridiculously　hot。　You　try　to　understand　where　the　high　temperatures　e　from　and　are　led　inexorably　to　one　or　another　kind　of　greenhouse　effect。　Decades　later　you　find　that　this　training　has　prepared　you　to　understand　and　help　predict　an　unexpected　threat　to　our　global　civilization。　I　know　many　other　instances　where　scientists　who　first　tried　to　puzzle　out　the　atmospheres　of　other　worlds　are　making　important　and　highly　practical　discoveries　about　this　one。　The　other　planets　are　a　superb　training　ground　for　students　of　the　Earth。　They　require　both　breadth　and　depth　of　knowledge；　and　they　challenge　the　imagination。

Those　who　are　skeptical　about　carbon　dioxide　greenhouse　warming　might　profitably　note　the　massive　greenhouse　effect　on　Venus。　No　one　proposes　that　Venus's　greenhouse　effect　derives　froth　imprudent　Venusians　who　burned　too　much　coal；　drove　fuel…inefficient　autos；　and　cut　down　their　forests。　My　point　is　different。　The　climatological　history　of　our　planetary　neighbor；　an　otherwise　Earthlike　planet　on　which　the　surface　became　hot　enough　to　melt　tin　or　lead；　is　worth　considering—especially　by　those　who　say　that　the　increasing　greenhouse　effect　on　Earth　will　be　self…correcting；　that　we　don't　really　have　to　worry　about　it；　or　（you　can　see　this　in　the　publications　of　some　groups　that　call　themselves　conservative）　that　the　greenhouse　effect　itself　is　a　〃hoax。〃

（3）　Nuclear　winter　is　the　predicted　darkening　and　cooling　of　the　Earth—mainly　from　fine　smoke　particles　injected　into　the　atmosphere　from　the　burning　of　cities　and　petroleum　facilities—that　is　predicted　to　follow　a　global　thermonuclear　war。　A　vigorous　scientific　debate　ensued　on　just　how　serious　nuclear　winter　might　be。　The　various　opinions　have　now　converged。　All　three…dimensional　general　circulation　puter　models　predict　that　the　global　temperatures　resulting　from　a　worldwide　thermonuclear　war　would　be　colder　than　those　in　the　Pleistocene　ice　ages。　The　implications　for　our　planetary　civilization—especially　through　the　collapse　of　agriculture—are　very　dire。　It　is　a　consequence　of　nuclear　war　that　was　somehow　overlooked　by　the　civil　and　military　authorities　of　the　United　States；　the　Soviet　Union；　Britain；　France；　and　China　when　they　decided　to　accumulate　well　over　60；000　nuclear　weapons。　Although　it's　hard　to　be　certain　about　such　things；　a　case　can　be　made　that　nuclear　Winter　played　a　constructive　role　（there　were　other　causes；　of　course）　in　convincing　the　nuclear…armed　nations；　especially　the　Soviet　Union；　of　the　futility　of　nuclear　war。

Nuclear　winter　was　first　calculated　and　named　in　1982/83　by　a　group　of　five　scientists；　to　which　I'm　proud　to　belong。　This　team　was　given　the　acronym　TTAPS　（for　Richard　P。　Turco；　（even　B。　Toon；　Thomas　Ackerman；　James　Pollack；　and　myself）。　Of　the　five　TTAPS　scientists；　two　were　planetary　scientists；　and　the　other　three　had　published　many　papers　in　planetary　science；　The　earliest　intimation　of　nuclear　winter　came　during　that　same　Mariner　9　mission　to　Mars；　when　there　was　a　global　dust　storm　and　we　were　unable　to　see　the　surface　of　the　planet；　the　infrared　spectrometer　on　the　spacecraft　found　the　high　atmosphere　to　be　warmer　and　the　surface　colder　than　they　ought　to　have　been。　Jim　Pollack　and　I　sat　down　and　tried　to　calculate　how　that　could　e　about。　Over　the　subsequent　twelve　years；　this　line　of　inquiry　led　from　dust　storms　on　Mars　to　volcanic　aerosols　on　Earth　to　the　possible　extinction　of　the　dinosaurs　by　impact　dust　to　nuclear　winter。　You　never　know　where　science　will　take　you。



PLANETARY　SCIENCE　fosters　a　broad　interdisciplinary　point　of　view　that　proves　enormously　helpful　in　discovering　and　attempting　to　defuse　these　looming　environmental　catastrophes。　When　you　cut　your　teeth　on　other　worlds；　you　gain　a　perspective　about　the　fragility　of　planetary　environments　and　about　what　other；　quite　different；　environments　are　possible。　There　may　well　be　potential　global　catastrophes　still　to　be　uncovered。　If　there　are；　I　bet　planetary　scientists　will　play　a　central　role　in　understanding　them。

Of　all　the　fields　of　mathematics；　technology；　and　science；　the　one　with　the　greatest　international　cooperation　（as　determined　by　how　often　the　co…authors　of　research　papers　hail　from　two　or　more　countries）　is　the　field　called　〃Earth　and　space　sciences。〃　Studying　this　world　and　others；　by　its　very　nature；　tends　to　be　non…local；　non…nationalist；　non…chauvinist。　Very　rarely　do　people　go　into　these　fields　because　they　are　internationalists。　Almost　always；　they　enter　for　other　reasons；　and　then　discover　that　splendid　work；　work　that　plements　their　own；　is　being　done　by　researchers　in　other　nations；　or　that　to　solve　a　problem；　you　need　data　or　a　perspective　（access　to　the　southern　sky；　for　example）　that　is　unavailable　in　your　country。　And　once　you　experience　such　cooperation—humans　from　different　parts　of　the　planet　working　in　a　mutually　intelligible　scientific　language　as　partners　on　matters　of　mon　concern—it's　hard　not　to　imagine　it　happening　on　other；　nonscientific　matters。　I　myself　consider　this　aspect　of　Earth　and　space　sciences　as　a　healing　and　unifying　force　in　world　politics；　but；　beneficial　or　not；　it　is　inescapable。

When　I　look　at　the　evidence；　it　seems　to　me　that　planetary　exploration　is　of　the　most　practical　and　urgent　utility　for　us　here　on　Earth。　Even　if　we　were　not　roused　by　the　prospect　of　exploring　other　worlds；　even　if　we　didn't　have　a　nanogram　of　adventuresome　spirit　in　us；　even　if　we　were　only　concerned　for　ourselves　and　in　the　narrowest　sense；　planetary　exploration　would　still　constitute　a　superb　investment。




CHAPTER　15　　　THE　GATES　OF　THE　WONDER　WORLD　OPEN

　

　

The　great　floodgates　of　the　wonder…world　swung　open。

—HERMAN　MELVILLE；　MOBY　DICK；　CHAPTER　1　（1851）

Sometime　ing　up；　perhaps　just　around　the　corner；　there　will　be　a　nation—more　likely；　a　consortium　of　nations—that　will　work　the　next　major　step　in　the　human　venture　into　space。　Perhaps　it　will　be　brought　about　by　circumventing　bureaucracies　and　making　efficient　use　of　present　technologies。　Perhaps　it　will　require　new　technologies；　transcending　the　great　blunderbuss　chemical　rockets。　The　crews　of　these　ships　will　set　foot　on　new　worlds。　The　first　baby　will　be　born　somewhere　up　there。　Early　steps　toward　living　off　the　land　will　be　made。　We　will　be　on　our　way。　And　the　future　will　remember。



TANTALIZING　AND　MAJESTIC；　Mars　is　the　world　next　door；　the　nearest　planet　on　which　an　astronaut　or　cosmonaut　could　safely　land。　Although　it　is　sometimes　as　warm　as　a　New　England　October；　Mars　is　a　chilly　place；　so　cold　that　some　of　its　thin　carbon　dioxide　atmosphere　freezes　out　as　dry　ice　at　the　winter　pole。

It　is　the　nearest　planet　whose　surface　we　can　see　with　a　small　telescope。　In　all　the　Solar　System；　it　is　the　planet　most　like　Earth。　Apart　from　flybys；　there　have　been　only　two　fully　successful　missions　to　Mars：　Mariner　9　in　1971；　and　Vikings　1　and　2　in　1976。　They　revealed　a　deep　rift　valley　that　would　stretch　from　New　York　to　San　Francisco；　immense　volcanic　mountains；　the　largest　of　which　towers　80；000　feet　above　the　average　altitude　of　the　Martian　surface；　almost　three　times　the　height　of　Mount　Everest；　an　intricate　layered　structure　in　and　among　the　polar　ices；　resembling　a　pile　of　discarded　poker　chips；　and　probably　a　record　of　past　climatic　change；　bright　and　dark　streaks　painted　down　on　the　surface　by　windblown　dust；　providing　high…speed　wind　maps　of　Mars　over　the　past　decades　and　centuries；　vast　globe…girdling　dust　storms；　and　enigmatic　surface　features。

Hundreds　of　sinuous　channels　and　valley　networks　dating　back　several　billion　years　can　be　found；　mainly　in　the　cratered　southern　highlands。　They　suggest　a　previous　epoch　of　more　benign　and　Earthlike　conditions—very　different　from　what　we　find　beneath　the　tenuous　and　frigid　atmosphere　of　our　time。　Some　ancient　channels　seem　to　have　been　carved　by　rainfall；　some　by　underground　sapping　and　collapse；　and　some　by　great　floods　that　gushed　up　out　of　the　ground。　Rivers　were　pouring　into　and　filling　great　thousand…kilometer…diameter　impact　basins　that　today　are　dry　as　dust。　Waterfalls　dwarfing　any　on　Earth　today　cascaded　into　the　lakes　of　ancient　Mars。　Vast　oceans；　hundreds　of　meters；　perhaps　even　a　kilometer；　deep　may　have　gently　lapped　shorelines　barely　discernible　today。　That　would　have　been　a　world　to　explore。　We　are　four　billion　years　late。*

*　Although　in　a　few　places；　such　as　the　slopes　of　the　elevation　called　Alba　Patera；　there　are　multibranched　valley　networks　that　by　parison　are　very　young。　Somehow；　even　in　the　most　recent　billion　years；　liquid　water　seems　to　have　flowed　here　and　there；　from　time　to　time；　through　the　deserts　of　Mars。

On　Earth　in　just　the　same　period；　the　first　microorganisms　arose　and　evolved。　Life　on　Earth　is　intimately　connected；　for　the　most　basic　chemical　reasons；　with　liquid　water。