pale blue dot -carl sagan-第17部分

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essed　firsthand　so　far　by　only　12　humans；　the　lunar　landing　crews　of　Apollos　11；　12；　and　14…17。

A　full　list　of　the　satellites　in　the　Solar　System；　known　as　of　this　writing；　is　given　in　the　acpanying　table。　（Nearly　half　of　them　were　discovered　by　Voyager。）　All　have　black　skies—except　Titan　of　Saturn　and　perhaps　Triton　of　Neptune；　which　are　big　enough　to　have　atmospheres。　And　all　asteroids　as　well。

Venus　has　about　90　times　more　air　than　Earth。　It　isn't　mainly　oxygen　and　nitrogen　as　here—it's　carbon　dioxide。　But　carbon　dioxide　doesn't　absorb　visible　light　either。　What　would　the　sky　look　like　from　the　surface　of　Venus　if　Venus　had　no　clouds。　With　so　much　atmosphere　in　the　way；　not　only　are　violet　and　blue　waves　scattered；　but　all　the　other　colors　as　well…green　yellow；　orange；　red。　The　air　is　so　thick；　though；　that　hardly　any　blue　light　makes　it　to　the　ground；　it's　scattered　back　to　space　by　successive　bounces　higher　up。　Thus；　the　light　that　does　reach　the　ground　should　be　strongly　reddened…like　an　Earth　sunset　all　over　the　sky。　Further；　sulfur　in　the　high　clouds　will　stain　the　sky　yellow。　Pictures　taken　by　the　Soviet　Venera　landers　confirm　that　the　skies　of　Venus　are　a　kind　of　yellow…orange。

　

SIXTY…TWO　WORLDS　FOR　THE　THIRD　MILLENNIUM：　KNOWN　MOONS　OF　THE　PLANETS　（AND　ONE　ASTEROID）—

LISTED　IN　ORDER　OF　DISTANCE　FROM　THEIR　PLANET

EARTH；　1
　MARS；　2
　IDA；　1
　JUPITER；　16
　SATURN；　18
　URANUS；　15
　NEPTUNE；　8
　PLUT0；　1
　
Moon
　Phobos
　Dactyl
　Metis
　Pan
　Cordelia
　Naiad
　Charon
　
　　Deimos
　　　Adrastea
　Atlas
　Ophelia
　Thalassa
　　　
　　　　　　Amalthea
　Prometheus
　Bianca
　Despina
　　　
　　　　　　Thebe
　Pandora
　Cressida
　Galatea
　　　
　　　　　　to
　Epimetheus
　Desdemona
　Larissa
　　　
　　　　　　Europa
　Janus
　Juliet
　Proteus
　　　
　　　　　　Ganymede
　Mimas
　Portia
　Triton
　　　
　　　　　　Callisto
　Enceladus
　Rosalind
　Nereid
　　　
　　　　　　Leda
　Tethys
　Belinda
　　　　　
　　　　　　Himalia
　Telesto
　Puck
　　　　　
　　　　　　Lysithea
　Calypso
　Miranda
　　　　　
　　　　　　Elara
　Diane
　Ariel
　　　　　
　　　　　　Ananke
　Helene
　Umbriel
　　　　　
　　　　　　Carme
　Rhea
　Titania
　　　　　
　　　　　　Pasiphae
　Titan
　Oberon
　　　　　
　　　　　　Sinope
　Hyperion
　　　　　　　
　　　　　　　　Iapetus
　　　　　　　
　　　　　　　　Phoebe
　　　　　　　

Mars　is　a　different　story。　It　is　a　smaller　world　than　Earth；　with　a　much　thinner　atmosphere。　The　pressure　at　the　surface　of　Mars　is；　in　fact；　about　the　same　as　the　altitude　in　the　Earth's　stratosphere　to　which　Simons　rose。　So　we　might　expect　the　Martian　sky　to　be　black　or　purple…black。　The　first　color　picture　from　the　surface　of　Mars　was　obtained　in　July　1976　by　the　American　Viking　1　lander—the　first　spacecraft　to　touch　down　successfully　on　the　surface　of　the　Red　Planet。　The　digital　data　were　dutifully　radioed　from　Mars　back　to　Earth；　and　the　color　picture　assembled　by　puter。　To　the　surprise　of　all　the　scientists　and　nobody　else；　that　first　image；　released　to　the　press；　showed　the　Martian　sky　to　be　a　fortable；　homey　blue—impossible　for　a　planet　with　so　insubstantial　an　atmosphere。　Something　had　gone　wrong。

The　picture　on　your　color　television　set　is　a　mixture　of　three　monochrome　images；　each　in　a　different　color　of　light—red；　green；　and　blue。　You　can　see　this　method　of　color　positing　in　video　projection　systems；　which　project　separate　beams　of　red；　green；　and　blue　light　to　generate　a　full…color　picture　（including　yellows）。　To　get　the　right　color；　your　set　needs　to　mix　or　balance　these　three　monochrome　images　correctly。　If　you　turn　up　the　intensity　of；　say；　blue；　the　picture　will　appear　too　blue。　Any　picture　returned　from　space　requires　a　similar　color　balance。　Considerable　discretion　is　sometimes　left　to　the　puter　analysts　in　deciding　this　balance。　The　hiking　analysts　were　not　planetary　astronomers；　and　with　this　first　color　picture　from　Mars　they　simply　mixed　the　colors　until　it　looked　〃right。〃　We　are　so　conditioned　by　our　experience　on　Earth　that　〃right；〃　of　course；　means　a　blue　sky。　The　color　of　the　picture　was　soon　corrected—using　color　calibration　standards　placed　for　this　very　purpose　on　board　the　spacecraft—and　the　resulting　posite　showed　no　blue　sky　at　all；　rather　it　was　something　between　ochre　and　pink。　Not　blue；　but　hardly　purple…black　either。

This　is　the　right　color　of　the　Martian　sky。　Much　of　the　surface　of　Mars　is　desert—and　red　because　the　sands　are　rusk。　There　are　occasional　violent　sandstorms　that　lift　fine　particles　from　the　surface　high　into　the　atmosphere。　It　takes　a　long　time　for　them　to　fall　out；　and　before　the　sky　has　fully　cleaned　itself；　there's　always　another　sandstorm。　Global　or　near…global　sandstorms　occur　almost　every　Martian　year。　Since　rusty　particles　are　always　suspended　in　this　sky；　future　generations　of　humans；　born　and　living　out　their　lives　on　Mars；　will　consider　that　salmon　color　to　be　as　natural　and　familiar　as　we　consider　our　homey　blue。　From　a　single　glance　at　the　daytime　sky；　they'll　probably　be　able　to　tell　how　long　it's　been　since　the　last　big　sandstorm。

The　planets　in　the　outer　Solar　System—Jupiter；　Saturn；　Uranus；　and　Neptune—　are　of　a　different　sort。　These　are　huge　worlds　with　giant　atmospheres　made　mainly　of　hydrogen　and　helium。　Their　solid　surfaces　are　so　deep　inside　that　no　sunlight　penetrates　there　at　all。　Down　there；　the　sky　is　black；　with　no　prospect　of　a　sunrise—not　ever。　The　perpetual　starless　night　is　perhaps　illuminated　on　occasion　by　a　bolt　of　lightning。　But　higher　in　the　atmosphere；　where　the　sunlight　reaches；　a　much　more　beautiful　vista　awaits。

On　Jupiter；　above　a　high…altitude　haze　layer　posed　of　ammonia　（rather　than　water）　ice　particles；　the　sky　is　almost　black。　Farther　down；　in　the　blue　sky　region；　are　multicolored　clouds—in　various　shades　of　yellow…brown；　and　of　unknown　position。　（The　candidate　materials　include　sulfur；　phosphorus；　and　plex　organic　molecules。）　Even　farther　down；　the　sky　will　appear　red…brown；　except　that　the　clouds　there　are　of　varying　thicknesses；　and　where　they　are　thin；　you　might　see　a　patch　of　blue。　Still　deeper；　we　gradually　return　to　perpetual　night。　Something　similar　is　true　on　Saturn；　but　the　colors　there　are　more　muted。

Uranus　and　especially　Neptune　have　an　uncanny；　austere　blue　color　through　which　clouds—some　of　them　a　little　whiter—are　carried　by　high…speed　winds。　Sunlight　reaches　a　paratively　clean　atmosphere　posed　mainly　of　hydrogen　and　helium　but　also　rich　in　methane。　Long　paths　of　methane　absorb　yellow　and　especially　red　light　and　let　the　green　and　blue　filter　through。　A　thin　hydrocarbon　haze　removes　a　little　blue。　There　may　be　a　depth　where　the　sky　is　greenish。

Conventional　wisdom　holds　that　the　absorption　by　methane　and　the　Rayleigh　scattering　of　sunlight　by　the　deep　atmosphere　together　account　for　the　blue　colors　on　Uranus　and　Neptune。　But　analysis　of　Voyager　data　by　Kevin　Baines　of　JPL　seems　to　show　that　these　causes　are　insufficient。　Apparently　very　deep—maybe　in　the　vicinity　of　hypothesized　clouds　of　hydrogen　sulfide—there　is　an　abundant　blue　substance。　So　far　no　one　has　been　able　to　figure　out　what　it　might　be。　Blue　materials　are　very　rare　in　Nature。　As　always　happens　in　science；　the　old　mysteries　are　dispelled　only　to　be　replaced　by　new　ones。　Sooner　or　later　we'll　find　out　the　answer　to　this　one；　too。



ALL　WORLDS　WITH　NONBLACK　SKIES　have　atmospheres。　If　you're　standing　on　the　surface　and　there's　an　atmosphere　thick　enough　to　see；　there's　probably　a　way　to　fly　through　it。　We're　now　sending　our　instruments　to　fly　in　the　variously　colored　skies　of　other　worlds。　Someday　we　will　go　ourselves。

Parachutes　have　already　been　used　in　the　atmospheres　of　Venus　and　Mars；　and　are　planned　for　Jupiter　and　Titan。　In　1985　two　French…Soviet　balloons　sailed　through　the　yellow　skies　of　Venus。　The　Vega　9　balloon；　about　4　meters　across；　dangled　an　instrument　package　13　meters　below。　The　balloon　inflated　in　the　night　hemisphere；　floated　about　54　kilometers　above　the　surface；　and　transmitted　data　for　almost　two　Earth　days　before　its　batteries　failed。　In　that　time　it　traveled　11；600　kilometers　（nearly　7；000　miles）　over　the　surface　of　Venus；　far　below。　The　Vega　2　balloon　had　an　almost　identical　profile。　The　atmosphere　of　Venus　has　also　been　used　for　aerobraking—changing　the　Magellan　spacecraft's　orbit　by　friction　with　the　dense　air；　this　is　a　key　future　technology　for　converting　flyby　spacecraft　to　Mars　into　orbiters　and　landers。

A　Mars　mission；　scheduled　to　be　launched　in　1998；　and　led　by　Russia；　includes　an　enormous　French　hot　air　balloon—looking　something　like　a　vast　jellyfish；　a　Portuguese　man…of…war。　It's　designed　to　sink　to　the　Martian　surface　every　chilly　twilight　and　rise　high　when　heated　by　sunlight　the　next　day。　The　winds　are　so　fast　that；　if　all　goes　well；　it　will　be　carried　hundreds　of　kilometers　each　day；　hopping　and　skipping　over　the　north　pole。　In　the　early　morning；　when　close　to　the　ground；　it　will　obtain　very　high　resolution　pictures　and　other　data。　The　balloon　has　an　instrumental　guide…rope；　essential　for　its　stability；　conceived　and　designed　by　a　private　membership　organization　based　in　Pasadena；　California；　The　Planetary　Society。

Since　the　surface　pressure　on　Mars　is　approximately　that　at　an　altitude　of　100；000　feet　on　Earth；　we　know　we　can　fly　airplanes　there。　The　U…2；　for　example；　or　the　SR…71　Blackbird　routinely　approaches　such　low　pressures。　Aircraft　with　even　larger　wingspans　have　been　designed　for　Mars。

The　dream　of　flight　and　the　dream　of　space　travel　are　twins；　conceived　by　similar　visionaries；　dependent　on　allied　technologies；　and　evolving　more　or　less　in　tandem。　As　certain　practical　and　economic　limits　to　flight　on　Earth　are　reached；　the　possibility　arises　of　flying　through　the　multihued　skies　of　other　worlds。



IT　IS　NOW　ALMOST　POSSIBLE　to　assign　color　binations；　based　on　the　colors　of　clouds　and　sky；　to　every　planet　in　the　Solar　System—from　the　sulfur…stained　skies　of　Venus　and　the　rusty　skies　of　Mars　to　the　aquamarine　of　Uranus　and　the　hypnotic　and　unearthly　blue　of　Neptune。　Sacre…jaunt；　sacre…rouge；　sacre…vert。　Perhaps　they　will　one　day　adorn　the　flags　of　distant　human　outposts　in　the　Solar　System；　in　that　time　when　the　new　frontiers　are　sweeping　out　from　the　Sun　to　the　stars；　and　the　explorers　are　surrounded　by　the　endless　black　of　space。　Sacre…noir。




CHAPTER　11　　　EVENING　AND　MORNING　STAR

　

　

This　is　another　world　which　is　not　of　men。

—LI　BAI；　〃QUESTION　AND　ANSWER　IN　THE　MOUNTAINS〃　（CHINA；　TANG　DYNASTY；　CA。　730）

You　can　see　it　shining　brilliantly　in　the　twilight；　chasing　the　Sun　down　below　the　western　horizon。　Upon　first　glimpsing　it　each　night；　people　were　accustomed　to　make　a　wish　（〃upon　a　star〃）。　Sometimes　the　wish　came　true。

Or　you　can　spy　it　in　the　east　before　dawn；　fleeing　the　rising　Sun。　In　these　two　incarnations；　brighter　than　anything　else　in　the　sky　except　only　the　Sun　and　the　Moon　it　was　known　as　the　evening　and　the　morning　star。　Our　ancestors　did　not　recognize　it　was　a　world；　the　same　world；　never　too　far　from　the　Sun　because　it　is　in　an　orbit　about　it　interior　to　the　Earth's。　Just　before　sunset　or　just　after　sunrise；　we　can　sometimes　see　it　near　some　fluffy　white　cloud；　and　then　discover　by　the　parison　that　Venus　has　a　color；　a　pale　lemon…yellow。

You　peer　through　the　eyepiece　of　a　telescope—even　a　big　telescope；　even　the　largest　optical　telescope　on　Earth—and　you　can　make　out　no　detail　at　all。　Over　the　months；　you　see　a　featureless　disk　methodically　going　through　phases；　like　the　Moon：　crescent　Venus；　full　Venus；　gibbous　Venus；　new　Venus。　There　is　not　a　hint　of　continents　or　oceans。

Some　of　the　first　astronomers　to　see　Venus　through　the　telescope　immediately　recognized　that　they　were　examining　a　world　enshrouded　by　clouds。　The　clouds；　we　now　know；　are　droplets　of　concentrated　sulfuric　acid；　stained　yellow　by　a　little　elemental　sulfur。　They　lie　high　above　the　ground。　In　ordinary　visible　light　there's　no　hint　of　what　this　planet's　surface；　some　50　kilometers　below　the　cloud　tops；　is　like；　and　for　centuries　the　best　we　had　were　wild　guesses。

You　might　conjecture　that　if　we　could　take　a　much　finer　look　there　might　be　breaks　in　the　clouds；　revealing　day　by　day；　in　bits　and　pieces；　the　mysterious　surface　ordinarily　hidden　from　our　view。　Then　the　time　of　guesses　would　be　over。　The　Earth　is　on　average　half　cloud…covered。　In　the　early　days　of　Venus　exploration；　we　saw　no　reason　that　Venus　should　be　100　percent　overcast。　If　instead　it　was　only　90　percent；　or　even　99　percent；　cloud…covered；　the　transient　patches　of　clearing　might　tell　us　much。

In　1960　and　1961；　Mariners　1　and　2；　the　first　American　spacecraft　designed　to　visit　Venus；　were　being　prepared。　There　were　those；　like　me；　who　thought　the　ships　should　carry　video　cameras　so　they　could　radio　pictures　back　to　Earth。　The　same　technology　would　be　used　a　few　years　later　when　Rangers　7；　8；　and　9　would　photograph　the　Moon　on　the　way　to　their　crash　landings—the　last　making　a　bull's…eye　in　the　crater　Alphonsus。　But　time　was　short　for　the　Venus　mission；　and　cameras　were　heavy。　There　were　those　who　maintained　that　cameras　weren't　really　scientific　instruments；　but　rather　catch…as…catch…can；　razzle…dazzle；　pandering　to　the　public；　and　unable　to　answer　a　single　straightforward；　well…posed　scientific　question。　I　thought　myself　that　whether　there　are　breaks　in　the　clouds　was　one　such　question。　I　argued　that　cameras　could　also　answer　questions　that　we　were　too　dumb　even　to　pose。　I　argued　that　pictures　were　the　only　way　to　show　the　public—who　were；　after　all；　footing　the　bill—the　excitement　of　robotic　missions。　At　any　rate；　no　camera　was　flown；　and　subsequent　missions　have；　for　this　particular　world；　at　least　partly　vindicated　that　judgment：　Even　at　high　resolution　from　close　flybys；　in　visible　light　it　turns　out　there　are　no　breaks　in　the　clouds　of　Venus；　any　more　than　in　the　clouds　of　Titan。*　These　worlds　are　permanently　overcast。

*　For　Titan；　imaging　revealed　a　succession　of　detached　hazes　above　the　main　layer　of　aerosols。　So　Venus　works　out　to　be　the　only　world　in　the　Solar　System　for　which　spacecraft　cameras　working　in　ordinary　visible　light　haven't　discovered　something　important。　Happily；　we've　now　returned　pictures　from　almost　every　world　we've　visited。　（NASA's　International　etary　Explorer；　which　raced　through　the　tail　of　et　Giacobini…Zimmer　in　1985；　flew　blind；　be　devoted　to　charged　particles　and　magnetic　fields。）

In　the　ultraviolet　there　is　detail；　but　due　to　transient　patches　of　high…altitude　overcast；　far　above　the　main　cloud　deck。　The　high　clouds　race　around　the　planet　much　faster　than　the　planet　itself　turns：　super…rotation。　We　have　an　even　smaller　chance　of　seeing　the　surface　in　the　ultraviolet。

When　it　became　clear　that　the　atmosphere　of　Venus　was　much　thicker　than　the　air　on　Earth—as　we　now　know；　the　pressure　at　the　surface　is　ninety　times　what　it　is　here—it　immediately　followed　that　in　ordinary　visible　light　we　could　not　possibly　see　the　surface；　even　if　there　were　breaks　in　the　clouds。　What　little　sunlight　is　able　to　make　its　tortuous　way　through　the　dense　atmosphere　to　the　surface　would　be　reflected　back；　all　right；　but　the　photons　would　be　so　jumbled　by　repeated　scattering　off　molecules　in　the　lower　air　that　no　image　of　surface　features　could　be　retained。　It　would　be　like　a　〃whiteout〃　in　polar　sn