pale blue dot -carl sagan-第26部分

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！


；　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　snowstorm。　However；　this　effect；　intense　Rayleigh　scattering；　declines　rapidly　with　increasing　wavelength；　in　the　near…infrared；　it　was　easy　to　calculate；　you　could　see　the　surface　if　there　were　breaks　in　the　clouds　or　if　the　clouds　were　transparent　there。

So　in　1970　Jim　Pollack；　Dave　Morrison；　and　I　went　to　the　McDonald　Observatory　of　the　University　of　Texas　to　try　to　observe　Venus　in　the　near…infrared。　We　〃hypersensitized〃　our　emulsions；　the　good　old…fashioned*　glass　photographic　plates　were　treated　with　ammonia；　and　sometimes　heated　or　briefly　illuminated；　before　being　exposed　at　the　telescope　to　light　from　Venus。　For　a　time　the　cellars　of　McDonald　Observatory　reeked　of　ammonia。　We　took　many　pictures。　None　showed　any　detail。　We　concluded　that　either　we　hadn't　gone　far　enough　into　the　infrared　or　the　clouds　of　Venus　were　opaque　and　unbroken　in　the　near　infrared。

*　Today　many　telescopic　images　are　obtained　with　such　electronic　contrivances　as　charge…coupled　devices　and　diode　arrays；　and　processed　by　puter—all　technologies　unavailable　to　astronomers　in　1970。

More　than　20　years　later；　the　Galileo　spacecraft；　making　a　close　flyby　of　Venus；　examined　it　with　higher　resolution　and　sensitivity；　and　at　wavelengths　a　little　further　into　the　infrared　than　we　were　able　to　reach　with　our　crude　glass　emulsions。　Galileo　photographed　great　mountain　ranges。　We　already　knew　of　their　existence；　though；　a　much　more　powerful　technique　had　earlier　been　employed：　radar。　Radio　waves　effortlessly　penetrate　the　clouds　and　thick　atmosphere　of　Venus；　bounce　off　the　surface；　and　return　to　Earth；　where　they　are　gathered　in　and　used　to　make　a　picture。　The　first　work　had　been　done；　chiefly；　by。　American　ground…based　radar　at　JPL's　Goldstone　tracking　station　in　the　Mojave　Desert　and　at　the　Arecibo　Observatory　in　Puerto　Rico；　operated　by　Cornell　University。

Then　the　U。S。　Pioneer　12；　the　Soviet　Venera　15　and　？6　and　the　U。S。　Magellan　missions　inserted　radar　telescopes　into　orbit　around　Venus　and　mapped　the　place　pole　to　pole。　Each　spacecraft　would　transmit　a　radar　signal　to　the　surface　and　then　catch　it　as　it　bounced　back。　From　how　reflective　each　patch　of　surface　was　and　how　long　it　took　the　signal　to　return　（shorter　from　mountains；　longer　from　valleys）；　a　detailed　map　of　the　entire　surface　was　slowly　and　painstakingly　constructed。

The　world　so　revealed　turns　out　to　be　uniquely　sculpted　by　lava　flows　（and；　to　a　much　lesser　degree；　by　wind）；　as　described　in　the　next　chapter。　The　clouds　and　atmosphere　of　Venus　have　now　bee　transparent　to　us；　and　another　world　has　been　visited　by　the　doughty　robot　explorers　from　Earth。　Our　experience　with　Venus　is　now　being　applied　elsewhere—especially　to　Titan；　where　once　again　impenetrable　clouds　hide　an　enigmatic　surface；　and　radar　is　beginning　to　give　us　hints　of　what　might　lie　below。



VENUS　HAD　LONG　BEEN　THOUGHT　of　as　our　sister　world。　It　is　the　nearest　planet　to　the　Earth。　It　has　almost　the　same　mass；　size；　density；　and　gravitational　pull　as　the　Earth　does。　It's　a　little　closer　to　the　Sun　than　the　Earth；　but　its　bright　clouds　reflect　more　sunlight　back　to　space　than　our　clouds　do。　As　a　first　guess　you　might　very　well　imagine　that；　under　those　unbroken　clouds；　Venus　was　rather　like　Earth。　Early　scientific　speculation　included　fetid　swamps　crawling　with　monster　amphibians；　like　the　Earth　in　the　Carboniferous　Period；　a　world　desert；　a　global　petroleum　sea；　and　a　seltzer　ocean　dotted　here　and　there　with　limestone…encrusted　islands。　While　based　on　some　scientific　data；　these　“models〃　of　Venus—the　first　dating　from　the　beginnings　of　the　century；　the　second　from　the　1930s；　and　the　last　two　from　the　raid…1950s—were　little　more　than　scientific　romances；　hardly　constrained　by　the　sparse　data　available。

Then；　in　1956；　a　report　was　published　in　The　Astrophysical　Journal　by　Cornell　H。　Mayer　and　his　colleagues。　They　had　pointed　a　newly　pleted　radio　telescope；　built　in　part　for　classified　research；　on　the　roof　of　the　Naval　Research　Laboratory　in　Washington；　D。C。；　at　Venus　and　measured　the　flux　of　radio　waves　arriving　at　Earth。　This　was　not　radar：　No　radio　waves　were　bounced　off　Venus。　This　was　listening　to　radio　waves　that　Venus　on　its　own　emits　to　space。　Venus　turned　out　to　be　much　brighter　than　the　background　of　distant　stars　and　galaxies。　This　in　itself　was　not　very　surprising。　Every　object　warmer　than　absolute　zero　（…273°C）　gives　off　radiation　throughout　the　electromagnetic　spectrum；　including　the　radio　region。　You；　for　example；　emit　radio　waves　at　an　effective　or　〃brightness〃　temperature　of　about　35°C；　and　if　you　were　in　surroundings　colder　than　you　are；　a　sensitive　radio　telescope　could　detect　the　faint　radio　waves　you　are　transmitting　in　all　directions。　Each　of　us　is　a　source　of　cold　static。

What　was　surprising　about　Mayer's　discovery　was　that　the　brightness　temperature　of　Venus　is　more　than　300°C；　far　higher　than　the　surface　temperature　of　the　Earth　or　the　measured　infrared　temperature　of　the　clouds　of　Venus。　Some　places　on　Venus　seemed　at　least　200°　hotter　than　the　normal　boiling　point　of　wate