pale blue dot -carl sagan-第33章

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*　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　water。　What　could　this　mean？

Soon　there　was　a　deluge　of　explanations。　I　argued　that　the　high　radio　brightness　temperature　was　a　direct　indication　of　a　hot　surface；　and　that　the　high　temperatures　were　due　to　a　massive　carbon　dioxide/water　vapor　greenhouse　effect—in　which　some　sunlight　is　transmitted　through　the　clouds　and　heats　the　Surface；　but　the　surface　experiences　enormous　difficulty　in　radiating　back　to　space　because　of　the　high　infrared　opacity　of　carbon　dioxide　and　water　vapor。　Carbon　dioxide　absorbs　at　a　range　of　wavelengths　through　the　infrared；　but　there　seemed　to　be　〃windows〃　between　the　CO2　absorption　bands　through　which　the　surface　could　readily　cool　off　to　space。　Water　vapor；　though；　absorbs　at　infrared　frequencies　that　correspond　in　part　to　the　windows　in　the　carbon　dioxide　opacity。　The　two　gases　together；　it　seemed　to　me；　could　pretty　well　absorb　almost　all　the　infrared　emission；　even　if　there　was　very　little　water　vapor—something　like　two　picket　fences；　the　slats　of　one　being　fortuitously　positioned　to　cover　the　gaps　of　the　other。

There　was　another　very　different　category　of　explanation；　in　which　the　high　brightness　temperature　of　Venus　had　nothing　to　do　with　the　ground。　The　surface　could　still　be　temperate；　clement；　congenial。　It　was　proposed　that　some　region　in　the　atmosphere　of　Venus　or　in　its　surrounding　magnetosphere　emitted　these　radio　waves　to　space。　Electrical　discharges　between　water　droplets　in　the　Venus　clouds　were　suggested。　A　glow　discharge　in　which　ions　and　electrons　rebined　at　twilight　and　dawn　in　the　upper　atmosphere　was　offered。　A　very　dense　ionosphere　had　its　advocates；　in　which　the　mutual　acceleration　of　unbound　electrons　（〃free…free　emission〃）　gave　off　radio　waves。　（One　proponent　of　this　idea　even　suggested　that　the　high　ionization　required　was　due　to　an　average　of　10；000　times　greater　radioactivity　on　Venus　than　on　Earth—perhaps　from　a　recent　nuclear　war　there。）　And；　in　the　light　of　the　discovery　of　radiation　from　Jupiter's　magnetosphere；　it　was　natural　to　suggest　that　the　radio　emission　came　from　an　immense　cloud　of　charged　particles　trapped　by　some　hypothetical　very　intense　Venusian　magnetic　field。

In　a　series　of　papers　I　published　in　the　middle　1960s；　many　in　collaboration　with　Jim　Pollack；*　these　conflicting　models　of　a　high　hot　emitting　region　and　a　cold　surface　were　subjected　to　a　critical　analysis。　By　then　we　had　two　important　new　clues：　the　radio　spectrum　of　Venus；　and　the　Mariner　2　evidence　that　the　radio　emission　was　more　intense　at　the　center　of　the　disk　of　Venus　than　toward　its　edge。　By　1967　we　were　able　to　exclude　the　alternative　models　with　some　confidence；　and　conclude　that　the　surface　of　Venus　was　at　a　scorching　and　un…Earthlike　temperature；　in　excess　of　400°C。　But　the　argument　was　inferential；　and　there　were　many　intermediate　steps。　We　longed　for　a　more　direct　measurement。

*　James　B。　Pollack　made　important　contributions　to　every　area　of　planetary　science。　He　was　my　first　graduate　student　and　a　colleague　ever　since。　He　converted　NASA's　Ames　Research　Center　into　a　world　leader　in　planetary　research　and　the　post…doctoral　training　of　planetary　scientists。　His　gentleness　was　as　extraordinary　as　his　scientific　abilities。　He　died　in　1994　at　the　height　of　his　powers。

In　October　1967—memorating　the　tenth　anniversary　of　Sputnik　1—the　Soviet　Venera　4　spacecraft　dropped　an　entry　capsule　into　the　clouds　of　Venus。　It　returned　data　from　the　hot　lover　atmosphere；　but　did　not　survive　to　the　surface。　One　day　later；　the　United　States　spacecraft　Mariner　5　flew　by　Venus；　its　radio　transmission　to　Earth　skimming　the　atmosphere　at　progressively　greater　depths。　The　rate　of　fading　of　the　signal　gave　information　about　atmospheric　temperatures。　Although　there　seemed　to　be　some　discrepancies　（later　resolved）　between　the　two　sets　of　spacecraft　data；　both　clearly　indicated　that　the　surface　of　Venus　is　very　hot。

Since　then　a　progression　of　Soviet　Venera　spacecraft　and　one　cluster　of　American　spacecraft　from　the　Pioneer　12　mission　have