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ds　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　entered　the　deep　atmosphere　or　landed　on　the　surface　and　measured　directly—essentially　by　sticking　out　a　thermometer—the　surface　and　near…surface　temperatures。　They　turn　out　to　be　about　470°C；　almost　900°F。　When　such　factors　as　calibration　errors　of　terrestrial　radio　telescopes　and　surface　emissivity　are　taken　into　account；　the　old　radio　observations　and　the　new　direct　spacecraft　measurements　turn　out　to　be　in　good　accord。

Early　Soviet　landers　were　designed　for　an　atmosphere　somewhat　like　our　own。　They　were　crushed　by　the　high　pressures　like　a　tin　can　in　the　grasp　of　a　champion　arm　wrestler；　or　a　World　War　II　submarine　in　the　Tonga　Trench。　Thereafter；　Soviet　Venus　entry　vehicles　were　heavily　reinforced；　like　modern　submarines；　and　successfully　landed　on　the　searing　surface。　When　it　became　clear　how　deep　the　atmosphere　is　and　how　thick　the　clouds；　Soviet　designers　became　concerned　that　the　surface　might　be　pitch…black。　Veneras　9　and　10　were　equipped　with　floodlights。　They　proved　unnecessary。　A　few　percent　of　the　sunlight　that　falls　on　the　top　of　the　clouds　makes　it　through　to　the　surface；　and　Venus　is　about　as　bright　as　on　a　cloudy　day　on　Earth。

The　resistance　to　the　idea　of　a　hot　surface　on　Venus　can；　I　suppose；　be　attributed　to　our　reluctance　to　abandon　the　notion　that　the　nearest　planet　is　hospitable　for　life；　for　future　exploration；　and　perhaps　even；　in　the　longer　term；　for　human　settlement。　As　it　turns　out　there　are　no　Carboniferous　swamps　no　global　oil　or　seltzer　oceans。　Instead；　Venus　is　a　stifling；　brooding　inferno。　There　are　some　deserts；　but　it's　mainly　a　world　of　frozen　lava　seas。　Our　hopes　are　unfulfilled。　The　call　of　this　world　is　now　more　muted　than　in　the　early　days　of　spacecraft　exploration；　when　almost　anything　was　possible　and　our　most　romantic　notions　about　Venus　might；　for　all　we　then　knew；　be　realized。



MANY　SPACECRAFT　CONTRIBUTED　to　our　present　understanding　of　Venus。　But　the　pioneering　mission　was　Mariner　2。　Mariner　1　failed　at　launch　and—as　they　say　of　a　racehorse　with　a　broken　leg—had　to　be　destroyed。　Mariner　2　worked　beautifully　and　provided　the　key　early　radio　data　on　the　climate　of　Venus。　It　made　infrared　observations　of　the　properties　of　the　clouds。　On　its　way　from　Earth　to　Venus；　it　discovered　and　measured　the　solar　wind—the　stream　of　charged　particles　that　flows　outward　from　the　Sun；　filling　the　magnetospheres　of　any　planets　in　its　way；　blowing　back　the　tails　of　ets；　and　establishing　the　distant　heliopause。　Mariner　2　was　the　first　successful　planetary　probe；　the　ship　that　ushered　in　the　age　of　planetary　exploration。

It's　still　in　orbit　around　the　Sun；　every　few　hundred　days　still　approaching；　more　or　less　tangentially；　the　orbit　of　Venus。　Each　time　that　happens；　Venus　isn't　there。　But　if　we　wait　long　enough；　Venus　will　one　day　be　nearby　and　Mariner　2　will　be　accelerated　by　the　planet's　gravity　into　some　quite　different　orbit。　Ultimately；　Mariner　2；　like　some　planetesimal　from　ages　past；　will　be　swept　up　by　another　planet；　fall　into　the　Sun；　or　be　ejected　from　the　Solar　System。

Until　then；　this　harbinger　of　the　age　of　planetary　exploration；　this　minuscule　artificial　planet；　will　continue　silently　orbiting　the　Sun。　It's　a　little　as　if　Columbus's　flagship；　the　Santa　Maria；　were　still　making　regular　runs　with　a　ghostly　crew　across　the　Atlantic　between　Cadiz　and　Hispaniola。　In　the　vacuum　of　interplanetary　space；　Mariner　2　should　be　in　mint　condition　for　many　generations。

My　wish　on　the　evening　and　morning　star　is　this：　that　late　in　the　twenty…first　century　some　great　ship；　on　its　regular　gravity…assisted　transit　to　the　outer　Solar　System；　intercepts　this　ancient　derelict　and　heaves　it　aboard；　so　it　can　be　displayed　in　a　museum　of　early　space　technology—on　Mars；　perhaps；　or　Europa；　or　Iapetus。




CHAPTER　12　　　THE　GROUND　MELTS

　


　

Midway　between　Thera　and　Therasia；　fires　broke　forth　from　the　sea　and　continued　for　four　days；　so　that　the　whole　sea　boiled　and　blazed；　and　the　fires　cast　up　an　island　which　was　gradually　elevated　as　though　by　levers　。　。　。　After　the　cessation　of　the　eruption；　the　Rhodians；　at　the　time　of　their　maritime　supremacy；　were　first　to　venture　upon　the　scene　and　to　erect　on　the　island　a　temple。

—STRABO；　GEOGRAPHY　（CA。　7　B。C）

All　over　the　Earth；　you　can　find　a　kind　of　mountain　with　one　striking　and　unusual　feature。　Any　child　can　recognize　it：　The　top　seems　sheared　or　squared　off：　If　you　climb　to　the　summit　or　fly　over　it；　you　discover　that　the　mountain　has　a　hole　or　crater　at　its　peak。　In　some　mountains　of　this　sort；　the　craters　are　small；　in　others；　they　are　almost　as　big　as　the　mountain　itself。　Occasionally；　the　craters　are　filled　with　water。　Sometimes　they're　filled　with　a　more　amazing　liquid：　You　tiptoe　10　the　edge；　and　see　vast；　glowing　lakes　of　yellow…red　liquid　and　fountains　of　fire。　These　holes　in　the　tops　of　mountains　are　called　calderas；　after　the　word　〃caldron；〃　and　the　mountains　on　which　they　sit　are　known；　of　course；　as　volcanos—after　Vulcan；　the　Roman　god　of　fire。　There　are　perhaps　600　active　volcanos　discovered　on　Earth。　Some；　beneath　the　oceans；　are　yet　to　be　found。

A　typical　volcanic　mountain　looks　safe　enough。　Natural　vegetation　runs　up　its　sides。　Terraced　fields　decorate　its　flanks。　Hamlets　and　shrines　nestle　at　its　base。　And　yet；　without　warning；　after　centuries　of　lassitude；　the　mountain　may　explode。　Barrages　of　boulders；　torrents　of　ash　drop　out　of　the　sky。　Rivers　of　molten　rock　e　pouring　down　its　sides。　All　over　the　Earth　people　imagined　that　an　active　volcano　was　an　imprisoned　giant　or　demon　struggling　to　get　out。

The　eruptions　of　Mt。　St。　Helens　and　Mt。　Pinatubo　are　recent　reminders；　but　examples　can　be　found　throughout　history。　In　1902　a　hot；　glowing　volcanic　cloud　swept　down　the　slopes　of　Mt。　Pelee　and　killed　35；000　people　in　the　city　of　St。　Pierre　on　the　Caribbean　island　of　Martinique。　Massive　mudflows　from　the　eruption　of　the　Nevado　del　Ruiz　volcano　in　1985　killed　more　than　25；000　Colombians。　The　eruption　of　Mt。　Vesuvius　in　the　first　century　buried　in　ash　the　hapless　inhabitants　of　Pompeii　and　Herculaneum　and　killed　the　intrepid　naturalist　Plin