pale blue dot -carl sagan- 第25部分

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　Pliny　the　Elder　as　he　made　his　way　up　the　side　of　the　volcano；　intent　on　arriving　at　a　better　understanding　of　its　workings。　（Pliny　was　hardly　the　last：　Fifteen　volcanologists　have　been　killed　in　sundry　volcanic　eruptions　between　1979　and　1993。）　The　Mediterranean　island　of　Santorin　（also　called　Thera）　is　in　reality　the　only　part　above　water　of　the　rim　of　a　volcano　now　inundated　by　the　sea。*　The　explosion　of　the　Santorin　volcano　in　1623　B。C。　may；　some　historians　think；　have　helped　destroy　the　great　Minoan　civilization　on　the　nearby　island　of　Crete　and　changed　the　balance　of　power　in　early　classical　civilization。　This　disaster　may　be　the　origin　of　the　Atlantis　legend　as　related　by　Plato；　in　which　a　civilization　was　destroyed　〃in　a　single　day　and　night　of　misfortune。〃　It　must　have　been　easy　back　then　to　think　that　a　god　was　angry。

*　The　eruption　of　a　nearby　submarine　volcano　and　the　rapid　construction　Of'　new　island　in　197　B。C。　are　described　by　Strabo　in　the　epigraph　to　this　chapter。

Volcanos　have　naturally　been　regarded　with　fear　and　awe。　When　medieval　Christians　viewed　the　eruption　of　Mt。　Hekla　in　Iceland　and　saw　churning　fragments　of　soft　lava　suspended　over　the　summit；　they　imagined　they　were　seeing　the　souls　of　the　damned　awaiting　entrance　to　Hell。　〃Fearful　howlings；　weeping　and　gnashing　of　teeth；〃　〃melancholy　cries　and　loud　wailings〃

were　dutifully　reported。　The　glowing　red　lakes　and　sulfurous　gases　within　the　Hekla　caldera　were　thought　to　be　a　real　glimpse　into　the　underworld　and　a　confirmation　of　folk　beliefs　in　Hell　sand；　by　symmetry；　in　its　partner；　Heaven）。

A　volcano　is；　in　fact；　an　aperture　to　an　underground　realm　much　vaster　than　the　thin　surface　layer　that　humans　inhabit；　and　far　more　hostile。　The　lava　that　erupts　from　a　volcano　is　liquid　rock—rock　raised　to　its　melting　point；　generally　around　1000°C。　The　lava　emerges　from　a　hole　in　the　Earth；　as　it　cools　and　solidifies；　it　generates　and　later　remakes　the　flanks　of　a　volcanic　mountain。

The　most　volcanically　active　locales　on　Earth　tend　to　be　along　ridges　on　the　ocean　floor　and　island　arcs—at　the　junction　of　two　great　plates　of　oceanic　crust—either　separating　from　each　other；　or　one　slipping　under　the　other。　On　the　seafloor　there　are　long　zones　of　volcanic　eruptions—acpanied　by　swarms　of　earthquakes　and　plumes　of　abyssal　smoke　and　hot　water—that　we　a　are　just　beginning　to　observe　with　robot　and　manned　submersible　vehicles。

Eruptions　of　lava　must　mean　that　the　Earth's　interior　is　extremely　hot。　Indeed；　seismic　evidence　shows　that；　only　a　few　hundred　kilometers　beneath　the　surface；　nearly　the　entire　body　of　the　Earth　is　at　least　slightly　molten。　The　interior　of　the　Earth　is　hot；　in　part；　because　radioactive　elements　there；　such　as　uranium；　give　off　heat　as　they　decay；　and　in　part　because　the　Earth　retains　some　of　the　original　heat　released　in　its　formation；　when　many　small　worlds　fell　together　by　their　mutual　gravity　to　make　the　Earth；　and　when　iron　drifted　down　to　form　our　planet's　core。

The　molten　rock；　or　magma；　rises　through　fissures　in　the　surrounding　heavier　solid　rocks。　We　can　imagine　vast　subterranean　caverns　filled　with　glowing；　red；　bubbling；　viscous　liquids　that　shoot　up　toward　the　surface　if　a　suitable　channel　is　by　chance　provided。　The　magma；　called　lava　as　it　pours　out　of　the　summit　caldera；　does　indeed　arise　from　the　underworld。　The　souls　of　the　damned　have　so　far　eluded　detection。

Once　the　volcano　is　fully　built　from　successive　outpourings；　and　the　lava　is　no　longer　spewing　up　into　the　caldera；　then　it　bees　just　like　any　other　mountain—slowly　eroding　because　of　rainfall　and　windblown　debris　and；　eventually；　the　movement　of　continental　plates　across　the　Earth's　surface。　〃How　many　years　can　a　mountain　exist　before　it　is　washed　to　the　sea？〃　asked　Bob　Dylan　in　the　ballad　〃Blowing　in　the　Wind。〃　The　answer　depends　on　which　planet　we're　talking　about。　For　the　Earth；　it's　typically　about　ten　million　years。　So　mountains；　volcanic　and　otherwise；　must　be　built　on　the　same　timescale；　otherwise　the　Earth　would　be　everywhere　smooth　as　Kansas。*　Volcanic　explosions　can　punch　vast　quantities　of　matter—mainly　fine　droplets　of　sulfuric　acid—into　the　stratosphere。　There；　for　a　year　or　two；　they　reflect　sunlight　back　to　space　and　cool　the　Earth。　This　happened　recently　with　the　Philippine　volcano；　Mt。　Pinatubo；　and　disastrously　in　1815…16　after　the　eruption　of　the　Indonesian　volcano　Mt。　Tambora；　which　resulted　in　the　famine…ridden　〃year　without　a　summer。〃　A　volcanic　eruption　in　Taupo；　New　Zealand；　in　the　year　177　cooled　the　climate　of　the　Mediterranean；　half　a　world　away；　and　dropped　fine　particles　onto　the　Greenland　ice　cap。　The　explosion　of　Mt。　Mazama　in　Oregon　（which　left　the　caldera　now　called　Crater　Lake）　in　4803　B。C。　had　climatic　consequences　throughout　the　northern　hemisphere。　Studies　of　volcanic　effects　on　the　climate　were　on　the　investigative　path　that　eventually　led　to　the　discovery　of　nuclear　winter。　They　provide　important　tests　of　our　use　of　puter　models　to　predi