pale blue dot -carl sagan-第19部分

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ed　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　predict　future　climate　change。　Volcanic　particles　injected　into　the　upper　air　are　also　an　additional　cause　of　thinning　of　the　ozone　layer。

*　Even　with　its　mountains　and　submarine　trenches；　our　planet　is　astonishingly　smooth。　If　the　Earth　were　the　size　of　a　billiard　ball；　the　largest　protuberances　would　be　less　than　a　tenth　of　a　millimeter　in　size—on　the　threshold　of　being　too　small　to　see　or　feel。

So　a　large　volcanic　explosion　in　some　unfrequented　and　obscure　part　of　the　world　can　alter　the　environment　on　a　global　scale。　Both　in　their　origins　and　in　their　effects；　volcanos　remind　us　of　how　vulnerable　we　are　to　minor　burps　and　sneezes　in　the　Earth's　internal　metabolism；　and　how　important　it　is　for　us　to　understand　how　this　subterranean　heat　engine　works。



IN　THE　FINAL　STAGES　of　formation　of　the　Earth—as　well　as　the　Moon；　Mars；　and　Venus—　impacts　by　small　worlds　are　thought　to　have　generated　global　magma　oceans。　Molten　rock　flooded　the　pre…existing　topography。　Great　floods；　tidal　waves　kilometers　high；　of　flowing；　red…hot　liquid　magma　welled　up　from　the　interior　and　poured　over　the　surface　of　the　planet；　burying　everything　in　their　path：　mountains；　channels；　craters；　perhaps　even　the　last　evidence　of　much　earlier；　more　clement　times。　The　geological　odometer　was　reset。　All　accessible　records　of　surface　geology　begin　with　the　last　global　magma　flood。　Before　the；　cool　and　solidify；　oceans　of　lava　may　be　hundreds　or　even　thousands　of　kilometers　thick。　In　our　time；　billions　of　years　later；　the　surface　of　such　a　world　may　be　quiet；　inactive；　with　no　hint　of　current　vulcanism。　Or　there　may　be—as　on　Earth—a　few　small…scale　but　active　reminders　of　an　epoch　when　the　entire　surface　was　flooded　with　liquid　rock。

In　the　early　days　of　planetary　geology；　ground…based　telescopic　observations　were　all　the　data　we　had。　A　fervent　debate　had　been　running　for　half　a　century　on　whether　the　craters　of　the　Moon　were　due　to　impacts　or　volcanos。　A　few　low　mounds　with　summit　calderas　were　found—almost　certainly　lunar　volcanos。　But　the　big　craters—bowl　or　pan…shaped　and　sitting　on　the　flat　ground　and　not　the　tops　of　mountains—were　a　different　story。　Some　geologists　saw　in　them　similarities　with　certain　highly　eroded　volcanos　on　Earth。　Others　did　not。　The　best　counter…argument　was　that　we　know　there　are　asteroids　and　ets　that　fly　past　the　Moon；　they　must　hit　it　sometimes；　and　the　collisions　must　make　craters。　Over　the　history　of　the　Moon　a　large　number　of　such　craters　should　have　been　punched　out。　So　if　the　craters　we　see　are　not　due　to　impacts；　where　then　are　the　impact　craters？　We　now　know　from　direct　laboratory　examination　of　lunar　craters　that　they　are　almost　entirely　of　impact　origin。　But　4　billion　years　ago　this　little　world；　nearly　dead　today；　was　bubbling　and　churning　away；　driven　by　primeval　vulcanism　from　sources　of　internal　heat　now　long　gone。

In　November　1971；　NASA's　Mariner　9　spacecraft　arrived　at　Mars　to　find　the　planet　pletely　obscured　by　a　global　dust　storm。　Almost　the　only　features　to　be　seen　were　four　circular　spots　rising　out　of　the　reddish　murk。　But　there　was　something　peculiar　about　them：　They　had　holes　in　their　tops。　As　the　storm　cleared；　we　were　able　to　see　unmistakably　that　we　had　been　viewing　four　huge　volcanic　mountains　penetrating　through　the　dust　cloud；　each　capped　by　a　great　summit　caldera。

After　the　storm　dissipated；　the　true　scale　of　these　volcanos　became　clear。　The　largest—appropriately　named　Olympus　Mons；　or　Mt。　Olympus；　after　the　home　of　the　Greek　gods—is　more　than　25　kilometers　（roughly　15　miles）　high；　dwarfing　not　only　the　largest　volcano　on　Earth　but　also　the　largest　mountain　of　any　sort；　Mt。　Everest；　which　stands　9　kilometers　above　the　Tibetan　plateau。　There　are　some　20　large　volcanos　on　Mars；　but　none　so　massive　as　Olympus　Mons；　which　has　a　volume　about　100　times　that　of　the　largest　volcano　on　Earth；　Mauna　Loa　in　Hawaii。

By　counting　the　accumulated　impact　craters　（made　by　small　impacting　asteroids；　and　readily　distinguished　from　summit　calderas）　on　the　flanks　of　the　volcanos；　estimates　of　their　ages　can　be　derived。　Some　Martian　volcanos　turn　out　to　be　a　few　billion　years　old；　although　none　dates　back　to　the　very　origin　of　Mars；　about　4。5　billion　years　ago。　Some；　including　Olympus　Mons；　are　paratively　new—perhaps　only　a　few　hundred　million　years　old。　It　is　clear　that　enormous　volcanic　explosions　occurred　early　in　Martian　history；　perhaps　providing　an　atmosphere　much　denser　than　the　one　Mars　holds　today。　What　would　the　place　have　looked　like　if　we　had　visited　it　then？

Some　volcanic　flows　on　Mars　（for　example；　in　Cerberus）　formed　as　recently　as　200　million　years　ago。　It　is；　I　suppose；　even　possible—although　there　is　no　evidence　either　way—that　Olympus　Mons；　the　largest　volcano　we　know　about　for　certain　in　the　Solar　System；　will　be　active　again。　Volcanologists；　a　patient　sort；　would　doubtless　wele　the　event。

In　1990…93　the　Magellan　spacecraft　returned　surprising　radar　data　about　the　landforms　of　Venus。　Cartographers　prepared　maps　of　almost　the　entire　planet；　with　fine　detail　down　to　about　100　meters；　the　goal…line…to…goal…line　distance　in　an　American　football　stadium。　More　data　were　radioed　home　by　Magellan　than　by　all　other　planetary　missions　bined。　Since　much　of　the　ocean　floor　remains　unexplored　（except　perhaps　for　still…classified　data　acquired　by　the　U。S。　and　Soviet　navies）；　we　may　know　more　about　the　surface　topography　of　Venus　than　about　any　other　planet；　Earth　included。　Much　of　the　geology　of　Venus　is　unlike　anything　seen　on　Earth　or　anywhere　else。　Planetary　geologists　have　given　these　landforms　names；　but　that　doesn't　mean　we　fully　understand　how　they're　formed。

Because　the　surface　temperature　of　Venus　is　almost　470°C　（900°F）；　the　rocks　there　are　much　closer　to　their　melting　points　than　are　those　at　the　surface　of　the　Earth。　Rocks　begin　to　soften　and　flow　at　much　shallower　depths　on　Venus　than　on　Earth。　This　is　very　likely　the　reason　that　many　geological　features　on　Venus　seem　to　be　plastic　and　deformed。

The　planet　is　covered　by　volcanic　plains　and　highland　plateaus。　The　geological　constructs　include　volcanic　cones；　probable　shield　volcanos；　and　calderas。　There　are　many　places　where　we　can　see　that　lava　has　erupted　in　vast　floods。　Some　plains　features　ranging　to　over　200　kilometers　in　size　are　playfully　called　〃ticks〃　and　〃arachnoids〃　（which　translates　roughly　as　〃spiderlike　things〃）—because　they　are　circular　depressions　surrounded　by　concentric　rings；　while　long；　spindly　surface　cracks　extend　radially　out　from　the　center。　Odd；　flat　〃pancake　domes〃—a　geological　feature　unknown　on　Earth；　but　probably　a　kind　of　volcano—are　perhaps　formed　by　thick；　viscous　lava　slowly　flowing　uniformly　in　all　directions。　There　are　many　examples　of　more　irregular　lava　flows。　Curious　ring　structures　called　〃coronae〃　range　up　to　some　2；000　kilometers　across。　The　distinctive　lava　flows　on　stifling　hot　Venus　offer　up　a　rich　menu　of　geological　mysteries。

The　most　unexpected　and　peculiar　features　are　the　sinuous　channels—with　meanders　and　oxbows；　looking　just　like　river　valleys　on　Earth。　The　longest　are　longer　than　the　greatest　rivers　on　Earth。　But　it　is　much　too　hot　for　liquid　water　on　Venus。　And　we　can　tell　from　the　absence　of　small　impact　craters　that　the　atmosphere　has　been　this　thick；　driving　as　great　a　greenhouse　effect；　for　as　long　as　the　present　surface　has　been　in　existence。　（If　it　had　been　much　thinner；　intermediate…sized　asteroids　would　not　have　burned　up　on　entry　into　the　atmosphere；　but　would　have　survived　to　excavate　craters　as　they　impact　this　planet's　surface。）　Lava　flowing　downhill　does　make　sinuous　channels　（sometimes　under　the　ground；　followed　by　collapse　of　the　roof　of　the　channel）。　But　even　at　the　temperatures　of　Venus；　the　lavas　radiate　heat；　cool；　slow；　congeal；　and　stop。　The　magma　freezes　solid。　Lava　channels　cannot　go　even　10　percent　of　the　length　of　the　long　Venus　channels　before　they　solidify。　Some　planetary　geologists　think　there　must　be　a　special　thin；　watery；　inviscid　lava　generated　on　Venus。　But　this　is　a　speculation　supported　by　no　other　data；　and　a　confession　of　our　ignorance。

The　thick　atmosphere　moves　sluggishly；　because　it's　so　dense；　though；　it's　very　good　at　lifting　and　moving　fine　particles。　There　are　wind　streaks　on　Venus；　largely　emanating　from　impact　craters；　in　which　the　prevailing　winds　have　scoured　piles　of　sand　and　dust　and　provided　a　sort　of　weather　vane　imprinted　on　the　surface。　Here　and　there　we　seem　to　see　fields　of　sand　dunes；　and　provinces　where　wind　erosion　has　sculpted　volcanic　landforms。　These　aeolian　processes　take　place　in　slow　motion；　as　if　at　the　bottom　of　the　sea。　The　winds　are　feeble　at　the　surface　of　Venus。　It　may　take　only　a　soft　gust　to　raise　a　cloud　of　fine　particles；　but　in　that　stifling　inferno　a　gust　is　hard　to　e　by。

There　are　many　impact　craters　on　Venus；　but　nothing　like　the　number　on　the　Moon　or　Mars。　Craters　smaller　than　a　few　across　are　oddly　missing。　The　reason　is　understood：　Small　asteroids　and　ets　are　broken　up　on　entry　into　the　dense　Venus　atmosphere　before　they　can　hit　the　surface。　The　observed　cutoff　in　crater　size　corresponds　very　well　to　the　present　density　of　the　atmosphere　of　Venus。　Certain　irregular　splotches　seen　on　the　Magellan　images　are　thought　to　be　the　remains　of　impactors　that　broke　up　in　the　thick　air　before　they　could　gouge　out　a　crater。

Most　of　the　impact　craters　are　remarkably　pristine　and　well　preserved；　only　a　few　percent　of　them　have　been　engulfed　by　subsequent　lava　flows。　The　surface　of　Venus　as　revealed　by　Magellan　is　very　young。　There　are　so　few　impact　craters　that　everything　older　than　about　500　million　years*　must　have　been　eradicated—on　a　planet　almost　certainly　4。5　billion　years　old。　There　is　only　one　plausible　erosive　agent　adequate　for　what　we　see：　vulcanism。　All　over　the　planet　craters；　mountains；　and　other　geological　features　have　been　inundated　by　seas　of　lava　that　once　welled　up　from　the　inside；　flowed　far；　and　froze。

*　The　age　of　the　Venus　surface；　as　determined　by　Magellan　radar　imagery；　puts　an　additional　nail　in　the　coffin　of　the　thesis　of　Immanuel　Velikovsky—who　around　1950　proposed；　to　surprising　media　acclaim；　that　3；500　years　ago　Jupiter　spat　ou