pale blue dot -carl sagan-第15章

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stion　is　bizarre；　though；　at　least　as　far　as　this　solar　system　is　concerned；　because　surface　oceans　of　liquid　water　exist　nowhere　else。　When　you　look　in　the　visible　and　near…infrared　spectrum　for　telltale　signatures　of　chemical　position；　sure　enough　you　discover　water　ice　in　the　polar　caps；　and　enough　water　vapor　in　the　air　to　account　for　the　clouds；　this　is　also　just　the　right　amount　that　must　exist　because　of　evaporation　if　the　oceans　are　in　fact　made　of　liquid　water。　The　bizarre　hypothesis　is　confirmed。

The　spectrometers　further　reveal　that　the　air　on　this　world　is　one　fifth　oxygen；　O2。　No　other　planet　in　the　Solar　System　has　anything　close　to　so　much　oxygen。　Where　does　it　all　e　from？　The　intense　ultraviolet　light　from　the　Sun　breaks　water；　H2O；　down　into　oxygen　and　hydrogen；　and　hydrogen；　the　lightest　gas；　quickly　escapes　to　space。　This　is　a　source　of　O2；　certainly；　but　it　doesn't　easily　account　for　so　much　oxygen。

Another　possibility　is　that　ordinary　visible　light；　which　the　Sun　pours　out　in　vast　amounts；　is　used　on　Earth　to　break　water　apart—except　that　there's　no　known　way　to　do　this　without　life。　There　would　have　to　be　plants；　life…forms　colored　by　a　pigment　that　strongly　absorbs　visible　light；　that　knows　how　to　split　a　water　molecule　by　saving　up　the　energy　of　two　photons　of　light；　that　retains　the　H　and　excretes　the　O；　and　that　uses　the　hydrogen　thus　liberated　to　synthesize　organic　molecules。　The　plants　would　have　to　be　spread　over　much　of　the　planet。　All　this　is　asking　a　lot。　If　you're　a　good　skeptical　scientist；　so　much　O2　would　not　be　proof　of　life。　But　it　certainly　might　be　cause　for　suspicion。

With　all　that　oxygen　you're　not　surprised　to　discover　ozone　（O3）　in　the　atmosphere；　because　ultraviolet　light　makes　ozone　out　of　molecular　oxygen　（O2）。　The　ozone　then　absorbs　dangerous　ultraviolet　radiation。　So　if　the　oxygen　is　due　to　life；　there's　a　curious　sense　in　which　the　life　is　protecting　itself。　But　this　life　〃night　be　mere　photosynthetic　plants。　A　high　level　of　intelligence　is　not　implied。

When　you　examine　the　continents　more　closely；　you　find　there　are；　crudely　speaking；　two　kinds　of　regions。　One　shows　the　spectrum　of　ordinary　rocks　and　minerals　as　found　on　many　worlds。　The　other　reveals　something　unusual：　a　material；　covering　vast　areas；　that　strongly　absorbs　red　light。　（The　Sun；　of　course；　shines　in　light　of　all　colors；　with　a　peak　in　the　yellow。）　This　pigment　might　be　just　the　agent　needed　if　ordinary　visible　light　is　being　used　to　break　water　apart　and　account　for　the　oxygen　in　the　air。　It's　another　hint；　this　time　a　little　stronger；　of　life；　not　a　bug　here　and　there；　but　a　planetary　surface　overflowing　with　life。　The　pigment　is　in　fact　chlorophyll：　It　absorbs　blue　light　as　well　as　red；　and　is　responsible　for　the　fact　that　plants　are　green。　What　you're　seeing　is　a　densely　vegetated　planet。

So　the　Earth　is　revealed　to　possess　three　properties　unique　at　least　in　this　solar　system—oceans；　oxygen；　life。　It's　hard　not　to　think　they're　related；　the　oceans　being　the　sites　of　origin；　and　the　oxygen　the　product；　of　abundant　life。

When　you　look　carefully　at　the　infrared　spectrum　of　the　Earth；　you　discover　the　minor　constituents　of　the　air。　In　addition　to　water　vapor；　there's　carbon　dioxide　（CO2）；　methane　（CH4）；　and　other　gases　that　absorb　the　heat　that　the　Earth　tries　to　radiate　away　to　space　at　night。　These　gases　warm　the　planet。　Without　them；　the　Earth　would　everywhere　be　below　the　freezing　point　of　water。　You've　discovered　this　world's　greenhouse　effect。

Methane　and　oxygen　together　in　the　same　atmosphere　is　peculiar。　The　laws　of　chemistry　are　very　clear：　In　an　excess　of　O2；　CH4　should　be　entirely　converted　into　H2O　and　CO2；　The　process　is　so　efficient　that　not　a　single　molecule　in　all　the　Earth's　atmosphere　should　be　methane。　Instead；　you　find　that　one　out　of　every　million　molecules　is　methane；　ail　immense　discrepancy。　What　could　it　mean？

The　only　possible　explanation　is　that　methane　is　being　injected　into　the　Earth's　atmosphere　so　quickly　that　its　chemical　reaction　with　Oz　can't　keep　pace。　Where　does　all　this　methane　e　from？　Maybe　it　seeps　out　of　the　deep　interior　of　the　Earth—but　quantitatively　this　doesn't　seem　to　work；　and　Mars　and　Venus　don't　have　anything　like　this　much　methane。　The　only　alternatives　are　biological；　a　conclusion　that　makes　no　assumptions　about　the　chemistry　of　life；　or　what　it　looks　like；　but　follows　merely　from　how　unstable　methane　is　in　an　oxygen　atmosphere。　In　fact；　the　methane　arises　from　such　sources　as　bacteria　in　bogs；　the　cultivation　of　rice；　the　burning　of　vegetation；　natural　gas　from　oil　wells；　and　bovine　flatulence。　In　an　oxygen　atmosphere；　methane　is　a　sign　of　life。

That　the　intimate　intestinal　activities　of　cows　should　be　detectable　from　interplanetary　space　is　a　little　disconcerting；　especially　when　so　much　of　what　we　hold　dear　is　not。　But　an　alien　scientist　flying　by　the　Earth　would；　at　this　point；　be　unable　to　deduce　bogs；　rice；　fire；　oil；　or　cows。　Just　life。

All　the　signs　of　life　that　we've　discussed　so　far　are　due　to　paratively　simple　forms　（the　methane　in　the　rumens　of　cows　is　generated　by　bacteria　that　homestead　there）。　Had　your　spacecraft　flown　by　the　Earth　a　hundred　million　years　ago；　in　the　age　of　the　dinosaurs　when　there　were　no　humans　and　no　technology；　you　would　still　have　seen　oxygen　and　ozone；　they　chlorophyll　pigment；　and　far　too　much　methane。　At　present；　though；　your　instruments　are　finding　signs　not　just　of　life；　but　of　high　technology—something　that　couldn't　possibly　have　been　detected　even　a　hundred　years　ago：

You　are　detecting　a　particular　kind　of　radio　wave　emanating　from　Earth。　Radio　waves　don't　necessarily　signify　life　and　intelligence　Many　natural　processes　generate　them。　You've　already　found　radio　emissions　from　other；　apparently　uninhabited　worlds—generated　by　electrons　trapped　in　the　strong　magnetic　fields　of　planets；　by　chaotic　motions　at　the　shock　front　that　separates　these　magnetic　fields　from　the　interplanetary　magnetic　field；　and　by　lightning。　（Radio　〃whistlers〃　usually　sweep　from　high　notes　to　low；　and　then　begin　again。）　Some　of　these　radio　emissions　are　continuous；　some　e　in　repetitive　bursts；　some　last　a　few　minutes　and　then　disappear。

But　this　is　different：　A　portion　of　the　radio　transmission　from　Earth　is　at　just　the　frequencies　where　radio　waves　begin　to　leak　out　of　the　planet's　ionosphere；　the　electrically　charged　region　above　the　stratosphere　that　reflects　and　absorbs　radio　waves。　There　is　a　constant　central　frequency　for　each　transmission；　added　to　which　is　a　modulated　signal　（a　plex　sequence　of　ons　and　offs）。　No　electrons　in　magnetic　fields；　no　shock　waves；　no　lightning　discharges　can　generate　something　like　this。　Intelligent　life　seems　to　be　the　only　possible　explanation。　Your　conclusion　that　the　radio　transmission　is　due　to　technology　on　Earth　holds　no　matter　what　the　ons　and　offs　mean：　You　don't　have　to　decode　the　message　to　be　sure　it　is　a　message。　（This　signal　is　really；　let　us　suppose；　munications　from　the　U。S。　Navy　to　its　distant　nuclear…armed　submarines。）

So；　as　an　alien　explorer；　you　would　know　that　at　least　one　species　on　Earth　has　achieved　radio　technology。　Which　one　is　it？　The　beings　that　make　methane？　Those　that　generate　oxygen？　The　ones　whose　pigment　colors　the　landscape　green？　Or　somebody　else；　somebody　more　subtle；　someone　not　otherwise　detectable　to　a　spacecraft　plummeting　by？　To　search　for　this　technological　species；　you　might　want　to　examine　the　Earth　at　finer　and　finer　resolution—seeking；　if　not　the　beings　themselves；　at　least　their　artifacts。

You　look　first　with　a　modest　telescope；　so　the　finest　detail　you　can　resolve　is　one　or　two　kilometers　across。　You　can　make　out　no　monumental　architecture；　no　strange　formations；　no　unnatural　reworking　of　the　landscape；　no　signs　of　life。　You　see　a　dense　atmosphere　in　motion。　The　abundant　water　must　evaporate　and　then　rain　back　down。　Ancient　impact　craters；　apparent　on　the　Earth's　nearby　Moon；　are　almost　wholly　absent。　There　must；　then；　be　a　set　of　processes　whereby　new　land　is　created　and　then　eroded　away　in　much　less　time　than　the　age　of　this　world。　Running　water　is　implicated。　As　you　look　with　finer　and　finer　definition　you　find　mountain　r