A Short History of Nearly Everything-第53章

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avored　their　wine　with　lead；　which　may　be　part　of　the　reasonthey　are　not　the　force　they　used　to　be。　as　we　have　seen　elsewhere；　our　own　performance　withlead　（not　to　mention　mercury；　cadmium；　and　all　the　other　industrial　pollutants　with　which　weroutinely　dose　ourselves）　does　not　leave　us　a　great　deal　of　room　for　smirking。　when　elementsdon’t　occur　naturally　on　earth；　we　have　evolved　no　tolerance　for　them；　and　so　they　tend　to　beextremely　toxic　to　us；　as　with　plutonium。　our　tolerance　for　plutonium　is　zero：　there　is　no　levelat　which　it　is　not　going　to　make　you　want　to　lie　down。

i　have　brought　you　a　long　way　to　make　a　small　point：　a　big　part　of　the　reason　that　earthseems　so　miraculously　acmodating　is　that　we　evolved　to　suit　its　conditions。　what　wemarvel　at　is　not　that　it　is　suitable　to　life　but　that　it　is　suitable　to　our　life—and　hardlysurprising；　really。　it　may　be　that　many　of　the　things　that　make　it　so　splendid　to　us—well…proportioned　sun；　doting　moon；　sociable　carbon；　more　magma　than　you　can　shake　a　stick　at；and　all　the　rest—seem　splendid　simply　because　they　are　what　we　were　born　to　count　on。　noone　can　altogether　say。

other　worlds　may　harbor　beings　thankful　for　their　silvery　lakes　of　mercury　and　driftingclouds　of　ammonia。　they　may　be　delighted　that　their　planet　doesn’t　shake　them　silly　with　itsgrinding　plates　or　spew　messy　gobs　of　lava　over　the　landscape；　but　rather　exists　in　apermanent　nontectonic　tranquility。　any　visitors　to　earth　from　afar　would　almost　certainly；　atthe　very　least；　be　bemused　to　find　us　living　in　an　atmosphere　posed　of　nitrogen；　a　gassulkily　disinclined　to　react　with　anything；　and　oxygen；　which　is　so　partial　to　bustion　thatwe　must　place　fire　stations　throughout　our　cities　to　protect　ourselves　from　its　livelier　effects。

but　even　if　our　visitors　were　oxygen…breathing　bipeds　with　shopping　malls　and　a　fondness　for3oxygen　itself　is　not　bustible；　it　merely　facilitates　the　bus　tion　of　other　things。　this　is　just　as　well；　for　ifoxygen　were　corn　bustible；　each　time　you　lit　a　match　all　the　air　around　you　would　bur　into　flame。　hydrogen　gas；on　the　other　hand；　is　extremely　corn　bustible；　as　the　dirigible　hindenburg　demonstrated　on　may　6；　193　inlakehurst；　new　jersey；　when　its　hydrogen　fuel　burst　explosive）　into　flame；　killing　thirty…six　people。

action　movies；　it　is　unlikely　that　they　would　find　earth　ideal。　we　couldn’t　even　give　themlunch　because　all　our　foods　contain　traces　of　manganese；　selenium；　zinc；　and　other　elementalparticles　at　least　some　of　which　would　be　poisonous　to　them。　to　them　earth　might　not　seem　awondrously　congenial　place　at　all。

the　physicist　richard　feynman　used　to　make　a　joke　about　a　posteriori　conclusions；　as　theyare　called。　“you　know；　the　most　amazing　thing　happened　to　me　tonight；”　he　would　say。　“isaw　a　car　with　the　license　plate　arw　357。　can　you　imagine？　of　all　the　millions　of　licenseplates　in　the　state；　what　was　the　chance　that　i　would　see　that　particular　one　tonight？

amazing！”　his　point；　of　course；　was　that　it　is　easy　to　make　any　banal　situation　seemextraordinary　if　you　treat　it　as　fateful。

so　it　is　possible　that　the　events　and　conditions　that　led　to　the　rise　of　life　on　earth　are　notquite　as　extraordinary　as　we　like　to　think。　still；　they　were　extraordinary　enough；　and　one　thingis　certain：　they　will　have　to　do　until　we　find　some　better。

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17　　　INTO　THE　TROPOSPHERE


thank　goodness　for　the　atmosphere。　it　keeps　us　warm。　without　it；　earth　would　be　alifeless　ball　of　ice　with　an　average　temperature　of　minus　60　degrees　fahrenheit。　in　addition；the　atmosphere　absorbs　or　deflects　ining　swarms　of　cosmic　rays；　charged　particles；ultraviolet　rays；　and　the　like。　altogether；　the　gaseous　padding　of　the　atmosphere　is　equivalentto　a　fifteen…foot　thickness　of　protective　concrete；　and　without　it　these　invisible　visitors　fromspace　would　slice　through　us　like　tiny　daggers。　even　raindrops　would　pound　us　senseless　if　itweren’t　for　the　atmosphere’s　slowing　drag。

the　most　striking　thing　about　our　atmosphere　is　that　there　isn’t　very　much　of　it。　it　extendsupward　for　about　120　miles；　which　might　seem　reasonably　bounteous　when　viewed　fromground　level；　but　if　you　shrank　the　earth　to　the　size　of　a　standard　desktop　globe　it　would　onlybe　about　the　thickness　of　a　couple　of　coats　of　varnish。

for　scientific　convenience；　the　atmosphere　is　divided　into　four　unequal　layers：　troposphere；stratosphere；　mesosphere；　and　ionosphere　（now　often　called　the　thermosphere）。　thetroposphere　is　the　part　that’s　dear　to　us。　it　alone　contains　enough　warmth　and　oxygen　to　allowus　to　function；　though　even　it　swiftly　bees　uncongenial　to　life　as　you　climb　up　through　it。

from　ground　level　to　its　highest　point；　the　troposphere　（or　“turning　sphere”）　is　about　ten　milesthick　at　the　equator　and　no　more　than　six　or　seven　miles　high　in　the　temperate　latitudes　wheremost　of　us　live。　eighty　percent　of　the　atmosphere’s　mass；　virtually　all　the　water；　and　thusvirtually　all　the　weather　are　contained　within　this　thin　and　wispy　layer。　there　really　isn’tmuch　between　you　and　oblivion。

beyond　the　troposphere　is　the　stratosphere。　when　you　see　the　top　of　a　storm　cloudflattening　out　into　the　classic　anvil　shape；　you　are　looking　at　the　boundary　between　thetroposphere　and　stratosphere。　this　invisible　ceiling　is　known　as　the　tropopause　and　wasdiscovered　in　1902　by　a　frenchman　in　a　balloon；　léon…philippe　teisserenc　de　bort。　pause　inthis　sense　doesn’t　mean　to　stop　momentarily　but　to　cease　altogether；　it’s　from　the　same　greekroot　as　menopause。　even　at　its　greatest　extent；　the　tropopause　is　not　very　distant。　a　fastelevator　of　the　sort　used　in　modern　skyscrapers　could　get　you　there　in　about　twenty　minutes；though　you　would　be　well　advised　not　to　make　the　trip。　such　a　rapid　ascent　withoutpressurization　would；　at　the　very　least；　result　in　severe　cerebral　and　pulmonary　edemas；　adangerous　excess　of　fluids　in　the　body’s　tissues。　when　the　doors　opened　at　the　viewingplatform；　anyone　inside　would　almost　certainly　be　dead　or　dying。　even　a　more　measuredascent　would　be　acpanied　by　a　great　deal　of　disfort。　the　temperature　six　miles　up　canbe　…70　degrees　fahrenheit；　and　you　would　need；　or　at　least　very　much　appreciate；supplementary　oxygen。

after　you　have　left　the　troposphere　the　temperature　soon　warms　up　again；　to　about　40degrees　fahrenheit；　thanks　to　the　absorptive　effects　of　ozone　（something　else　de　bortdiscovered　on　his　daring　1902　ascent）。　it　then　plunges　to　as　low　as　…130　degrees　fahrenheit　inthe　mesosphere　before　skyrocketing　to　2；700　degrees　fahrenheit　or　more　in　the　aptly　namedbut　very　erratic　thermosphere；　where　temperatures　can　vary　by　a　thousand　degrees　from　dayto　night—though　it　must　be　said　that　“temperature”　at　such　a　height　bees　a　somewhatnotional　concept。　temperature　is　really　just　a　measure　of　the　activity　of　molecules。　at　sealevel；　air　molecules　are　so　thick　that　one　molecule　can　move　only　the　tiniest　distance—aboutthree…millionths　of　an　inch；　to　be　precise—before　banging　into　another。　because　trillions　ofmolecules　are　constantly　colliding；　a　lot　of　heat　gets　exchanged。　but　at　the　height　of　thethermosphere；　at　fifty　miles　or　more；　the　air　is　so　thin　that　any　two　molecules　will　be　milesapart　and　hardly　ever　e　in　contact。　so　although　each　molecule　is　very　warm；　there　are　fewinteractions　between　them　and　thus　little　heat　transference。　this　is　good　news　for　satellitesand　spaceships　because　if　the　exchange　of　heat　were　more　efficient　any　man…made　objectorbiting　at　that　level　would　burst　into　flame。

even　so；　spaceships　have　to　take　care　in　the　outer　atmosphere；　particularly　on　return　trips　toearth；　as　the　space　shuttle　columbia　demonstrated　all　too　tragically　in　february　2003。

although　the　atmosphere　is　very　thin；　if　a　craft　es　in　at　too　steep　an　angle—more　thanabout　6　degrees—or　too　swiftly　it　can　strike　enough　molecules　to　generate　drag　of　anexceedingly　bustible　nature。　conversely；　if　an　ining　vehicle　hit　the　thermosphere　attoo　shallow　an　angle；　it　could　well　bounce　back　into　space；　like　a　pebble　skipped　across　water。

but　you　needn’t　venture　to　the　edge　of　the　atmosphere　to　be　reminded　of　what　hopelesslyground…hugging　beings　we　are。　as　anyone　who　has　spent　time　in　a　lofty　city　will　know；　youdon’t　have　to　rise　too　many　thousands　of　feet　from　sea　level　before　your　body　begins　toprotest。　even　experienced　mountaineers；　with　the　benefits　of　fitness；　training；　and　bottledoxygen；　quickly　bee　vulnerable　at　height　to　confusion；　nausea；　exhaustion；　frostbite；hypothermia；　migraine；　loss　of　appetite；　and　a　great　many　other　stumbling　dysfunctions。　in　ahundred　emphatic　ways　the　human　body　reminds　its　owner　that　it　wasn’t　designed　to　operateso　far　above　sea　level。

“even　under　the　most　favorable　circumstances；”　the　climber　peter　habeler　has　written　ofconditions　atop　everest；　“every　step　at　that　altitude　demands　a　colossal　effort　of　will。　youmust　force　yourself　to　make　every　movement；　reach　for　every　handhold。　you　are　perpetuallythreatened　by　a　leaden；　deadly　fatigue。”　in　the　other　side　of　everest；　the　british　mountaineerand　filmmaker　matt　dickinson　records　how　howard　somervell；　on　a　1924　british　expeditionup　everest；　“found　himself　choking　to　death　after　a　piece　of　infected　flesh　came　loose　andblocked　his　windpipe。”　with　a