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　world。　Running　water　is　implicated。　As　you　look　with　finer　and　finer　definition　you　find　mountain　ranges；　river　valleys；　and　many　other　indications　that　the　planet　is　geologically　active。　There　are　also　odd　places　surrounded　by　vegetation；　but　which　are　themselves　denuded　of　plants。　They　look　like　discolored　smudges　on　the　landscape。

When　you　examine　the　Earth　at　about　100…meter　resolution；　everything　changes。　The　planet　is　revealed　to　be　covered　with　straight　lines；　squares；　rectangles；　circles—sometimes　huddling　along　river　banks　or　nestling　on　the　lower　slopes　of　mountains；　sometimes　stretching　over　plains；　but　rarely　in　deserts　or　high　mountains；　and　absolutely　never　in　the　oceans。　Their　regularity；　plexity；　and　distribution　would　be　hard　to　explain　except　by　life　and　intelligence；　although　a　deeper　understanding　of　function　and　purpose　might　be　elusive。　Perhaps　you　would　conclude　only　that　the　dominant　life…forms　have　a　simultaneous　passion　for　territoriality　and　Euclidean　geometry。　At　this　resolution　you　could　not　see　them；　much　less　know　them。

Many　of　the　devegetated　smudges　are　revealed　to　have　an　underlying　checkerboard　geometry。　These　are　the　planets　cities。　Over　much　of　the　landscape；　and　not　just　in　the　cities；　there　is　a　profusion　of　straight　lines；　squares；　rectangles；　circles。　The　dark　smudges　of　the　cities　are　revealed　to　be　highly　geometrized；　with　only　a　few　patches　of　vegetation—themselves　with　highly　regular　boundaries—left　intact。　There　are　occasional　triangles；　and　in　one　city　there　is　even　a　pentagon。

When　you　take　pictures　at　a　meter　resolution　or　better；　you　find　that　the　crisscrossing　straight　lines　within　the　cities　and　the　long　straight　lines　that　join　them　with　other　cities　are　filled　with　streamlined；　multicolored　beings　a　few　meters　in　length；　politely　running　one　behind　the　other；　in　long；　slow　orderly　procession。　They　are　very　patient。　One　stream　of　beings　stops　so　another　stream　can　continue　at　right　angles。　Periodically；　the　favor　is　returned。　At　night；　they　turn　on　two　bright　lights　in　front　so　they　can　see　where　they're　going。　Some；　a　privileged　few；　go　into　little　houses　when　their　workday　is　done　and　retire　for　the　night。　Most　are　homeless　and　sleep　in　the　streets。

At　last！　You've　detected　the　source　of　all　the　technology。　the　dominant　life…forms　on　the　planet。　The　streets　of　the　cities　and　the　roadways　of　the　countryside　are　evidently　built　for　their　benefit。　You　might　believe　that　you　were　really　beginning　to　understand　life　on　Earth。　And　perhaps　you'd　be　right。

If　the　resolution　improved　just　a　little　further；　you'd　discover　tiny　parasites　that　occasionally　enter　and　exit　the　dominant　organisms。　They　play　some　deeper　role；　though；　because　a　stationary　dominant　organism　will　often　start　up　again　just　after　it's　reinfected　by　a　parasite；　and　stop　again　just　before　the　parasite　is　expelled。　This　is　puzzling。　But　no　one　said　life　on　Earth　would　be　easy　to　understand。

All　the　images　you've　taken　so　far　are　in　reflected　sunlight—that　is；　on　the　day　side　of　the　planet。　Something　most　interesting　is　revealed　when　you　photograph　the　Earth　at　light：　The　planet　is　lit　up。　The　brightest　region；　near　the　Arctic　Circle；　is　illuminated　by　the　aurora　borealis—generated　not　by　life　；　but　by　electrons　and　protons　from　the　Sun；　beamed　down　by　the　Earth's　magnetic　field。　Everything　else　you　see　is　due　to　life。　The　lights　recognizably　outline　the　same　continents　you　can　make　out　in　daytime；　and　many　correspond　to　cities　you've　already　mapped。　The　cities　are　concentrated　near　coastlines。　They　tend　to　be　sparser　in　continental　interiors。　Perhaps　the　dominant　organisms　are　desperate　for　seawater　（or　maybe　oceangoing　ships　were　once　essential　for　merce　and　emigration）。

Some　of　the　lights；　though；　are　not　due　to　cities。　In　North　Africa；　the　Middle　East；　and　Siberia；　for　example；　there　are　very　bright　lights　in　a　paratively　barren　landscape　due；　it　turns　out；　to　burnoff　in　oil　and　natural　gas　wells。　In　the　Sea　of　Japan　on　the　day　you　first　look；　there　is　a　strange；　triangular…shaped　area　of　light。　In　daytime　it　corresponds　to　open　ocean。　This　is　no　city。　What　could　it　be？　It　is　in　fact　the　Japanese　squid　fishing　fleet；　using　brilliant　illumination　to　attract　schools　of　squid　up　to　their　deaths。　On　other　days；　this　pattern　of　light　wanders　all　over　the　Pacific　Ocean；　seeking　prey。　In　effect；　what　you　have　discovered　here　is　sushi。

It　seems　sobering　to　me　that　from　space　you　can　so　readily　detect　some　of　the　odds　and　ends　of　life　on　Earth—the　gastrointestinal　habits　of　ruminants；　Japanese　cuisine；　the　means　of　municating　with　nomadic　submarines　that　carry　death　for　200　cities—while　so　much　of　our　monumental　architecture；　our　greatest　engineering　works；　our　efforts　to　care　for　one　another；　are　almost　wholly　invisible。　It's　a　kind　of　parable。



BY　THIS　POINT　your　expedition　to　the　Earth　must　be　considered　highly　successful。　You've　characterized　the　environment；　you've　detected　life；　you've　found　manifestations　of　intelligent　beings；　you　may　even　have　identified　the　dominant　species；　the　one　transfixed　with　geometry　and　rectilinearity。　Surely　this　planet　is　worth　a　longer　and　more　detailed　study。　That's　why　you've　now　inserted　your　spacecraft　into　orbit　around　the　Earth。

Looking　down　on　the　planet；　you　uncover　new　puzzles。　All　over　the　Earth；　smokestacks　are　pouring　carbon　dioxide　and　toxic　chemicals　into　the　air。　So　are　the　dominant　beings　who　run　on　the　roadways。　But　carbon　dioxide　is　a　greenhouse　gas。　As　you　watch；　the　amount　of　it　in　the　atmosphere　increases　steadily；　year　after　year。　The　same　is　true　of　methane　and　other　greenhouse　gases。　If　this　keeps　up；　the　temperature　of　the　planet　is　going　to　increase。　Spectroscopically；　you　discover　another　class　of　molecules　being　injected　into　the　air；　the　chlorofluorocarbons。　Not　only　are　they　greenhouse　gases；　but　they　are　also　devastatingly　effective　in　destroying　the　protective　ozone　layer。

You　look　more　closely　at　the　center　of　the　South　American　continent；　which—as　you　know　by　now—is　a　vast　rain　forest。　Every　night　you　see　thousands　of　fires。　In　the　daytime；　you　find　the　region　covered　with　smoke。　Over　the　years；　all　over　the　planet；　you　find　less　and　less　forest　and　more　and　more　scrub　desert。

You　look　down　on　the　large　island　of　Madagascar。　The　rivers　are　colored　brown；　generating　a　vast　stain　in　the　surrounding　ocean。　This　is　topsoil　being　washed　out　to　sea　at　a　rate　so　high　that　in　another　few　decades　there　will　be　none　left。　The　same　thing　is　happening；　you　note；　at　the　mouths　of　rivers　all　over　the　planet。

But　no　topsoil　means　no　agriculture。　In　another　century；　what　will　they　eat？　What　will　they　breathe？　How　will　they　cope　with　a　changing　and　more　dangerous　environment？

From　your　orbital　perspective；　you　can　see　that　something　has　unmistakably　gone　wrong。　The　dominant　organisms；　whoever　they　are—who　have　gone　to　so　much　trouble　to　rework　the　surface—are　simultaneously　destroying　their　ozone　layer　and　their　forests；　eroding　their　topsoil；　and　performing　massive；　uncontrolled　experiments　on　their　planet's　climate。　Haven't　they　noticed　what's　happening？　Are　they　oblivious　to　their　fate？　Are　they　unable　to　work　together　on　behalf　of　the　environment　that　sustains　them　all？

Perhaps；　you　think；　it's　time　to　reassess　the　conjecture　that　there's　intelligent　life　on　Earth。


　

LOOKING　FOR　LIFE　ELSE　WHERE　：　A　CALIBRATION　

Spacecraft　from　the　Earth　have　now　flown　by　dozens　of　planets；　moons；　ets；　and　asteroids—equipped　with　cameras；　instruments　for　measuring　heat　and　radio　waves；　spectrometers　to　determine　position；　and　a　host　of　other　devices。　We　have　found　not　a　hint　of　life　anywhere　else　in　the　Solar　System。　But　you　might　be　skeptical　about　our　ability　to　detect　life　elsewhere；　especially　life　different　from　the　kind　we　know。　Until　recently　we　had　never　performed　the　obvious　calibration　test：　to　fly　a　modern　interplanetary　spacecraft　by　the　Earth　and　see　whether　we　could　detect　ourselves。　This　all　changed　on　December　8；　1990。

Galileo　is　a　NASA　spacecraft　designed　to　explore　the　giant　planet　Jupiter；　its　moons；　and　its　rings。　It's　named　after　the　heroic　Italian　scientist　who　played　so　central　a　role　in　toppling　the　geocentric　pretension。　It　is　he　who　first　saw　Jupiter　as　a　world；　and　who　discovered　its　four　big　moons。　To　get　to　Jupiter；　the　spacecraft　had　to　fly　close　by　Venus　（once）　and　the　Earth　（twice）　and　be　accelerated　by　the　gravities　of　these　planets—otherwise　there　wasn't　enough　oomph　to　get　it　where　it　was　going。　This　necessity　of　trajectory　design　permitted　us；　for　the　first　time；　to　look　systematically　at　the　Earth　from　an　alien　perspective。

Galileo　passed　only　960　kilometers　（about　600　miles）　above　the　Earth's　surface。　With　some　exceptions—including　pictures　showing　features　finer　than　1　kilometer　across；　and　the　images　of　the　Earth　at　night—much　of　the　spacecraft　data　described　in　this　chapter　were　actually　obtained　by　Galileo。　With　Galileo　we　were　able　to　deduce　an　oxygen　atmosphere；　water；　clouds；　oceans；　polar　ice；　life；　and　intelligence。　The　use　of　instruments　and　protocols　developed　to　explore　the　planets　to　monitor　the　environmental　health　of　our　own—something　NASA　is　now　doing　in　earnest—was　described　by　the　astronaut　Sally　Ride　as　〃Mission　to　Planet　Earth。〃

Other　members　of　the　NASA　scientific　team　who　worked　with　me　on　Galileo's　detection　of　life　on　Earth　were　Drs。　W。　Reid　Thompson；　Cornell　University；　Robert　Carlson；　JPL；　Donald　Gurnett；　University　of　Iowa；　and　Charles　Hord；　University　of　Colorado。

Our　success　in　detecting　life　on　Earth　with　Galileo；　without　making　any　assumptions　beforehand　about　what　kind　of　life　it　must　be；　increases　our　confidence　that　when　we　fail　to　find　life　on　other　planets；　that　negative　result　is　meaningful。　Is　this　judgment　anthropocentric；　geocentric；　provincial？　I　don't　think　so。　We're　not　looking　only　for　our　kind　of　biology。　Any　widespread　photosynthetic　pigment；　any　gas　grossly　out　of　equilibrium　with　the　rest　of　the　atmosphere；　any　rendering　of　the　surface　into　highly　geometrized　patterns；　any　steady　constellation　of　lights　on　the　night　hemisphere；　any　non…astrophysical　sources　of　radio　emission　would　betoken　the　presence　of　life。　On　Earth　w