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tches　the　optical　constants　of　Titan。　So　we　can　fairly　claim　to　have　bottled　the　haze　of　Titan—formed　high　in　its　atmosphere；　slowly　falling　out；　and　accumulating　in　copious　amounts　on　its　surface。　What　is　this　stuff　made　of？

It's　very　hard　to　know　the　exact　position　of　a　plex　organic　solid。　For　example；　the　chemistry　of　coal　is　still　not　fully　understood；　despite　a　long…standing　economic　incentive。　But　we've　found　out　some　things　about　Titan　tholin。　It　contains　many　of　the　essential　building　blocks　of　life　on　Earth。　Indeed；　if　you　drop　Titan　tholin　into　water　you　make　a　large　number　of　amino　acids；　the　fundamental　constituents　of　proteins；　and　nucleotide　bases　also；　the　building　blocks　of　DNA　and　RNA。　Some　of　the　amino　acids　so　formed　are　widespread　in　living　things　on　Earth。　Others　are　of　a　pletely　different　sort。　A　rich　array　of　other　organic　molecules　is　present　also；　some　relevant　to　life；　some　not。　During　the　past　four　billion　years；　immense　quantities　of　organic　molecules　sedimented　out　of　the　atmosphere　onto　the　surface　of　Titan。　If　it's　all　deep…frozen　and　unchanged　in　the　intervening　aeons；　the　amount　accumulated　should　be　at　least　tens　of　meters　（a　hundred　feet）　thick；　outside　estimates　put　it　at　a　kilometer　deep。

But　at　180°C　below　the　freezing　point　of　water；　you　might　very　well　think　that　amino　acids　will　never　be　made。　Dropping　tholins　into　water　may　be　relevant　to　the　early　Earth；　but　not；　it　would　seem；　to　Titan。　However；　ets　and　asteroids　must　on　occasion　e　crashing　into　the　surface　of　Titan。　（The　other　nearby　moons　of　Saturn　show　abundant　impact　craters；　and　the　atmosphere　of　Titan　isn't　thick　enough　to　prevent　large；　high…speed　objects　from　reaching　the　surface。）　Although　we've　never　seen　the　surface　of　Titan；　planetary　scientists　nevertheless　know　something　about　its　position。　The　average　density　of　Titan　lies　between　the　density　of　ice　and　the　density　of　rock。　Plausibly　it　contains　both。　Ice　and　rock　are　abundant　on　nearby　worlds；　some　of　which　are　made　of　nearly　pure　rice。　If　the　surface　of　Titan　is　icy；　a　high…speed　etary　impact　will　temporarily　melt　the　ice。　Thompson　and　I　estimate　that　any　given　spot　on　Titan's　surface　has　a　better　than　50…50　chance　of　having　once　been　melted；　with　an　average　lifetime　of　the　impact　melt　and　slurry　of　almost　a　thousand　years。

This　makes　for　a　very　different　story。　The　origin　of　life　on　Earth　seems　to　have　occurred　in　oceans　and　shallow　tidepools。　Life　on　Earth　is　made　mainly　of　water；　which　plays　an　essential　physical　and　chemical　role。　Indeed；　it's　hard；　for　us　water…besotted　creatures　to　imagine　life　without　water。　If　on　our　planet　the　origin　of　life　took　less　than　a　hundred　million　years；　is　there　any　chance　that　on　Titan　it　took　a　thousand？　With　tholins　mixed　into　liquid　water…even　for　only　a　thousand　years　the　surface　of　Titan　may　be　much　further　along　toward　the　origin　of　life　than　we　thought。



DESPITE　ALL　THIS　we　understand　pitifully　little　about　Titan。　This　was　brought　home　forcefully　to　me　at　a　scientific　symposium　on　Titan　held　in　Toulouse；　France；　and　sponsored　by　the　European　Space　Agency　（ESA）。　While　oceans　of　liquid　water　are　impossible　on　Titan；　oceans　of　liquid　hydrocarbons　are　not。　Clouds　of　methane　（CH4）；　the　most　abundant　hydrocarbon；　are　expected　not　far　above　the　surface。　Ethane　（C2H6）；　the　next　most　abundant　hydrocarbon；　must　condense　out　at　the　surface　in　the　same　way　that　water　vapor　bees　a　liquid　near　the　surface　of　the　Earth；　where　the　temperature　is　generally　between　the　freezing　and　melting　points。　Vast　oceans　of　liquid　hydrocarbons　should　have　accumulated　over　the　lifetime　of　Titan。　They　would　lie　far　beneath　the　haze　and　clouds。　But　that　doesn't　mean　they　would　be　wholly　inaccessible　to　us—because　radio　waves　readily　penetrate　the　atmosphere　of　Titan　and　its　suspended；　slowly　falling　fine　particles。

In　Toulouse；　Duane　O。　Muhleman　of　the　California　Institute　of　Technology　described　to　us　the　very　difficult　technical　feat　of　transmitting　a　set　of　radio　pulses　from　a　radio　telescope　in　California's　Mojave　Desert；　so　they　reach　Titan；　penetrate　through　the　haze　and　clouds　to　its　surface；　are　reflected　back　into　space；　and　then　returned　to　Earth。　Here；　the　greatly　enfeebled　signal　is　picked　up　by　an　array　of　radio　telescopes　near　Socorro；　New　Mexico。　Great。　If　Titan　has　a　rocky　or　icy　surface；　a　radar　pulse　reflected　off　its　surface　should　be　detectable　on　Earth。　But　if　Titan　were　covered　with　hydrocarbon　oceans；　Muhleman　shouldn't　see　a　thing：　Liquid　hydrocarbons　are　black　to　these　radio　waves；　and　no　echo　would　have　been　returned　to　Earth。　In　fact；　Muhleman's　giant　radar　system　sees　a　reflection　when　some　longitudes　of　Titan　are　turned　toward　Earth；　and　not　at　other　longitudes。　All　right；　you　might　say；　so　Titan　has　oceans　and　continents；　and　it　was　a　continent　that　reflected　the　signals　back　to　Earth。　But　if　Titanic　in　this　respect　like　the　Earth—for　some　meridians　（through　Europe　and　Africa；　say）　mainly　continent；　and　for　others　（through　the　central　Pacific；　say）　mainly　ocean—then　we　must　confront　another　problem：

The　orbit　of　Titan　around　Saturn　is　not　a　perfect　circle。　it's　noticeably　squashed　out；　or　elliptical。　If　Titan　has　extensive　oceans；　though；　the　giant　planet　Saturn　around　which　it　orbits　will　raise　substantial　tides　on　Titan；　and　the　resulting　tidal　friction　will　circularize　Titan's　orbit　in　much　less　than　the　age　of　the　Solar　System。　In　a　1982　scientific　paper　called　〃The　Tide　in　the　Seas　of　Titan；〃　Stanley　Dermott；　now　at　the　University　of　Florida；　and　I　argued　that　for　this　reason　Titan　must　be　either　an　all…ocean　or　an　all…land　world。　Otherwise　the　tidal　friction　in　places　where　the　ocean　is　shallow　would　have　taken　its　toll。　Lakes　and　islands　might　be　permitted；　but　anything　more　and　Titan　would　have　a　very　different　orbit　than　the　one　we　see。

We　have；　then；　three　scientific　arguments—one　concluding　that　this　world　is　almost　entirely　covered　with　hydrocarbon　oceans；　another　that　it's　a　mix　of　continents　and　oceans；　and　a　third　requiring　us　to　choose；　counseling　that　Titan　can't　have　extensive　oceans　and　extensive　continents　at　the　same　time。　It　will　be　interesting　to　see　what　the　answer　turns　out　to　be。

What　I've　just　told　you　is　a　kind　of　scientific　progress　report。　Tomorrow　there　might　be　a　new　finding　that　clears　up　these　mysteries　and　contradictions。　Maybe　there's　something　wrong　with　Muhleman's　radar　results；　although　it's　hard　to　see　what　it　might　be：　His　system　tells　him　he's　seeing　Titan　when　it's　nearest；　when　he　ought　to　be　seeing　Titan。　Maybe　there's　something　wrong　with　Dermott's　and　my　calculation　about　the　tidal　evolution　of　the　orbit　of　Titan；　but　no　one　has　been　able　to　find　any　errors　so　far。　And　it's　Bard　to　see　how　ethane　can　avoid　condensing　out　at　the　surface　of　Titan。　Maybe；　despite　the　low　temperatures；　over　billions　of　years　there's　been　a　change　in　the　chemistry；　maybe　some　bination　of　ets　impacting　from　the　sky　and　volcanoes　and　other　tectonic　events；　helped　along　by　cosmic　rays；　can　congeal　liquid　hydrocarbons；　turning　them　into　some　plex　organic　solid　that　reflects　radio　waves　back　to　space。　Or　maybe　something　reflective　to　radio　waves　is　floating　on　the　ocean　surface。　But　liquid　hydrocarbons　are　very　underdense：　Every　known　organic　solid；　unless　extremely　frothy；　would　sink　like　a　stone　in　the　seas　of　Titan。

Dermott　and　I　now　wonder　whether；　when　we　imagined　continents　and　oceans　on　Titan；　we　were　too　transfixed　by　our　experience　on　our　own　world；　too　Earth…chauvinist　in　our　thinking。　Battered；　cratered　terrain　and　abundant　impact　basins　cover　other　moons　in　the　Saturn　system。　If　we　pictured　liquid　hydrocarbons　slowly　accumulating　on　one　of　those　worlds；　we　would　wind　up　not　with　global　oceans；　but　with　isolated　large　craters　filled；　although　not　to　the　brim；　with　liquid　hydrocarbons。　Many　circular　seas　of　petroleum；　some　over　a　hundred　miles　across；　would　be　splattered　across　the　surface—but　no　perceptible　waves　would　be　stimulated　by　distant　Saturn　and；　it　is　conventional　to　think；　no　ships；　no　swimmers；　no　surfers；　and　no　fishing。　Tidal　friction　should；　we　calculate；　be　negligible　in　such　a　case；　and　Titan's　stretched…out；　elliptical　orbit　would　not　have　bee　so　circular。　We　can't　know　for　sure　until　we　start　getting　radar　or　near…infrared　images　of　the　surface。　But　perhaps　this　is　the　resolution　of　our　dilemma：　Titan　as　a　world　of　large　circular　hydrocarbon　lakes；　more　of　them　in　some　longitudes　than　in　others。

Should　we　expect　an　icy　surface　covered　with　deep　tholin　sediments；　a　hydrocarbon　ocean　with　at　most　a　few　organic　encrusted　islands　poking　up　here　and　there；　a　world　of　crater　lakes；　or　something　more　subtle　that　we　haven't　yet　figured　out？　This　isn't　just　an　academic　question；　because　there's　a　real　spacecraft　being　designed　to　go　to　Titan。　In　a　joint　NASA/ESA　program；　a　spacecraft　called　Cassini　will　be　launched　in　October　1997—if　all　goes　well。　With　two　flybys　of　Venus；　one　of　Earth；　and　one　of　Jupiter　for　gravitational　assists；　the　ship　will；　after　a　seven…year　voyage；　be　injected　into　orbit　around　Saturn。　Each　time　the　spacecraft　es　close　to　Titan；　the　moon　will　be　examined　by　an　array　of　instruments；　including　radar。　Because　Cassini　will　be　so　much　closer　to　Titan；　it　will　be　able　to　resolve　many　details　on　Titan's　surface　indetectable　to　Muhleman's　pioneering　Earth…based　system。　It's　also　likely　that　the　surface　can　be　viewed　in　the　near　infrared。　Maps　of　the　hidden　surface　of　Titan　may　be　in　our　hands　sometime　in　the　summer　of　2004。

Cassini　is　also　carrying　an　entry　probe；　fittingly　called　Huygens；　which　will　detach　itself　from　the　main　spacecraft　and　plummet　into　Titan's　atmosphere。　A　great　parachute　will　be　deployed。　The　instrument　package　will　slowly　settle　through　the　organic　haze　down　into　the　lower　atmosphere；　through　the　methane　clouds。　It　will　examine　organic　chemistry　as　it　descends；　and—if　it　survives　the　landing—on　the　surface　of　this　world　as　well。

Nothing　is　guaranteed。　But　the　mission　is　technically　feasible；　hardware　is　being　built；　an　impressive　coterie　of　specialists；　including　many　young　European　scientists；　are　hard　at　work　on　it；　and　all　the　nations　responsible　seem　mitted　to　the　project。　Perhaps　it　will　actually　e　about。　Pe