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re¡¡of¡¡organic¡¡gases¡£¡¡The¡¡simplest¡¡hydrocarbons¡¡on¡¡Titan¡¡are¡¡manufactured¡¡by¡¡ultraviolet¡¡light¡¡from¡¡the¡¡Sun¡£¡¡But¡¡for¡¡all¡¡the¡¡other¡¡gas¡¡products£»¡¡those¡¡made¡¡most¡¡readily¡¡by¡¡electrons¡¡in¡¡the¡¡laboratory¡¡correspond¡¡to¡¡those¡¡discovered¡¡by¡¡Voyager¡¡on¡¡Titan£»¡¡and¡¡in¡¡the¡¡same¡¡proportions¡£¡¡The¡¡correspondence¡¡is¡¡one¡¡to¡¡one¡£¡¡The¡¡next¡¡most¡¡abundant¡¡gases¡¡that¡¡we've¡¡found¡¡in¡¡the¡¡laboratory¡¡will¡¡be¡¡looked¡¡for¡¡in¡¡future¡¡studies¡¡of¡¡Titan¡£¡¡The¡¡most¡¡plex¡¡organic¡¡gases¡¡we¡¡make¡¡have¡¡six¡¡or¡¡seven¡¡carbon¡¡and/or¡¡nitrogen¡¡atoms¡£¡¡These¡¡product¡¡molecules¡¡are¡¡on¡¡their¡¡way¡¡to¡¡forming¡¡tholins¡£



WE¡¡HAD¡¡HOPED¡¡FOR¡¡A¡¡BREAK¡¡In¡¡the¡¡weather¡¡as¡¡Voyager¡¡1¡¡approached¡¡Titan¡£¡¡A¡¡long¡¡distance¡¡away£»¡¡it¡¡appeared¡¡as¡¡a¡¡tiny¡¡disk£»¡¡at¡¡closest¡¡approach£»¡¡our¡¡camera's¡¡field¡¡of¡¡view¡¡was¡¡filled¡¡by¡¡a¡¡small¡¡province¡¡of¡¡Titan¡£¡¡If¡¡there¡¡had¡¡been¡¡a¡¡break¡¡in¡¡the¡¡haze¡¡and¡¡clouds£»¡¡even¡¡only¡¡a¡¡few¡¡miles¡¡across£»¡¡as¡¡we¡¡scanned¡¡the¡¡disk¡¡we¡¡would¡¡have¡¡seen¡¡something¡¡of¡¡its¡¡hidden¡¡surface¡£¡¡But¡¡there¡¡was¡¡no¡¡hint¡¡of¡¡a¡¡break¡£¡¡This¡¡world¡¡is¡¡socked¡¡in¡£¡¡No¡¡one¡¡on¡¡Earth¡¡knows¡¡what's¡¡on¡¡Titan's¡¡surface¡£¡¡And¡¡an¡¡observer¡¡there£»¡¡looking¡¡up¡¡ill¡¡ordinary¡¡visible¡¡light£»¡¡would¡¡have¡¡no¡¡idea¡¡of¡¡the¡¡glories¡¡that¡¡await¡¡upon¡¡ascending¡¡through¡¡the¡¡haze¡¡and¡¡beholding¡¡Saturn¡¡and¡¡its¡¡magnificent¡¡rings¡£

From¡¡measurements¡¡by¡¡Voyager£»¡¡by¡¡the¡¡International¡¡Ultraviolet¡¡Explorer¡¡observatory¡¡in¡¡Earth¡¡orbit£»¡¡and¡¡by¡¡ground¡­based¡¡telescopes£»¡¡we¡¡know¡¡a¡¡fair¡¡amount¡¡about¡¡the¡¡orange¡­brown¡¡haze¡¡particles¡¡that¡¡obscure¡¡the¡¡surface£º¡¡which¡¡colors¡¡of¡¡light¡¡they¡¡like¡¡to¡¡absorb£»¡¡which¡¡colors¡¡they¡¡pretty¡¡much¡¡let¡¡pass¡¡through¡¡them£»¡¡how¡¡much¡¡they¡¡bend£»¡¡the¡¡light¡¡that¡¡does¡¡pass¡¡through¡¡them£»¡¡and¡¡how¡¡big¡¡they¡¡are¡£¡¡£¨They're¡¡mostly¡¡the¡¡size¡¡of¡¡the¡¡particles¡¡in¡¡cigarette¡¡smoke¡££©¡¡The¡¡¡¨optical¡¡properties¡¨¡¡will¡¡depend£»¡¡of¡¡course£»¡¡on¡¡the¡¡position¡¡of¡¡the¡¡haze¡¡particles¡£

In¡¡collaboration¡¡with¡¡Edward¡¡Arakawa¡¡of¡¡Oak¡¡Ridge¡¡National¡¡Laboratory¡¡in¡¡Tennessee£»¡¡Khare¡¡and¡¡I¡¡have¡¡measured¡¡the¡¡optical¡¡properties¡¡of¡¡Titan¡¡tholin¡£¡¡It¡¡turns¡¡out¡¡to¡¡be¡¡a¡¡dead¡¡ringer¡¡for¡¡the¡¡real¡¡Titan¡¡haze¡£¡¡No¡¡other¡¡candidate¡¡material£»¡¡mineral¡¡or¡¡organic£»¡¡matches¡¡the¡¡optical¡¡constants¡¡of¡¡Titan¡£¡¡So¡¡we¡¡can¡¡fairly¡¡claim¡¡to¡¡have¡¡bottled¡¡the¡¡haze¡¡of¡¡Titan¡ªformed¡¡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¡ãC¡¡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¡ªbecause¡¡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£º¡¡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¡ªfor¡¡some¡¡meridians¡¡£¨through¡¡Europe¡¡and¡¡Africa£»¡¡say£©¡¡mainly¡¡continent£»¡¡and¡¡for¡¡others¡¡£¨through¡¡the¡¡central¡¡Pacific£»¡¡say£©¡¡mainly¡¡ocean¡ªthen¡¡we¡¡must¡¡confront¡¡another¡¡problem£º

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¡ªone¡¡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¡¡o
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