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randa's　interior。　Or　perhaps　we　are　seeing　the　results　of　the　primordial　collision　that　is　thought　to　have　knocked　Uranus　over。　Or；　just　conceivably；　maybe　Miranda　was　once　utterly　destroyed；　dismembered；　blasted　into　smithereens　by　a　wild　careening　world；　with　many　collision　fragments　still　left　in　Miranda's　orbit。　The　shards　and　remnants；　slowly　colliding；　gravitationally　attracting　one　another；　may　have　re…aggregated　into　just　such　a　jumbled；　patchy；　unfinished　world　as　Miranda　is　today。

For　me；　there's　something　eerie　about　the　pictures　of　dusky　Miranda；　because　I　can　remember　so　well　when　it　‘vas　only　a　faint　point　of　light　almost　lost　in　the　glare　of　Uranus；　discovered　through　great　difficulty　by　dint　of　the　astronomer's　skills　and　patience。　In　only　half　a　lifetime　it　has　gone　from　an　undiscovered　world　to　a　destination　whose　ancient　and　idiosyncratic　secrets　have　been　at　least　partially　revealed。




CHAPTER　9　　　AN　AMERICAN　SHIP　AT　THE　FRONTIERS　OF　THE　SOLAR　SYSTEM

　


。　。　。　by　the　shore　of　Triton's　Lake　。　。　。

I　will　clear　my　breast　of　secrets。

—EURIPIDES；　ION　（CA。　413　B。C。）

Neptune　was　the　final　port　of　call　in　Voyager　2's　grand　tour　of　the　Solar　System。　Usually；　it　is　thought　of　as　the　penultimate　planet；　with　Pluto　the　outermost。　But　because　of　Pluto's　stretched…out；　elliptical　orbit；　Neptune　has　lately　been　the　outermost　planet；　and　will　remain　so　until　1999。　Typical　temperatures　in　its　upper　clouds　are　about　…240°C；　because　it　is　so　far　from　the　warming　rays　of　the　Sun。　It　would　be　colder　still　except　for　the　heat　welling　up　from　its　interior。　Neptune　glides　along　the　hem　of　interstellar　night。　It　is　so　far　away　that；　in　its　sky；　the　Sun　appears　as　little　more　than　an　extremely　bright　star。

How　far？　So　far　away　that　it　has　yet　to　plete　a　single　trip　around　the　Sun；　a　Neptunian　year；　since　its　discovery　in　1846。*　It's　so　far　away　that　it　cannot　be　seen　with　the　naked　eye。　It's　so　far　away　that　it　takes　light—faster　than　which　nothing　can　go—more　than　five　hours　to　get　from　Neptune　to　Earth。

*　It　takes　so　long　to　circuit　the　Sun　because　its　orbit　is　so　vast；　23　billion　miles　around；　and　because　the　force　of　the　Sun's　gravity—which　keeps　it　from　flying　out　into　interstellar　space—is　at　that　distant　paratively　feeble；　less　than　a　thousandth　what　it　is　in　the　Earth's　vicinity。

When　Voyager　2　raced　through　the　Neptune　system　in　1989；　its　cameras；　spectrometers；　particle　and　field　detectors；　and　other　instruments　were　feverishly　examining　the　planet；　its　moons；　and　its　rings。　The　planet　itself；　like　its　cousins　Jupiter；　Saturn；　and　Uranus；　is　a　giant。　Every　planet　is　an　Earthlike　world　at　heart—but　the　four　gas　giants　wear　elaborate；　cumbersome　disguises。　Jupiter　and　Saturn　are　great　gas　worlds　with　relatively　small　rocky　and　icy　cores。　But　Uranus　and　Neptune　are　fundamentally　rock　and　ice　worlds　swaddled　in　dense　atmospheres　that　hide　them　from　view。

Neptune　is　four　times　bigger　than　the　Earth。　When　we　look　down　on　its　cool；　austere　blueness；　again　we　are　seeing　only　atmosphere　and　clouds—no　solid　surface。　Again；　the　atmosphere　is　made　mainly　of　hydrogen　and　helium；　with　a　little　methane　and　traces　of　other　hydrocarbons。　There　may　also　be　some　nitrogen。　The　bright　clouds；　which　seem　to　be　methane　crystals；　float　above　thick；　deeper　clouds　of　unknown　position。　From　the　motion　of　the　clouds　we　discovered　fierce　winds；　approaching　the　local　speed　of　sound。　A　Great　Dark　Spot　was　found；　curiously　at　almost　the　same　latitude　as　the　Great　Red　Spot　on　Jupiter。　The　azure　color　seems　appropriate　for　a　planet　named　after　the　god　of　the　sea。

Surrounding　this　dimly　lit；　chilly；　stormy；　remote　world　is—here　also—a　system　of　rings；　each　posed　of　innumerable　orbiting　objects　ranging　in　size　from　the　fine　particles　in　cigarette　smoke　to　small　trucks。　Like　the　rings　of　the　other　Jovian　planets；　those　of　Neptune　seem　to　be　evanescent—it　is　calculated　that　gravity　and　solar　radiation　will　disrupt　them　in　much　less　than　the　age　of　the　Solar　System。　If　they　are　destroyed　quickly；　we　must　see　them　only　because　they　were　made　recently。　But　how　can　rings　be　made？

The　biggest　moon　in　the　Neptune　system　is　called　Triton。*　Nearly　six　of　our　days　are　required　for　it　to　orbit　Neptune；　which—alone　among　big　moons　in　the　Solar　System—it　does　in　the　opposite　direction　to　which　its　planet　spins　（clockwise　if　we　say　Neptune　rotates　counterclockwise）。　Triton　has　a　nitrogen…rich　atmosphere；　somewhat　similar　to　Titan's；　but；　because　the　air　and　haze　are　much　thinner；　we　can　see　its　surface。　The　landscapes　are　varied　and　splendid。　This　is　a　world　of　ices—nitrogen　ice；　methane　ice；　probably　underlain　by　more　familiar　water　ice　and　rock。　There　are　impact　basins；　which　seem　to　have　been　flooded　with　liquid　before　refreezing　（so　there　once　were　lakes　on　Triton）；　impact　craters；　long　crisscrossing　valleys；　vast　plains　covered　by　freshly　fallen　nitrogen　snow；　puckered　terrain　that　resembles　the　skin　of　a　cantaloupe；　and　more　or　less　parallel；　long；　dark　streaks　that　seem　to　have　been　blown　by　the　wind　and　then　deposited　on　the　icy　surface　despite　how　sparse　Triton's　atmosphere　is　（about　1/10；000　the　thickness　of　the　Earth's）。

*　Robert　Goddard；　the　inventor　of　the　modern　liquid…fueled　rocket；　envisioned　a　time　when　expeditions　to　the　stars　would　be　outfitted　on　and　launched　from　Triton。　This　was　in　a　1927　afterthought　to　a　1918　handwritten　manuscript　called　〃The　Last　Migration。〃　Considered　much　too　daring　for　publication；　it　was　deposited　in　a　friend's　safe。　The　cover　page　bears　a　warning：　〃The＇se＇　notes　should　be　read　thoroughly　only　by　an　optimist。〃

All　the　craters　on　Triton　are　pristine—as　if　stamped　out　by　some　vast　milling　device。　There　are　no　slumped　walls　or　muted　relief。　Even　with　the　periodic　falling　and　evaporation　of　snow；　it　seems　that　nothing　has　eroded　the　surface　of　Triton　in　billions　of　years。　So　the　craters　that　were　gouged　out　during　the　formation　of　Triton　must　have　all　been　filled　in　and　covered　over　by　some　early　global　resurfacing　event。　Triton　orbits　Neptune　in　the　opposite　direction　to　Neptune's　rotation—unlike　the　situation　with　the　Earth　and　its　moon；　and　with　most　of　the　large　moons　in　the　Solar　System。　If　Triton　had　formed　out　of　the　same　spinning　disk　that　made　Neptune；　it　ought　to　be　going　around　Neptune　in　the　same　direction　that　Neptune　rotates。　So　Triton　was　not　made　from　the　original　local　nebula　around　Neptune；　but　arose　somewhere　else—perhaps　far　beyond　Pluto—and　was　by　chance　gravitationally　captured　when　it　passed　too　close　to　Neptune。　This　event　should　have　raised　enormous　solid…body　tides　in　Triton；　melting　the　surface　and　sweeping　away　all　the　past　topography。

In　some　places　the　surface　is　as　bright　and　white　as　freshly　fallen　Antarctic　snows　（and　may　offer　a　skiing　experience　unrivaled　in　all　the　Solar　System）。　Elsewhere　there's　a　tint；　ranging　from　pink　to　brown。　One　possible　explanation：　Freshly　fallen　snows　of　nitrogen；　methane；　and　other　hydrocarbons　are　irradiated　by　solar　ultraviolet　light　and　by　electrons　trapped　in　the　magnetic　field　of　Neptune；　through　which　Triton　plows。　We　know　that　such　irradiation　will　convert　the　snows　（like　the　corresponding　gases）　to　plex；　dark；　reddish　organic　sediments；　ice　tholins—nothing　alive；　but　here　too　posed　of　some　of　the　molecules　implicated　in　the　origin　of　life　on　Earth　four　billion　years　ago。

In　local　winter；　layers　of　ice　and　snow　build　up　on　the　surface。　（Our　winters；　mercifully；　are　only　4　percent　as　long。）　Through　the　spring；　they　are　slowly　transformed；　more　and　more　reddish　organic　molecules　accumulating。　By　summertime；　the　ice　and　snow　have　evaporated；　the　gases　so　released　migrate　halfway　across　the　planet　to　the　winter　hemisphere　and　there　cover　the　surface　with　ice　and　snow　again。　But　the　reddish　organic　molecules　do　not　vaporize　and　are　not　transported—a　lag　deposit；　they　are　next　winter　covered　over　by　new　snows；　which　are　in　turn　irradiated；　and　by　the　following　summer　the　accumulation　is　thicker。　As　time　goes　on；　substantial　amounts　of　organic　matter　are　built　up　on　the　surface　of　Triton；　which　may　account　for　its　delicate　color　markings。

The　streaks　begin　in　small；　dark　source　regions；　perhaps　when　the　warmth　of　spring　and　summer　heats　subsurface　volatile　snows。　As　they　vaporize；　gas　es　gushing　out　as　in　a　geyser；　blowing　off　less…volatile　surface　snows　and　dark　organics。　Prevailing　low…speed　winds　carry　away　the　dark　organics；　which　slowly　sediment　out　of　the　thin　air；　are　deposited　on　the　ground；　and　generate　the　appearance　of　the　streaks。　This；　at　least；　is　one　reconstruction　of　recent　Tritonian　history。

Triton　may　have　large；　seasonal　polar　caps　of　smooth　nitrogen　ice　underlying　layers　of　dark　organic　materials。　Nitrogen　snows　seem　recently　to　have　fallen　at　the　equator。　Snowfalls；　geysers；　windblown　organic　dust；　and　high…altitude　hazes　were　entirely　unexpected　on　a　world　with　so　thin　an　atmosphere。

Why　is　the　air　so　thin？　Because　Triton　is　so　far　from　the　Sun。　Were　you　somehow　to　pick　this　world　up　and　move　it　into　orbit　around　Saturn；　the　nitrogen　and　methane　ices　would　quickly　evaporate；　a　much　denser　atmosphere　of　gaseous　nitrogen　and　methane　would　form；　and　radiation　would　generate　an　opaque　tholin　haze。　It　would　bee　a　world　very　like　Titan。　Conversely；　if　you　moved　Titan　into　orbit　about　Neptune；　almost　all　its　atmosphere　would　freeze　out　as　snows　and　ices；　the　tholin　would　fall　out　and　not　be　replaced；　the　air　would　clear；　and　the　surface　would　bee　visible　in　ordinary　light。　It　would　bee　a　world　very　like　Triton。

These　two　worlds　are　not　identical。　The　interior　of　Titan　seems　to　contain　much　more　ice　than　that　of　Triton；　and　much　less　rock。　Titan's　diameter　is　almost　twice　that　of　Triton。　Still；　if　placed　at　the　same　distance　from　the　Sun　they　would　look　like　sisters。　Alan　Stern　of　the　Southwest　Research　Institute　suggests　that　they　are　two　members　of　a　vast　collection　of　small　worlds　rich　in　nitrogen　and　methane　that　formed　in　the　early　Solar　System。　Pluto；　yet　to　be　visited　by　a　spacecraft；　appears　to　be　another　member　of　this　group。　Many　more　may　await　discovery　beyond　Pluto。　The　thin　atmospheres　and　icy　surfaces　of　all　these　worlds　are　being　irradiated—by　cosmic　rays；　if　nothing