pale blue dot -carl sagan-第52章

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founded　fantasy；　could　be　more　soberly　treated　without　ceasing　to　be　interesting　and　could　show　to　even　the　most　adventurous　of　the　young　that　a　world　without　war　need　not　be　a　world　without　adventurous　and　hazardous　glory。*　To　this　kind　of　contest　there　is　no　limit。　Each　victory　is　only　a　prelude　to　another；　and　no　boundaries　can　be　set　to　rational　hope。

*　Russell's　phrase　is　noteworthy：　〃adventurous　and　hazardous　glory。〃　Even　if　we　could　make　human　spaceflight　risk…free…and　of　course　we　cannot…it　might　be　counterproductive。　The　hazard　is　an　inseparable　ponent　of　the　glory。

In　the　long　run；　these—more　than　any　of　the　〃practical〃　justifications　considered　earlier—may　be　the　reasons　we　will　go　to　Mars　and　other　worlds。　In　the　meantime；　the　most　important　step　we　can　take　toward　Mars　is　to　make　significant　progress　on　Earth。　Even　modest　improvements　in　the　social；　economic；　and　political　problems　that　our　global　civilization　now　faces　could　release　enormous　resources；　both　material　and　human；　for　other　goals。

There's　plenty　of　housework　to　be　done　here　on　Earth；　and　our　mitment　to　it　must　be　steadfast。　But　we're　the　kind　of　species　that　needs　a　frontier—for　fundamental　biological　reasons。

Every　time　humanity　stretches　itself　and　turns　a　new　corner；　it　receives　a　jolt　of　productive　vitality　that　can　carry　it　for　centuries。

There's　a　new　world　next　door。　And　we　know　how　to　get　there。




CHAPTER　17　　　ROUTINE　INTERPLANETARY　VIOLENCE

　

　

It　is　a　law　of　nature　that　Earth　and　all　other　bodies　should　remain　in　their　proper　places　and　be　moved　from　them　only　by　violence。

—ARISTOTLE　（384…322　B。C。）；　PHYSICS

There　was　something　funny　about　Saturn。　When；　in　1610；　Galileo　used　the　world's　first　astronomical　telescope　to　view　the　planet—then　the　most　distant　world　known—he　found　two　appendages；　one　on　either　side。　He　likened　them　to　〃handles。〃　Other　astronomers　called　them　〃ears。〃　The　Cosmos　holds　many　wonders；　but　a　planet　with　jug　ears　is　dismaying。　Galileo　went　to　his　grave　with　this　bizarre　matter　unresolved。

As　the　years　passed；　observers　found　the　ears　。　。　。　Well；　waxing　and　waning。　Eventually；　it　became　clear　that　what　Galileo　had　discovered　was　an　extremely　thin　ring　that　surrounds　Saturn　at　its　equator　but　touches　it　nowhere。　In　some　years；　because　of　the　changing　orbital　positions　of　Earth　and　Saturn；　the　ring　had　been　seen　edge…on　and；　because　of　its　thinness；　it　seemed　to　disappear。　In　other　years；　it　had　been　viewed　more　face…oil；　and　the　〃ears〃　grew　bigger。　But　what　does　it　mean　that　there's　a　ring　around　Saturn？　A　thin；　flat；　solid　plate　with　a　hole　cut　out　for　the　planet　to　fit　into？　Where　does　that　e　from？

This　line　of　inquiry　will　shortly　take　us　to　world…shattering　collisions；　to　two　quite　different　perils　for　our　species；　and　to　a　reason—beyond　those　already　described—that　we　must；　for　our　very　survival；　be　out　there　among　the　planets。

We　now　know　that　the　rings　（emphatically　plural）　of　Saturn　are　a　vast　horde　of　tiny　ice　worlds；　each　on　its　separate　orbit；　each　bound　to　Saturn　by　the　giant　planet's　gravity。　In　size；　these　worldlets　range　from　particles　of　fine　dust　to　houses。　None　is　big　enough　to　photograph　even　from　close　flybys。　Spaced　out　in　an　exquisite　set　of　fine　concentric　circles；　something　like　the　grooves　on　a　phonograph　record　（which　in　reality　make；　of　course；　a　spiral）；　the　rings　were　first　revealed　in　their　true　majesty　by　the　two　Voyager　spacecraft　in　their　1980/81　flybys。　In　our　century；　the　Art　Deco　rings　of　Saturn　have　bee　an　icon　of　the　future。

At　a　scientific　meeting　in　the　late　1960s；　I　was　asked　to　summarize　the　outstanding　problems　in　planetary　science。　One；　I　suggested；　was　the　question　of　why；　of　all　the　planets；　only　Saturn　had　rings。　This；　Voyager　discovered；　is　a　nonquestion。　All　four　giant　planets　in　our　Solar　System—　Jupiter；　Saturn；　Uranus；　and　Neptune—in　fact　have　rings。　But　no　one　knew　it　then。

Each　ring　system　has　distinctive　features。　Jupiter's　is　tenuous　and　made　mainly　of　dark；　very　small　particles。　The　bright　rings　of　Saturn　are　posed　mainly　of　frozen　water；　there　are　thousands　of　separate　rings　here；　some　twisted；　with　strange；　dusky；　spoke…like　markings　forming　and　dissipating。　The　dark　rings　of　Uranus　seem　to　be　posed　of　elemental　carbon　and　organic　molecules—something　like　charcoal　or　chimney　soot；　Uranus　has　nine　main　rings；　a　few　of　which　sometime　seem　to　〃breathe；〃　expanding　and　contracting。　Neptune's　rings　are　the　most　tenuous　of　all；　varying　so　much　in　thickness　that；　when　detected　from　Earth；　they　appear　only　as　arcs　and　inplete　circles。　A　number　of　rings　seem　to　be　maintained　by　the　gravitational　tugs　of　two　shepherd　moons；　one　a　little　nearer　and　the　other　a　little　farther　from　the　planet　than　the　ring。　Each　ring　system　displays　its　own；　appropriately　unearthly；　beauty。

How　do　rings　form？　One　possibility　is　tides：　If　an　errant　world　passes　close　to　a　planet；　the　interloper's　near　side　is　gravitationally　pulled　toward　the　planet　more　than　its　far　side；　if　it　es　close　enough；　if　its　internal　cohesion　is　low　enough；　it　can　be　literally　torn　to　pieces。　Occasionally　we　see　this　happening　to　ets　as　they　pass　too　close　to　Jupiter；　or　the　Sun。　Another　possibility；　emerging　from　the　Voyager　reconnaissance　of　the　outer　Solar　System；　is　this：　Rings　are　made　when　worlds　collide　and　moons　are　smashed　to　smithereens。　Both　mechanisms　may　have　played　a　role。

The　space　between　the　planets　is　traversed　by　an　odd　collection　of　rogue　worldlets；　each　in　orbit　about　the　Sun。　A　few　are　as　big　as　a　county　or　even　a　state；　many　more　have　surface　areas　like　those　of　a　village　or　a　town。　More　little　ones　are　found　than　big　ones；　and　they　range　in　size　down　to　particles　of　dust。　Some　of　them　travel　on　long；　stretched…out　elliptical　paths；　which　make　them　periodically　cross　the　orbit　of　one　or　more　planets。

Occasionally；　unluckily；　there's　a　world　in　the　way。　The　collision　can　shatter　and　pulverize　both　the　interloper　and　the　moon　that's　hit　（or　at　least　the　region　around　ground　zero）。　The　resulting　debris—ejected　from　the　moon　but　not　so　fast…moving　as　to　escape　from　the　planet's　gravity—may　form；　for　a　time；　a　new　ring。　It's　made　of　whatever　the　colliding　bodies　were　made　of；　but　usually　more　of　the　target　moon　than　the　rogue　impactor。　If　the　colliding　worlds　are　icy；　the　net　result　will　be　rings　of　ice　particles；　if　they're　made　of　organic　molecules；　the　result　will　be　rings　of　organic　particles　（which　will　slowly　be　processed　by　radiation　into　carbon）。　All　the　mass　in　the　rings　of　Saturn　is　no　more　than　'would　result　from　the　plete　impact　pulverization　of　a　single　icy　moon。　The　disintegration　of　small　moons　can　likewise　account　for　the　ring　systems　of　the　three　other　giant　planets。

Unless　it's　very　close　to　its　planet；　a　shattered　moon　gradually　reaccumulates　（or　at　least　a　fair　fraction　of　it　does）。　The　pieces；　big　and　small；　still　in　approximately　the　same　orbit　as　the　moon　was　before　the　impact；　fall　together　helter…skelter。　What　used　to　be　a　piece　of　the　core　is　now　at　the　surface；　and　vice　versa。　The　resulting　hodgepodge　surfaces　might　seem　very　odd。　Miranda；　one　of　the　moons　of　Uranus；　looks　disconcertingly　jumbled　and　may　have　had　such　an　origin。

The　American　planetary　geologist　Eugene　Shoemaker　proposes　that　many　moons　in　the　outer　Solar　System　have　been　annihilated　and　reformed—not　just　once　but　several　times　each　over　the　4。5　billion　years　since　the　Sun　and　the　planets　condensed　out　of　interstellar　gas　and　dust。　The　picture　emerging　from　the　Voyager　reconnaissance　of　the　outer　Solar　System　is　of　worlds　whose　placid　and　lonely　vigils　are　spasmodically　troubled　by　interlopers　from　space；　of　world…shattering　collisions；　and　of　moons　re…forming　from　debris；　reconstituting　themselves　like　phoenixes　from　their　own　ashes。

But　a　moon　that　lives　very　close　to　a　planet　cannot　re…form　if　it　is　pulverized—the　gravitational　tides　of　the　nearby　planet　prevent　it。　The　resulting　debris；　once　spread　out　into　a　ring　system；　might　be　very　long…lived—at　least　by　the　standard　of　a　human　lifetime。　Perhaps　many　of　the　small；　inconspicuous　moons　now　orbiting　the　giant　planets　will　one　day　blossom　forth　into　vast　and　lovely　rings。