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　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。

These　ideas　are　supported　by　the　appearance　of　a　number　of　satellites　in　the　Solar　System。　Phobos；　the　inner　moon　of　Mars；　has　a　large　crater　named　Stickney；　Mimas；　an　inner　moon　of　Saturn；　has　a　big　one　named　Herschel。　These　craters—like　those　on　our　own　Moon　and；　indeed；　throughout　the　Solar　System—are　produced　by　collisions。　An　interloper　smashes　into　a　bigger　world　and　makes　an　immense　explosion　at　the　point　of　impact；　A　bowl…shaped　crater　is　excavated；　and　the　smaller　impacting　object　is　destroyed。　If　the　interlopers　that　dug　out　the　Stickney　and　Herschel　craters　had　been　only　a　little　larger；　they　would　have　had　enough　energy　to　blow　Phobos　and　Mimas　to　bits。　These　moons　barely　escaped　the　cosmic　wrecking　ball。　Many　others　did　not。

Every　time　a　world　is　smashed　into；　there's　one　less　interloper—something　like　a　demolition　derby　on　the　scale　of　the　Solar　System；　a　war　of　attrition。　The　very　fact　that　many　such　collisions　have　occurred　means　that　the　rogue　worldlets　have　been　largely　used　up。　Those　on　circular　trajectories　around　the　Sun；　those　that　don't　intersect　the　orbits　of　other　worlds；　will　be　unlikely　to　smash　into　a　planet。　Those　on　highly　elliptical　trajectories；　those　that　cross　the　orbits　of　other　planets；　Will　sooner　or　later　collide　or；　by　a　near　miss；　be　gravitationally　ejected　from　the　Solar　System。

The　planets　almost　certainly　accumulated　from　worldlets　which　in　turn　had　condensed　out　of　a　great　flat　cloud　of　gas　and　dust　Surrounding　the　Sun—the　sort　of　cloud　that　can　now　be　seen　around　young　nearby　stars。　So；　in　the　early　history　of　the　solar　System　before　collisions　cleaned　things　up；　there　should　have　been　many　more　worldlets　than　we　see　today。

Indeed；　there　is　clear　evidence　for　this　in　our　own　backyard：　If　we　count　up　the　interloper　worldlets　in　our　neighborhood　in　space；　we　can　estimate　how　often　they'll　hit　the　Moon。　Let　us　make　the　very　modest　assumption　that　the　population　of　interlopers　has　never　been　smaller　than　it　is　today。　We　can　then　calculate　how　many　craters　there　should　be　on　the　Moon。　The　number　we　figure　turns　out　to　be　much　less　than　the　number　we　see　on　the　Moon's　ravaged　highlands。　The　unexpected　profusion　of　craters　on　the　Moon　speaks　to　us　of　an　earlier　epoch　when　the　Solar　System　was　in　wild　turmoil；　churning　with　worlds　on　collision　trajectories。　This　makes　good　sense；　because　they　formed　from　the　aggregation　of　much　smaller　worldlets—which　themselves　had　grown　out　of　interstellar　dust。　Four　billion　years　ago；　the　lunar　impacts　were　hundreds　of　times　more　frequent　than　they　are　today；　and　4。5　billion　years　ago；　when　the　planets　were　still　inplete；　collisions　happened　perhaps　a　billion　times　more　often　than　in　our　becalmed　epoch。

The　chaos　may　have　been　relieved　by　much　more　flamboyant　ring　systems　than　grace　the　planets　today。　If　they　had　small　moons　in　that　time；　the　Earth；　Mars；　and　the　other　small　planets　may　also　have　been　adorned　with　rings。

The　most　satisfactory　explanation　of　the　origin　of　our　own　Moon；　based　on　its　chemistry　（as　revealed　by　samples　returned　from　the　Apollo　missions）；　is　that　it　was　formed　almost　4。5　billion　years　ago；　when　a　world　the　size　of　Mars　struck　the　Earth。　Much　of　our　planet's　rocky　mantle　was　reduced　to　dust　and　hot　gas　and　blasted　into　space。　Some　of　the　debris；　in　o：…bit　around　the　Earth；　then　gradually　reaccumulated—atom　by　atom；　boulder　by　boulder。　If　that　unknown　impacting　world　had　been　only　a　little　larger；　the　result　would　have　been　the　obliteration　of　the　Earth。　Perhaps　there　once　were　other　worlds　in　our　Solar　System—perhaps　even　worlds　on　which　life　was　stirring—hit　by　some　demon　worldlet；　utterly　demolished；　and　of　which　today　we　have　not　even　an　intimation。

The　emerging　picture　of　the　early　Solar　System　does　not　resemble　a　stately　progression　of　events　designed　to　form　the　Earth。　Instead；　it　looks　as　if　our　planet　was　made；　and　survived；　by　mere　lucky　chance；*　amid　unbelievable　violence。　Our　world　does　not　seem　to　have　been　sculpted　by　a　master　craftsman。　Here　too；　there　is　no　hint　of　a　Universe　made