The Solar System 81

THE SOLAR SYSTEM

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BIMtN IN IHI X lOt I NI IM MII Of xO0>C locig-loxi »tar. Oxir xol.ir x> >icm began to c'x>l*e when etjuoding debris from a ncarhy Mipctno' a c\ploxion paxxcd ihrough a va\l leniwo' ckiod of gax and dux| morę ihan 4.6 billton ycarx ago. Comprcxxion. crcatcd by ihc dyinjr xlar‘x xhock waxcx. triggered Ihc eollapxc of the pax cloud. and itx o»n 'clf-gra' iiy pallcd il W-pet ber w ilh an mcx itablc iox-rcaxc >n dcn»it> and temperaturę. Wnhrn the Nink of a coxmte eye. the x ix*ud hxd hccome a roiatinp dixk uf pax and vafvri/x-dduxt pramx. fhe hubwould bectwnca

new xur. the wan: the d«'k wouhJ jenerale the

pJanelxaml their v»iclUtcx. the aMcriwd'. and the comct x itut make tip oor xol*/ xy xtem.

Ax ihc dok coofcd. x.inxmx materiał' x‘on-dcnxcd from the cloud: liny cr.nr.- of xilicatcx. ti*cialx. .md kex of uater. carSon dk>Mdc. methune. and ammonia When lhexc e«*ll«dcd. thcy<x»alexxed.

Soononly targerbodio remamed. Ihcncwly formcd Min. xhininp at fuli hrilliance. had drivcn moxl of ihc rcmaming duxt and pax mto inlcrMcIlar xpace by meanxof ihe xoUr wind— Mre.imx of rapidly moxing ułomie particie*— and ihc pei'ixienl prexxurc of itxowo Irghl Sur-xivmg hodtcy eontinucd to colt-Jc and grow.

HeeaułC they cxx»łexeed at differenl dixUnx'cx

from the xun'x heal. Ihey xahed in ecrnipootion. Today in ihe dark fnnget of the xyx»cm. Satc-

ly withm the xun'x graxitat»iivil graxp. arc oS-jecb eompoxed rooxłl> of tx'Cx— the cometx. They circlc unlil diilurbed by a puxxinp xtar. Tbeir akered pulh may Sring łbem nearer to the xun. after wb>eh mx>xt return to remoic rcgionx oftheiroripn

Separating the inner and ooter pł.metx arc the aucrotdx. rock fragment* ranping in xi/c fiom 1.000 km to lcxx than one km aeroxx. Ax(croidx. likc coroctx. arc próbabły xuixivinp IVilxam from ihc carly xolar xyxtcm.

Kour tcrrcxirvd. oi canhlikc. planctx mh.iNi the inner part of oor x»>lar xy xtcm. Lach ix a xol-id xpł»ere wiih a meullie x’orc xurroondcd by xhdlx of xilieatc*. rclaiocly dcplctcd of "da-tdcx. Ali weic Cralcred hy mctcoritcx and aloe woh 'oktanom and carthquakcx tn limex (Xixt, Voleanoex cxhałcd Ihc g.ixex Ihat Seeamc the atmoxphercx ctoting today on Vcnux. Larth. and Marx. I hc atmoxphere of Marx. prinuoly earSon dioyidc. hax a xurfax'e prcvuirc lexx than onc-hurtdrcdth th.it of Kurth. whilc Ihc atmo. xphcrc of Venux. alxo roo*tly CO.. produeex a cruOwng xurface pre»xure nearly a hundred timc» oor owti Mcrcury ix airlexx. xo xmall and ełoxe to the xun th.it itx p.ixex wcrc nuickly h»xt IOXfXKC.

The piani outer planetx coMaln 99 pcrcent of ail materiał in the xołar xyxtem. c\etudmp the xun. Lach tx a hupe xphcro*d of paxex—hydro-pen and heliom wilh xome methanc. ammiwua. walcr. and olher sotuliło Bcncuth ekiud by-«>. paxeoux hydrogen hecomc' incrcaxingly dcnxe—al one point bcx\>m«ng liguid and. al xl.ll greuter deplhx in Jupiter and Salurn. chang-mg lo a meiallie xlalc. Ali four planclx proKiNy h.nc CO«Cx of xiliealex. meiaS. and water. K\ccpł for L’ranux. ihc pani pLanclx radiale mx>rc heal hax'k into xpaee ihan they abxofb

from Ihe xvn. Thix rcsiducof primordi.il cncrgy.

Icft over from iheir crealion. i\ like the warmlh fromdyingcoalx.

Many of Ihc rvful.iT vitellilcx of Ihc pani płunclx—iho»c ihat formcd along wilh ihc planet ax oppoxcd tobcing caplurctl later—arc largo cnough lo Sc conxidercd planet' dkfltriw. The fout larfc mOMH of Jupiter h*vc xilicale C«JC*x and. with Ihc c\ccplion of lo. arc Mir-roundcd by xhcll'of iec and pox\ibly Iniuid Waler. lo. ragirtg with Mil fur and 'ubfur diotidc xolcani\m. ix the ooły body olher Ihan EaUk know n to c\hib«l activc vołeanoex.

Huto. wilh itx łatgc vildlitc. Charon. lakę' on ihc chanictcriMic* of a doubte planet. Pluło hax a thin ,umo'phcrc of ractlune g»x. and bolh bodtcx may b< compowbJ of k<% xoch ax melb-ane. Orbiling at the ouler edge of Ihc pfauKUry realm. Huto m»y wcII Sc a tranMtion object bc-tween planet'and comct'

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