Our Unknown Solar System assignment 1Word格式文档下载.docx
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Our Unknown Solar System assignment 1Word格式文档下载.docx
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Onceuponatime,4.6billionyearsago,somethingwasbrewinginanunremarkablebackwateroftheMilkyWay(银河).Theragbagofstuffthatsuffusestheinconsequential,in-betweenbitsofallgalaxies(星系,银河系,银河)-hydrogen(氢)andhelium(氦)gaswithjustasprinklingof(少量,一点儿)soliddust-hadbeguntocondenseandformmolecules.Unabletoresistitsownweight,partofthisnewlyformedmolecularcloudcollapsedinonitself.Intheensuingheatandconfusion,astarwasborn-oursun.
Wedon'
tknowexactlywhatkick-startedthisprocess.Perhaps,withpleasingsymmetry,itwastheshockwavefromtheexplosivedeaththroesofanearbystar.Itwasnot,atanyrate,aparticularlyunusualevent.IthadhappenedcountlesstimessincetheMilkyWayitselfcameintoexistenceabout13billionyearsago,andinourtelescopeswecanseeitstillgoingonindistantpartsofourgalaxytoday.Asstarsgo,thesunisnothingoutoftheordinary.
Andyet,asfarasweknow,itisunique.Fromathindiscofstuffleftoverfromitsbirth,eightplanetsformed,trappedinorbitbyitsgravity.Oneofthoseplanetssettledintoapeculiarlytranquilrelationshipwithitsstaranditsfellowplanets.Eventually,creaturesemergedonitthatbegantowonderhowtheirneighbourhoodcametobeasitis-andcouldformulatethefollowingsixenduringmysteriesofourfamiliar,andyetdeeplymysterious,solarsystem.
1.THEBEGINNING
1:
Howwasthesolarsystembuilt?
Lookingattheplanetsofthesolarsystem,youcouldbeforgivenforthinkingthatiftheydobelongtothesamefamily,itisbyadoptionratherthankinship.Notso:
thestoryofthesolarsystem'
sbirthrevealsthattheyarebloodsiblings,allcreatedfromthesamemolecularcloudwhosecollapseformedthesun.Youmightalsothinkthatthesedisparatebodiesarescatteredacrossthesolarsystemwithoutrhymeorreason.Butmoveanypieceofthesolarsystemtoday,ortrytoaddanythingmore,andthewholeconstructionwouldbethrownfatallyoutofkilter.Sohowexactlydidthisdelicatearchitecturecometobe?
Whenoursunformed,itswallowedabout99.8percentofthedebriscloud(星云碎片)aroundit.Accordingtothegenerallyfavouredpicture,theleanpickingsthatremainedweresculptedbygravityintoathindiscofgasanddustencircling(包围,环绕)thenewbornstar'
smidriff(seeillustration).Asthedustgrainsofthisdiscorbitedthesun,theycollidedandprogressivelycoagulatedintoeverlargerbodies.Inthedisc'
sinnermostregion,theignitionandburningofhydrogeninthesunmadethingsveryhot,sothatonlymetalsandsilicatemineralswithhighmeltingpointswerepresentinsolidform.Bodiesinthatregioncouldonlyreachacertainsize-producingthefoursmallrockyplanetsoftheinnersolarsystem:
Mercury,Venus,EarthandMars.
Nosuchstringent(严格的)limitations(限制)existedfurtherout,beyondthe"
iceline"
wheremethane(甲烷,沼气)andwaterarealsopresentassolids.Here,thedevelopingplanetscouldgrowbigger,andbecomelargeenoughtostartaccreting(共生)gasmolecules-mainlyhydrogen-beforeenergyfromthesun'
sincreasingglarerippedthosemoleculesapart.That,ultimately,washowthegasgiantsJupiterandSaturncametobeand,furtheroutinstillcolderclimes,theicegiantsUranusandNeptune.Thatisthereasonastronomers(天文学家)expecttheseplanetstootopossessrockyheartsbeneaththeirfluidcoats.
Sofar,sostraightforward.Butwhenitcomesto(当提到)certaindetails,theaccretionmodelbecomesratherhand-wavy,saysAlessandroMorbidellioftheCô
ted'
AzurObservatoryinNice,France.Forastart,noonereallyknowsexactlyhowmetre-sizedboulderscoalescedintobodiestensofkilometresacross.Solidobjectsthatsmallwouldhavebeenbuffettedaroundbythepressureofthegassurroundingthemandsentspirallingintothesunbeforetheycouldevergettogether.Apromisingidearecentlyproposedisthatlocalpatchesofturbulenceinthegasprovidedvorticesoflowerpressureinwhichtheboulderscouldcollectandcoagulate.
Asimilarproblembedevilsthegasgiants,whosesolidcoresmusthavecoalescedinthepresenceofgastheywouldlateraccrete.Theriskofsuchplanetsbeingbouncedtowardsthesunisillustratedbythe"
hotJupiters"
seeninotherplanetarysystems.TheseareplanetsroughlythesizeofJupiterbutorbitingaroundtheirstarsatthedistanceofEarthorcloser(see"
Isthesolarsystemunique?
"
).Hadanythinglikethathappenedintheearlyyearsofoursolarsystem,theEarthandotherinnerplanetscouldwellhavebeenslingshottedoutofthesolarsystemaltogether-althoughthat'
snocertainconclusion.
AccordingtoPhilArmitageoftheUniversityofColoradoinBoulder,there'
snotmuchsignofanysuchdramainourneighbourhood.Ifevidencesuchasouroverlargemoonisanyindication,theinnersolarsystemdidremainachoppyplaceforitsfirst100millionyearsastherockyplanetsconsolidated(see"
Whyarethesunandmoonthesamesizeinthesky?
),butitsoonsettleddown.AndaccordingtoatheorydevelopedbyMorbidelliandcolleagues,therewasarearrangementandexpansionoftheoutersolarsystemafewhundredmillionyearsafterthesunwasborn,whenaparticularconjunctionoftheorbitsofJupiterandSaturngaveagravitationalshovethatpropelledUranusandNeptuneouttothedistantorbitstheyoccupytoday.SomeofthesmallbodiesthatscatteredonthewayfellbacktowardsJupiter,whoseimmensegravitymayhaveejectedsomeofthemfromthesolarsystem.Deepinspace,theseunaccretedfragmentscollectedasthehypotheticalOortcloud("
Wheredocometscomefrom?
).
Theknock-oneffectofthislastgravitationaltwitchofthesolarsystemmayhavebeenadisturbanceintheasteroidbeltbetweenJupiterandMars,creatingtheLateHeavyBombardmentthatshoweredEarthwithmeteoritessome4billionyearsago,500to600millionyearsafterthesunformed.Sincethen,however,theobjectsthatconstituteoursolarsystemhavesettledintoatranquil,ifsensitive,balance-toourowninestimableadvantage.
2.Whyarethesunandmoonthesamesizeinthesky?
Itisoneofthemostgloriouspiecesofnaturaltheatre.AssumingyouspendyourlifeonthesamepartoftheEarth'
ssurface,youmightwitnessitonce-ifyouareparticularlyluckyorverylong-lived,perhapstwice.Butatotalsolareclipseisworththewait.Attheheightoftotality,thefitofsunandmoonissoperfectthatbeadsofsunlightcanonlypenetratetousthroughtheruggedvalleysonthelunarsurface,creatingthestunning"
diamondring"
effect.
Itisallthankstoastrikingcoincidence.Thesunisabout400timesaswideasthemoon,butitisalso400timesfurtheraway.Thetwothereforelookthesamesizeinthesky-auniquesituationamongoursolarsystem'
seightplanetsand166knownmoons.Earthisalsotheonlyplanettoharbourlife.Purecoincidence?
Almostundoubtedly,saymostastronomers.Butperhapsitisnotsomuchofoneasthebarenumberssuggest.Ourmoonisdifferent.Themanymoonsofthelargeouterplanets-Jupiter,Saturn,UranusandNeptune-arethoughttohaveoriginatedthroughoneoftwoprocesses:
fromtheaccretionofadiscofmaterialintheplanet'
sgravityfield,inaminiatureversionoftheformationofthesolarsystem'
splanets,orthroughthelatergravitationalcaptureofpassingsmallbodies.ThesecondpossibilityisalsothoughttoaccountforMars'
stwosmallsatellites,DeimosandPhobos,theonlyothermoonsintheinnersolarsystem.
ButourmoonissimplytoobigrelativetoEarth'
sownsizetohaveformedeasilybyeitheroftheseprocesses.Planetaryscientistsbelievetherecanbeonlyoneexplanation:
inthefirst100millionyearsofthesolarsystem,whenunattacheddebriswasstillzingingaroundtheinnersolarsystem,aMars-sizedobjectsmashedintoEarth.Thatimpactradicallyremodelledourplanet,expellingahugeamountofdebristhateventuallycongealedintoouroversizedmoon.
Andhere'
sthebestbit.SuchabigmoonisabigboonforlifeonEarth.AsEarthspinsonitsownaxis,ithasanaturaltendencytowobble,owingtothevaryingpullonitfromotherbodiessuchasthesun.Theunseenhandofthemoon'
sgravitygentlydampsthatwobble,preventingrotationalinstabilitieswhichwouldotherwisehavecauseddramaticchangesinEarth'
sclimaticzonesovertime.Suchinstabilitieswouldhavemadeitmuchmoretrickyforlifetogetstartedonourplanet.
Earth'
spositioninthe"
habitablezone"
aroundthesunwhereliquidwaterisabundantisundoubtedlythesinglemostimportantfactorinitsfecundity.Butthepresenceofalargemoon-onelargeenoughtocausetotaleclipses-mightalsohavebeencrucial.Ifso,thathasimportantconsequencesforthesearchforlifeonotherplanets.
Sincetheimpactthatcreatedit,themoonhasbeenmovingsteadilyawayfromus,currentlyabout3.8centimetresperyear.Thedinosaursdidnotseeeclipseslikewedo:
themoonwastooclose200millionyearsago,morethanbigenoughintheskytoblockouttheentiresun.Equally,anyoccupantsofEarthinacoupleofhundredmillionyears'
timewillnotseetotaleclipsesatall,asthemoonwillappeartoosmall.
Ourluckseemstheresultoftwocoincidenttimescales:
thatoftherecessionofanimpact-formedmoon,andthatfortheevolutionofintelligentlife.Ifyoushouldbefortunateenoughtoexperienceatotaleclipseinyourlifetime,con
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