Our Unknown Solar System assignment 1Word格式文档下载.docx

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