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Chapter 6 TransNeptunian objects（第1页）

Chapter6　Traus

&ivelysparsepopulationofasteroids，kaurs，existsbetweenJupiteraune。Somearedarkandred，similartothetarry（tholiypeasteroids，butothersarebluer，suggestingthatmuchoftheirsurfacesmaybefreshlyexposedice。Becausetheirorbitseclosetothegiaheyareingferthanabouttenmillionyears。Probably，taursareTNOsthathavebeenscatteredinerhapsbyaterwithheriionslasprobablyheminwardsuntiltheybeeshort-periodets，spendiheinnerSolarSystem，wheretheyareheatedbytheSuheirvolatilesiaculartails。

Sixtrojashavebeendiscoveredeptune’sleadingLagrangianpoint。Dynamicalargumevastdisbianpoints）arojaimesmorehanJupiter’s。

&uheKuiperbeltaNOs。OnefamilyofKuiperbeltobjectstravelin3:2orbitalresohuheclass，whicludesPluto，areknolutinos’，nottobefusedwithPlutoid，whichistheofficialIAUtermforanyToberankedasadlutoidsbeplutinos，classicalKuiperbeltobjects（lagorbitalresohune），orScatteredDiskobjeai。ClassicalKuiperbeltobjeowivelyas‘os’（‘QB1-os’）becausethefirstKuiperbeltobjecttobediscoveredafterPlutoboretheprovisioion1992QB1。

Plutoand

&iesofmostTNOsarepoorlykoanditssatellitearesuffitlylargeaohavebeeelescopicallyforseveraldecades。Frehane，anddioxidehavebeeroscopiPluto，atelesagesrevealdarkpatchesthatareprobablytholin-richresidues。Pluto’sdensitysuggeststhatrockmustbeabout70%ofitstotalmass，andmostlikelyitisinternallydiffereharockydfeasiblyaniron-riercore）overlainbyamalyofwater-icetoppedbyamorevolatile-richcrust。

Nearperihelion（ere1989），Plutohasamospherepossiblyderiton’s。BecausePluto’sgravityissoweak，animaginaryshelleng99%ofitsatmospherewouldextendto300kilometresabovethesurface，whereasfortheEarththeequivaleisonly40kilometres。MuchofPluto’satmosphereisexpectedtoseontothesurfacewhiledistaheSunincreasesfrom4。5billioperihelionto7。4billioaphelionin2113。Itisapitythatwemissedthecetostudy

Plutofredurihefly-pastbyNASA’sNewHorizonmissiontoPlutowillo2015，bywhichtimemuospheremayhavesedahe‘perma’surfaeathaseasonalshroudeo’s6。4-dayrotatiohesameastheorbitalperiodestsatellite，，whichalsorotatessynously。Thisrelationshipisaresulttides，aPlutoandahesamefacestowardseachother。Plutoismoreevenlymatsizeandmasstothaordlasestsatellite。assisabout12%ofPluto’s，anditorbitsatadistanlyabout17PlutoradiifromPluto’sparison，theMoon’smassisonly1。2%ofEarth’sanditsorbitalradiusis60Earthradii。’sproximitytoPlutoexplainswhyitremaiil1978。Pluto’stwosmallersatellites，NixandHydra，weredis2005。TheseorbitinPluto’sorbitalplaneio4:1and6:1orbitalresoh。

SeenfromPluto’ssurfawouldlookeighttimeswiderthantheMoondoesfromEarth。Becausetheirrelativemassesaresosimilar，theiroreofmass（their‘barytre’）isobutatapointihetwobodies。Althoughdoubleasteroidssuchas（90）AntiopeanddoubleKuiperbeltobjectssuchas2001QW332（200-kilometrediametertwins）areknown，Pluto-isthemostevenlymatchedpairamongbhtotasplasord>

’ssurfaier-icewithtraonia。ItsdensityislessthanPluto，butstillsuffitforasubstantialrockycore。ayturnouttobearelativelypassive，heavilycrateredglobe，lutomightimpressusallbybeinggeologicallyactive，asthevarietyinitssurfacematerialssuggests。

Oheremaybeareasonfororeaitslarger。ThisisbecausePluto’saxialinis119。6°（beihan90°，thismeansitsrotatirade）。’sorbitisexaPluto’sequatorialplane，sosharesthehighinrelativetotheirjointorbitabouttheSuidalpullsontheSunandPlutomightfeasiblybestroocausemeltihinahisisthewearefacedwiththeintriguiofaEuropa-likesurfacefordeveiallylife-bearihit。Thebesthintwehavesofariraofobtainedin2007whidwater-ithatisstilliallineform，intrasttotheamorphoussubmicroscopicstateoficethathasbeeosolarultravioletradiationandientformorethahousahesimplestexplanationforthisisgeysers，sprayingoutfreshitheinterior，liketheplumesonEnceladus。

&

&sPlutoaotherTNOsasrahetime。Ofthese，Eris，Makemake，andHaumeaareofficiallyreizedasdlaerisflatteherbecauseofitsrapidrotatiohan4hfrom。TheseareclassicalKuiperbeltobjects，exceptErisand2007OR10（ScatteredDiskobjects），2002TC302（5:2orbitalresohune），Ixion（plutino），andSedna，whiodditywaybeyoeredDisk，inahighlyellipticalorbithelionat975AU。

&helargesttraus

ApartfromPluto，thesizesoftheseobjectsarepoorlykhoseforwhigleroundnumberisgiveheirdimeesbasedonassumptionsabouttheiralbedeoftheisunlighttheyreflect）。Iftheyarelessrefleassumed，theymustbelarger，butiftheyaremorereflective，thebesmaller。Sizeestimatesbeimprovedbymeasuringthermalradiationfromtheirsurfaces，buttheyaresocold（–230°Corless）thatthisbealybytelesspace，abovetheEarth’satmiveies，itisualloftheseobjectswillsurviveien’lists。

TNeinred（probablywidespreadtholinsacrosstheirsurfaces）toblue-grey（exposediorphouseaisoheblue-greyosmass（derivedfromtheorbitsofitssatellites）showsthatitsdensityisgreaterthanPluto’s，soitmusthavearelativelyhigh。OnQuaoar，eidammoniahydratehavebeeroscopically，suggestiresurfagargumentssimilartothoseadvancedfor）。Thiswouldrequireeithergeologicalaajorimpattogeasuffitlywidespreadtodomirum。

&ween2%and3%ofTohavesatellites，whichissimilartotheabueroidswithsatellites。TheproportiohelargerTNOsandposesproblemsintryingtoatfin。

IfNASA’sNewHorizonmissionremaierits2015fly-byofPluto-，itwillbedirewardstoamoredistantThasyettobedetermiideallywillbeablue-greyobjetrastwithPluto’sreddishnature。

Atrau?

MostastrothatwehavediscoveredallthelargeobjegingtoourSolarSystem。ly，thereothingofplaarysizehidingintheKuiperbelt。IfsuobjectwerepreseheKuiperbeltwouldbeunstable。However，thereremaintwopossibilitiesforanoutlyi（popularlydubbed‘Plahathaveegoneaway。Othereisah-massobjeindetricorbitbetween80and170AUfromtheSun。Thepresenceofsuchalargebiwardsbyaterwithune）couldexplainanobservedsuddendrop-offiionoftheKuiperbeltbeyond48AU，knownasthe‘Kuipercliff’。Itmightalsoatfortheextremesgevidencedbyobjectssua。

Thesedpossibilityarisesbeg-periodetsiiallyfromaparticularregioherthaninginfromraions。IthasbeeheseweredislodgedfromtheOortCloudbyaJupiter-massbodyabout32，000AUfromtheSun。Thiswouldbehard，butnotimpossible，todetectbytelescope。A‘pla’sofaroutbegravitationallyboucouldbejustadererthroughiellarspace，possiblyestheplaemofaar。