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C-tipe asteroïdes en koolstofhoudende chondrietmeteoriete

C-tipe asteroïdes en koolstofhoudende chondrietmeteoriete


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Ek het net gewonder, wat is die verband tussen C-tipe asteroïdes en koolstofhoudende chondriet (CC) meteoriete? Kom CC-meteoriete van C-tipe asteroïdes af?


Die verskil tussen asteroïdes, komete en meteore word bespreek in Wat is die verskil tussen asteroïdes, komete en meteore?

Sommige groepe asteroïdes is gekorreleer met meteoriet-tipes:

C-tipe - koolstofhoudende chondriet meteoriete
S-tipe - Klipperige meteoriete
M-tipe - Ystermeteoriete
V-tipe - HED-meteoriete

Die S- en C-tipe asteroïde-klassifikasies is afkomstig van die taksonomie wat David Tholen voorstel.

Meteore word gewoonlik beskou as asteroïdes wat die aarde se atmosfeer binnedring. Meteoriete is meteoroorblyfsels wat die grond bereik. In wese begin koolstofagtige chondrietmeteor (e) as C-tipe asteroïdes; hulle is van dieselfde goed gemaak.


C-tipe asteroïde

C-tipe (koolstofhoudend) asteroïdes is die mees algemene variëteit en vorm ongeveer 75% van die bekende asteroïdes. [1] Hulle word gekenmerk deur 'n baie lae albedo omdat hulle 'n groot hoeveelheid koolstof bevat, benewens gesteentes en minerale. Hulle kom meestal voor aan die buitenste rand van die asteroïedegordel, 3,5 astronomiese eenhede (AU) van die son af, waar 80% van die asteroïdes van hierdie tipe is, terwyl slegs 40% van die asteroïdes op 2 AE van die son C-tipe is. . [2] Die verhouding C-tipes kan eintlik groter wees as dit, omdat C-tipes baie donkerder (en dus minder waarneembaar) is as die meeste ander asteroïdesoorte, behalwe D-tipes en ander wat meestal aan die uiterste buitekant van die asteroïde gordel.


Evolusie van die Aarde

9.10.4.2 Geologiese rekords

Koolstofhoudende chondriete, met hul mengsel van smektietklei en diversiteit van gereduseerde organiese verbindings (Pizzarello et al., 2001), is bewyse vir abiotiese sintese van organiese verbindings in die teenwoordigheid van klei en verminderde ysterminerale soos sideriet vroeg tydens die ontstaan ​​van die sonnestelsel. Die koolstofagtige kleierige matriks het oliviene chrondules wat 4,56 Ga oud is (Amelin et al., 2002) en voorafgaande aan die dwarsdeure van kalsiet (Endress et al., 1996) en halite (Whitby et al., 2000), nie jonger as 4,51 Ga nie. Die kenmerkende koolstof-isotopiese fraksie van rubisco, 'n belangrike ensiem in fotosintese, word aangedui deur ligte koolstof-isotopiese verhoudings van amorfe organiese materiaal in die 3.8 Ga Isua-groensteenband van Wes-Groenland (Rosing en Frei, 2004), en mikrofossiele wat vergelykbaar is met sianobakterieë is gevind in chert van die 3.5 Ga Apex Chert van die Warrawoona-groep naby Marble Bar, Wes-Australië (Schopf, 1983). Die oorsprong van die lewe het dus gedurende die 700 miljoen jaar interval tussen 4,5 en 3,8 Ma of ongeveer die eerste 15% van die geskiedenis van ons planeet plaasgevind. Hierdie tydperk is 'n donker tyd in die Aarde se geskiedenis, waarvan die sedimentêre rekords deur metamorfose en subduksie vernietig word. Oppervlaktes van die Maan en Mars wat dateer uit hierdie vroeë tyd in die geskiedenis van die sonnestelsel, openbaar ook 'n ander rede vir die gebrek aan geologiese rekords: die groot bombardement van groter planete deur planeetdiere en ander puin van die ontwikkelende sonnestelsel. Daar word vermoed dat een van hierdie baie groot impak by ongeveer 4,4 Ga ons Maan geskep het deur 'n Mars-grootte impak ('Theia') wat homself en 'n groot fraksie van die Aarde smelt (Halliday, 2004). Die smelt van klippe sou die lewe sowel as enige voorgangsklei of organiese materiaal vernietig het (Maher en Stevenson, 1988). 'N Ander impedansie vir die lewe en sy voorgangers in oppervlakomgewings sou ultraviolet en ander vorme van proteïen-denaturerende straling gewees het van 'n jong son wat deur 'n volledig gevormde atmosfeer en osoonlaag gefiltreer is (Sagan en Pollack, 1974).


Die unieke Flensburg-koolstofagtige kondrietmeteoriet

Flensburg-meteoriet met swart samesmeltingskors. Krediet: A. Bischoff en M. Patzek, Universiteit van Münster.

Die Flensburg-meteoriet het op 12 September 2019 op die aarde geval in 'n gebeurtenis wat deur honderde ooggetuies van Nederland, België, Duitsland, Denemarke en die Verenigde Koninkryk waargeneem is. 'N Dag nadat die vuurbal aan die dag van die hemel gesien is, is 'n klein meteoriet van 24,5 gram in die tuin van 'n huis in Flensburg, Duitsland, naby die grens van Denemarke gevind. Die huiseienaar het die ontdekking van die meteoriet by die Internasionale Meteor-organisasie aangemeld.

Die Flensburg-meteoriet is ondersoek deur 'n kosmochemiese navorsingsgroep wat deur Addi Bischoff van die Universiteit van Münster (Duitsland) gekoördineer is met deelnemende wetenskaplikes uit Europa, Australië en die Verenigde State. Die span het uitgebreide laboratoriumontledings gedoen om die minerale, chemiese en isotopiese samestelling van die meteoriet te beskryf. Hierdie meteoriet word geklassifiseer as 'n unieke soort koolstofhoudende chondriet, 'n klas meteoriete wat beskou word as die mees primitiewe, vlugtige materiale uit die vroeë sonnestelsel. Goedbewaarde monsters van koolstofhoudende chondriete (CC) is deels relatief skaars in meteorietversamelings omdat dit brose (swak) gesteentes is wat moontlik nie deur die Aarde se atmosfeer gaan nie. Die interne struktuur van die Flensburg-meteoriet bestaan ​​uit relikokondrules, trosse sulfied- en magnetietkorrels, en karbonaatminerale in 'n fyn korrelmatriks wat oorheers word deur gehidreerde filosilikaatminerale en organiese verbindings. Die ouderdom van mangaan-chroom (53 Mn- 53 Cr) van die karbonate in Flensburg toon dat die wateragtige verandering aan die moederliggaam plaasgevind het minder as drie miljoen jaar na die vorming van die eerste vaste liggame in die sonnestelsel. Hierdie monsters verteenwoordig die oudste karbonate wat ontleed is in enige meteoriet waarin baie vroeë waterige aktiwiteit behoue ​​gebly het. Ander chemiese en isotopiese samestellings van Flensburg dui ook aan dat dit uniek is in vergelyking met bekende CC-meteorietgroepe.

Die Flensburg-meteoriet kan een van die vele klein, waterryke ouerliggame wat in die vroeë geskiedenis van die sonnestelsel water aan die aarde gelewer het, beproef. Die studie van Flensburg sal ook belangrik wees vir die interpretasie van monsters wat deur asteroïdes Ryugu en Bennu deur die ruimtetuig Hayabusa2 en OSIRIS-REx teruggestuur word. Ryugu en Bennu is a-steroïede van die C-soort, wat as soortgelyk aan CC's beskou word, en albei toon bewyse vir die teenwoordigheid van gehidreerde minerale soos dié in Flensburg. Die ontdekking van unieke CC's soos Flensburg brei ons kennis uit oor die verskeidenheid CC-materiale in die sonnestelsel en het die potensiaal om geskikte analoë vir C-tipe asteroïdes soos Ryugu en Bennu te bied. LEES MEER


Chondriete vorm die mees algemene tipe klipperige meteoriet (die ander hoofsoort staan ​​bekend as achondriete) en is verantwoordelik vir ongeveer 86% van alle meteorietvalle. Hulle bevat gewoonlik kondules, korrels silikaatmateriaal van millimeter grootte wat vermoedelik regstreeks uit die sonnevel gekondenseer is, en dit lyk asof dit feitlik onveranderd gebly het sedert hul vorming in die vroeë Sonnestelsel.

Afgesien van die afwesigheid van waterstof en helium, is die primêre chemiese samestelling van chondriete dieselfde as dié van die oorspronklike sonnevel, wat daarop dui dat hulle nie 'n periode van langdurige of uiterste verhitting sedert hul ontstaan ​​ondergaan het nie. Dit dui daarop dat hulle afkomstig is van nie-gedifferensieerde voorwerpe, soos kleiner asteroïdes, en dat die waargenome variasies in samestelling waarskynlik vasgestel is as die ouerliggaam wat op verskillende afstande van die son afkomstig is van die sonnevel.

Met hierdie samestellingsvariasies kan chondriete onderverdeel word in drie hoofgroepe:

  1. Koolstofhoudende chondriete is die mees ongerepte, en toon geen bewyse dat hulle ooit verhit is nie. Dit kan tot 20 gew.% Water bevat, sowel as aansienlike hoeveelhede koolstof (hoofsaaklik as organiese verbindings soos aminosure) en geoksideerde elemente. Met die hoogste hoeveelheid vlugtige elemente, word vermoed dat dit afkomstig is van asteroïdes wat op betreklik groot afstande van die son af is.
  2. Gewone Chondrites bevat ook geoksideerde en vlugtige elemente, maar in 'n mindere mate as die koolstofhoudende chondriete. Daar word vermoed dat hul ouer-asteroïdes in die binneste asteroïde gordel gevorm het.
  3. Die hoofelementele bestanddeel van Enstatiet Chondrites is yster in sy metaal- of sulfiedtoestand. Dit is in teenstelling met gewone en koolstofhoudende chondriete waarin yster meestal as oksiede voorkom en in silikate vasgebind is. Die hoë metaal, lae suurstofinhoud vir enstatietchondriete dui daarop dat hierdie meteoriete moontlik in die binneste Sonnestelsel ontstaan ​​het.

Alhoewel die primêre samestelling daarvan ooreenstem met die sonnevel, is die chemiese en isotopale samestelling van chondriete deur sekondêre prosesse verander. Die mate waarin die chondriet verander is, word aangedui deur 'n heelgetal tussen 1 en 7. Die chondriettipes 1 en 2 is spesifiek chemies verander in die teenwoordigheid van oorvloedige water, terwyl tipes 3 tot 7 geleidelik meer hitte-geïnduseerde veranderings aan beide die chondrules en die chondrite self.

Bestudeer sterrekunde aanlyn aan die Swinburne Universiteit
Alle materiaal is © Swinburne Universiteit van Tegnologie, behalwe waar aangedui.


Evaluering van die biologiese potensiaal in monsters wat teruggestuur word van planetêre satelliete en klein sonnestelselliggame: raamwerk vir besluitneming (1998)

Asteroïdes, net soos komete, is die oorblywende bevolking van planeetdiere en die klein oerliggame waaruit die planete ophoop. Algemene asteroïdesoorte word in Tabel 4.1 beskryf. Oor die algemeen is die asteroïdes wat beskou word as die relikwie planeetdiere wat in en buite die asteroïde gordel gevorm word (wat tussen 2,2 en 3,2 AE van die son af geleë is), so ver van die son af as die Trojane, wat op afstand van Jupiter wentel. Diegene wat op meer afgeleë plekke gevorm word, word gewoonlik komete genoem, of ten minste beskou as komete. Hierdie hoofstuk ondersoek die oorsprong, samestelling en omgewingstoestande van drie hoofklasse asteroïdes: ongedifferensieerde, primitiewe (C-tipe) asteroïdes ongedifferensieerde metamorfose asteroïdes en gedifferensieerde asteroïdes.

Die historiese definisies van asteroïdes hou verband met die afwesigheid of teenwoordigheid van 'n kometiese aktiwiteit, wat vlugtige verbindings (veral waterys) naby of aan die oppervlak van die voorwerp vereis. Watson et al. (1963) het getoon dat waterys maklik sublimeer in kort tydperke in vergelyking met die ouderdom van die sonnestelsel tot op afstand van Jupiter. Daar kan dus verwag word dat voorwerpe gemaak van 'n oermengsel van ys en vuurvaste stowwe wat op asteroïdale afstande van die son gevorm is, oppervlakkige ys verloor het en dat hulle sluimerende "asteroïdes is, "terwyl voorwerpe wat in die binneste sonnestelsel gebuig word vanaf baie meer afgeleë bergingslokale (die Kuiper-gordel, verstrooide skyf en Oort-wolk) sulke vlugtige stowwe op of naby hul oppervlaktes behou en 'kometêre' aktiwiteit toon (comae en sterte) totdat hulle geleef het duisende jare in die binneste sonnestelsel.

Terwyl die afwesigheid van vlugtige oppervlaktes naby die oppervlak om hierdie rede nie die afwesigheid van vlugtige stowwe op diepte binne asteroïdes waarborg nie, bepaal ander faktore of asteroïdes aansienlike hoeveelhede vlugtige stowwe het of nie. Binne 'n afstand van die son kan vlugtige stowwe nooit op planeetdiere gekondenseer word nie. Dit bly 'n kwessie van vermoede en huidige navorsing waar hierdie grens bestaan ​​het (bv. Was die aarde "nat" gevorm [bv. Dreibus en W & aumlnke, 1987] of al die vlugtige stowwe afkomstig was van laat aanvaarde planeetdiere [Chyba, 1987], bv. , komete, van veel verder?). Daarbenewens kan die daaropvolgende termiese evolusie en miskien afhang van die liggaamsgrootte en / of afstand van die son af, en ongetwyfeld sommige asteroïede uitdroog, miskien het dit die meeste asteroïede uitgedroog. Ander prosesse (bv. Doeltreffende megaregolitiese omverwerping van rommel-asteroïdes deur herhaalde botsings) het vlugtige stowwe moontlik ook toegelaat om te sublimeer. Spektrale refleksie-studies en ander afstandwaarnemingstegnieke toon dat sommige asteroïdes waarskynlik bestaan ​​uit 'n droë reeks minerale (bv. Metaalliggame). Alhoewel dit vermoedelik is, sal die meeste navorsers nie verbaas wees as baie asteroïdes vanaf die middel van die asteroïde gordel tot by die Trojane kortliks soos komete lyk soos skaars katastrofiese, ontwrigtende botsings wat begrawe vlugtige stowwe blootstel nie. ('N Minderheid, maar 'n beduidende fraksie van voorwerpe wat asteroïdes genoem word, veral in baan om die aarde, kan sluimerende of dooie komete wees.)

Vir byna twee eeue het dit vir die wetenskaplike gemeenskap redelik gelyk dat meteoriete van asteroïdes kan kom. Maar al in die 1970's was daar geen bekende fisiese meganismes om asteroïedfragmente na die aarde te vervoer nie. Byvoorbeeld, botsings wat voldoende is om sulke ingrypende baanveranderings te bewerkstellig, sou die asteroïdale materiaal verdamp. Die probleem is nou opgelos, behalwe vir die besonderhede (Wisdom, 1985). Dwarsdeur die asteroïedegordel is daar gebiede waar resonante swaartekragversteurings deur planete, hoofsaaklik Jupiter en Saturnus, dinamies chaotiese sones vorm. Wanneer inter-asteroïdale botsings naby die grense van hierdie sones fragmente daarin stuur, word wentel-eksentrisiteite vinnig vermeerder en die fragmente kruis die wentelbane van die ander planete en die aarde insluitend. Fragmente bereik die aarde hoofsaaklik vanaf die binneste dele van die asteroïde gordel, veral naby die 3: 1-waardigheid met Jupiter (dié in die buitenste gordel bereik Jupiter eerste en word gewoonlik uit die sonnestelsel uitgegooi). Aardoïde-kruisende asteroïdes het dikwels steeds aphelia in die asteroïde gordel, en dus ly hulle steeds botsings met asteroïede in die hoofriem. Die kleiner fragmente wat die aarde as meteoriete teëkom, is die gevolg van 'n multi-generasie botsing waterval en kom direk van die resonansies in die hoofband en van kraters en botsings waarby Asteroïdes deurkruis.

Enkele meteoriete kom van die Maan en Mars (Warren, 1994). Sommige meteoriete kan afkomstig wees van komete (Campins, 1997) of van asteroïede wat verder geleë is, maar niemand is tot dusver geïdentifiseer as waarskynlike kandidate nie, en daar is fisiese redes wat daarteen versag (die hoë snelheid en die swak sterkte van komete lei tot die boonste atmosfeer. disintegrasie van inkomende kometêre meteoroïede is nie effektiewe dinamiese meganismes vir die aflewering van asteroïdale puin uit streke buite die 5: 2-resonansie geïdentifiseer nie). Die botsing waterval genereer ook fyner materiale so klein soos interplanetêre stof, wat vervoer word deur Poynting-Robertson en ander stralingskragte (Burns et al., 1979). Die relatiewe bydrae van komete en asteroïdes tot deeltjies van verskillende groottes in die interplanetêre stofkompleks is nie bekend nie, maar albei bronne dra 'n beduidende fraksie by (Bradley et al., 1988).

TABEL 4.1 Algemene asteroïdesoorte

Ongedifferensieerde C-agtige tipes

Baie lae albedo, plat lang van 0,4 mikromabsorpsieband in UV en soms naby 3 micrometer

Lae albedo C-agtig maar helderder, meer neutraal

Lae albedo C-agtige maar helderder, sterk UV

Ongedifferensieerde gemetamorfiseerde tipes

Matige albedo, sterk absorpsie naby 1 & microm en 2 & microm

Matige albedo, rooierig in sigbare, swak tot matige absorpsie naby 1 & microm en 2 & microm

Gedifferensieerde tipes

Matige albedo, effens rooierige helling

Hoë albedo, soos S-tipes, maar sterker en bykomende absorpsies

Hoë albedo, sterk absorpsies as gevolg van olivien

Matige albedo, rooierig van sigbare, swak tot matige absorpsie naby 1 & microm 2 & microm

Hoë albedo, plat of effens rooierig

Baie lae albedo, effens rooierige helling

Baie lae albedo, rooierige helling

OPMERKING: Daar is ander asteroïdesoorte gedefinieer wat nie in hierdie tabel opgeneem word nie, byvoorbeeld F-soorte. Sommige is onderafdelings van die gelyste soorte. Ander is skaars, nuwe soorte, wat gewoonlik net onder die bevolking van baie klein asteroïdes gesien word. Sommige geklassifiseerde asteroïdes kan atipies van hul klas wees (bv. M-tipes met 3 en mikromabsorpsie-eienskappe) of kan verskillende meteorietanaloge hê, hangende die beskikbaarheid van beter data (bv. M-tipes wat nie deur radar ondersoek word vir hoë radarreflektiwiteit nie, kan ongedifferensieerd wees. enstatiet chondriete as 'n meteoriet analoog).

Die vroeë geskiedenis van asteroïdes is in onsekerheid gehul. Daar word geglo dat 'n planeet weens die invloed van massiewe Jupiter nooit in die asteroïedegordel toegeneem het nie. Die neigings en eksentrisiteite van asteroïde-bane beteken deesdae dat asteroïdes gewoonlik teen 5 km / sek. Bots, wat lei tot kraterasie en katastrofiese fragmentasie, nie tot aanwas nie. Maar om die grootte van die groter asteroïede te laat groei (honderde tot byna 1 000 km in deursnee), moes die snelhede een keer baie laer gewees het, soos in ander planeetaanwasgebiede. Daar is groot leemtes in die verspreiding van asteroïdes waar planeetdiere eens moes bestaan, maar nie meer nie, as gevolg van resonante versteurings deur Jupiter (dit sluit nie net die Kirkwood-vergelykingsgapings in die asteroïedegordel in nie, maar ook die sogenaamde sekulêre resonansies, en die groot hoeveelhede ruimte buite 3,2 AE waar asteroïdes nou skaars is en vermoedelik vroeg in die sonnestelselgeskiedenis uit die weg geruim is). Miskien het botsings (en naby gravitasie-ontmoetings, as dit groot genoeg was) deur die nou verdwynde liggame en ander in Jupiter se aanwasgebied (Jupiter-verspreide planetesimale) die snelhede van die oorblywende hoofgordelsteroïede verhoog. Alternatiewelik kan joviaanse resonansies, wat gedurende die laat stadiums van Jupiter se groei deur die asteroïedestreek migreer, dit moontlik gedoen het terwyl asteroïdale planeetdiere die snelhede van baie asteroïede aangevul, uitgewis en / of opgepomp het (Ruzmaikina et al., 1989). Die asteroïdes word ook uitgeput deur fragmentasie van botsings, maar onlangse navorsing dui daarop (maar nog nie finaal nie) dat botsings ondoeltreffend is om asteroïdes te ontwrig (Asphaug et al., 1998) en waarskynlik nie meer as 'n geringe rol gespeel het om die oorspronklike planeet te verminder nie- massa in die asteroïdale streek tot ongeveer die 0,01 persent wat vandag nog oorbly.

Wat ook al die presiese scenario vir asteroïede is, die asteroïde-bevolking is ongeveer 4 miljard jaar lank ongeveer soos vandag, met die grootste liggame wat nooit veel groter is as die 950 km Ceres nie. Sedertdien ontwikkel die asteroïdes geleidelik deur botsings. Voldoende energieke botsings breek asteroïdes in stukke en gee die fragmente snelhede wat die ontsnappingssnelheid oorskry, sodat dit in soortgelyke maar afsonderlike heliosentriese wentelbane gaan (groepe asteroïdes in soortgelyke wentelbane word 'families' genoem). Die meeste botsings het egter nie die energie om 'n asteroïde te ontwrig nie, maar is meer as voldoende om die samestellende rotsagtige materiale te verpletter. Daarom word verwag dat die meeste asteroïdes (ten minste 'n groter as 'n paar kilometer in deursnee) na gravitasiegebonde "rommelstapels" verpletter word (Melosh en Ryan, 1997). Uitsonderings kan die veel sterker oorblywende metaalkern wees van gedifferensieerde liggame (dit wil sê liggame wat eenmaal gesmelt het tot die mate dat metaal in hul kerne gesink het en die minste digte silikate op hul oppervlaktes as lawa uitgebars het).

Daar moet op gelet word dat die grootteverspreiding van asteroïdes sodanig is dat die grootste deel van die massa (dus botsende kinetiese energie van impakte) in die grootste voorwerpe is. Die grootste skade word dus op die grootste ruimtelike weegskade aangerig, wat tot growwe rommel lei, analoog aan die maan-megaregoliet. Terwyl oppervlakkige regoliete op die groter asteroïdes bestaan, wat analoog kan wees aan die fynkorrelige maanregoliet, word asteroïde-interieurs nie op fyn skaal "tuingemaak" nie, maar eerder deurmekaar geruk. Daarom, hoewel 'n beduidende fraksie van meteoriete gedeeltes bevat wat sonwindgasse en kosmiese straalspore bevat, wat aandui dat hulle eenmalig vir 'n kort tydjie naby die oppervlak van hul moederliggaam was, moet baie ander materiaal in asteroïdes verwag word. om nooit naby genoeg aan die oppervlak te wees om deur kosmiese strale gesteriliseer te word nie, ten spyte van die botsing van die botsing.

Meteoriete getuig van die botsingsprosesse. Baie is gesteentes (ook bekend as breccias) waarvan die eienskappe die botsings weerspieël wat die asteroïede gesteentes breek, verpletter en aanmekaar sweis. Alhoewel gelokaliseerde smelt soms voorkom, het smelting slegs 'n klein fraksie van meteoritiese materiale beïnvloed en was dit waarskynlik nog nooit voldoende om die hele, of selfs 'n wesenlike deel, van 'n asteroïde te onderskei nie (Keil et al., 1997). Daar is bewyse dat sommige asteroïdes tot baie hoë temperature verhit is (sien hieronder), maar dit het alles baie vroeg in hul geskiedenis gebeur, miskien met die verval van 26 Al binne die eerste miljoen jaar.

Selfs die oorblywende asteroïdale ouerliggame van meteoriete toon bewyse van hoë temperature. Sommige asteroïdes het nie net tot geochemiese differensiasie gesmelt nie (metale kerne, mantels en basaltkorsies gevorm), maar baie ander is ook gemetamorfeer by temperature wat slegs 'n paar honderd grade was om te smelt (McSween et al., 1988), en selfs baie van die mees primitiewe, minste ontwikkelende asteroïdes toon tekens van vroeë hoë temperature. Byvoorbeeld, die meeste koolstofhoudende chondritiese meteoriete het vroeg in die geskiedenis van hul moederliggaam uitgebreide wateragtige veranderinge ondervind (Zolensky en McSween, 1988). Hedendaagse asteroïdes is egter te klein om warm streke na die vroeë periode van vorming te hê of te onderhou. (Die enigste meteoriete wat bekend is met die jong ouderdom van die vorming, kom nou van Mars af.) Asteroïdes word soms in 'n baan wat na die son nader, verdryf, wat hulle egter sal verhit. Sulke liggame sal waarskynlik binne 'n paar minute vernietig word.

miljoen jaar van sulke baanverandering (byvoorbeeld deur in die son te duik of op 'n planeet neer te stort). Daar is eenvoudig geen lewensvatbare scenario om die warm temperatuur binne 'n asteroïde te handhaaf nie.

Die meeste meteoriete wat op die aardoppervlak voorkom, kom gedurende die laaste honderdduisende tot honderde miljoene jare van liggame op metrumskaal wat bevry is van groter liggame (hetsy in die omliggende aarde of in die hoofsteroïedgordel). Daar kan dikwels van sulke meteoriete verwag word om deur bestraling gesteriliseer te word, afhangende van hoe lank hulle as klein, onafhanklike liggame om die Son wentel. Met seldsame tussenposes (duisende tot baie miljoene jare) is dit moontlik dat die aarde deur baie groot liggame geraak word, wat honderde meter tot kilometer groot of groter is (bv. Gehrels, 1994). Sulke voorwerpe word nie deur die aarde se atmosfeer beïnvloed nie, en die projektielmateriaal word verdamp, gesmelt of andersins ernstig beskadig as dit op die aarde se oppervlak raak, op maniere wat waarskynlik die lewensbestaan ​​wat daarin kan voorkom, onherkenbaar is. By tussengroottes en treffersfrekwensies is daar egter liggame meter tot tien meter groot wat die Aarde redelik gereeld tref en kan materiaal wat nooit aan kosmiese bestraling onderwerp is nie, redelik sag op die aardoppervlak (teen terminale snelheid) aflewer. .

In die bespreking van verskillende soorte asteroïdes en meteoriete in hierdie hoofstuk, word aanvaar dat daar 'n fraksie van asteroïdale materiale bestaan ​​wat nooit aan 'n dodelike dosis kosmiese bestraling onderwerp is nie en dus 'n sluimerende lewe kan bevat. Die moontlikheid bly egter dat natuurlike bestraling van langlewende radionukliede voldoende kan wees om die gesteriliseerde oorblywende lewe selfs in die begrawe, beskermde gedeeltes van asteroïede te hê (sien hoofstuk 1 ook Clark et al., 1998). As dit die geval is, is baie van die moontlike gevaarlike situasies wat bespreek word, uitgesluit. Die taakgroep merk op dat die meeste asteroïdes nie-vlugtige materiaal bevat met ongeveer kosmiese samestelling op klein ruimtelike skale en dat hul binneste eenvormig aan sulke lae vlak bestraling onderwerp sou gewees het. Die uitsondering is gedeeltes van sterk gedifferensieerde voorwerpe wat sterk uitgeput is in radioaktiewe elemente of gedeeltes van ongedifferensieerde voorwerpe wat ys sakke bevat.

Die verband tussen asteroïdes van verskillende soorte en verskillende soorte meteoriete is onvolledig en nie sonder omstredenheid bepaal nie. Die volgende algemene opsommings moet voldoende wees vir die doel van hierdie verslag. Vanweë die botsingsmengingsproses onder asteroïdes, wat lei tot die aanwesigheid van 'n klein persentasie litiese fragmente (xenoliete) van ander liggame, kan verwag word dat byna enige asteroïde 'n klein komponent van 'n ander soort bevat (sien hoofstuk 2). Hierdie faktor is nie word hieronder bespreek, wat slegs die inheemse materiale van die verskillende soorte asteroïdes en meteorietouerliggame in ag neem.

ONGESKEIDTE, PRIMITIEWE (C-TIPE) ASTEROIDE

A-steroïede van C-tipe is baie swart voorwerpe, wat gewoonlik slegs 3 tot 5 persent van die sonlig voorkom. Hulle het weerkaatsingsspektra wat relatief neutraal van kleur is in die sigbare en naby-infrarooi kleur, behalwe 'n prominente absorpsiefunksie (slegs in sommige van hulle) naby 3 mikrometer, hoofsaaklik as gevolg van hidrasiewater (Jones et al., 1990) . Hierdie kenmerke is ongeveer tipies van laboratoriumspektra van koolstofagtige chondritiese meteoriete (Feierberg et al., 1981). Daar is geringe afwykings in ooreenstemmende meteoriet- en asteroïdespektrums, en ook klein variasies tussen asteroïdespektrums wat gelei het tot addisionele klasse asteroïdes (G- en B-tipes word saamgevoeg met die C-tipe asteroïdes).

Alhoewel dit nie noukeurig bewys word nie, is dit waarskynlik dat die C-tipe asteroïdes (wat oorweldigend die meeste in die hoofband is, veral die middelste en buitenste dele) in verskillende meteorietversamelings 1 deur koolstofhoudende kondriete voorgestel word (Feierberg et al. ., 1981). Die monsterneming van sulke asteroïede deur koolstofagtige meteoriete is waarskynlik bevooroordeeld teenoor nabygeleë resonansies (soos die 3: 1), eerder as die buitenste gordelasteroïede; dit is waarskynlik dat die oorgrote meerderheid koolstofhoudende kondriete uit minder as tien O-liggame van C-tipe, hoewel klein waarskynlikheid van 'n baie groter monster waarskynlik is. Omdat die relevante spektrale reflektansiedata nie baie van die uitgesproke absorpsie-eienskappe het wat diagnosties is vir die samestelling nie, is die vergelyking

C-tipe asteroïdes word waarskynlik in meteorietversamelings voorgestel deur petrologiese tipe 1 en 2 koolstofhoudende kondriete. Die tipe 1 en tipe 2 koolstofhoudende chondriete is die waterige veranderende meteoriete soos voorgestel deur die CM2 Murchison en CI1 Orgueil. Die tipe 3 koolstofhoudende chondriete soos CM3 Allende het slegs baie klein waterige veranderings. Op grond van spektrale kriteria kan tipe 3 koolstofhoudende chondriete as S-tipe asteroïdes geklassifiseer word (Gaffey et al., 1993a, b).

tussen koolstofhoudende chondritiese meteoriete en C-tipe asteroïdes is minder sterk as vir sommige ander spektraalsoorte.

Die meeste C-tipe asteroïdes wat vandag waargeneem word, is waarskynlik fragmente van botsings tussen vroeëre generasies van ietwat groter asteroïdes, hoewel die werklike grootteverspreiding in die oer-asteroïde gordel onseker is. Die eienskappe van koolstofhoudende chondriete dui daarop dat die diameters van hul ouervoorwerpe in die orde van 100 km is, maar groter voorwerpe kan nie uitgesluit word nie. Let op dat die grootste C-tipe vandag, Ceres, byna 1 000 km in deursnee is.

Op grond van meteoritiese bewyse is C-tipe asteroïdes van stof afgevat, waarvan die chemiese samestelling naby was aan die gemiddelde materiaal van die protosolêre stelsel vir die nie-vlugtige elemente, plus vlugtige materiaal in die vorm van waterys (en moontlik selfs vlugtiger ys) en organiese materiaal (Bunch en Chang, 1980). Die chemiese en fisiese vorm van daardie organiese materiaal is onbekend, maar ten minste was sommige waarskynlik teenwoordig as molekulêre fragmente in ysmantels wat stofkorrels bedek, en daar word voorspel dat sulke strukture in digte interstellêre wolke sal vorm soos wat die sonnestelsel gebaar het ( Greenberg, 1984). In hierdie sin kan asteroïdes van die C-soort dieselfde wees as die nie-vlugtige fraksie komete.

Koolstof-chondriet petrologie onthul dat ten minste sommige C-tipe asteroïdes kort na aanwas verhit is tot bo die smeltpunt van waterys (Zolensky en McSween, 1988). Die verwarmingsmiddel is nie bekend nie, maar dit kan die verval van varsgemaakte 26 Al en / of 60 Fe wees, of moontlik induktiewe verwarming veroorsaak deur 'n sterk vroeë sonwind. Die vloeibare water wat hieruit voortspruit, is miskien uiteindelik in die ruimte verlore, maar het, ten minste in die groter asteroïdes, lank genoeg gebly om 'n sekondêre, gehidreerde litologie te produseer uit die primêre watervrye silikate, oksiede, sulfiede en metaal wat oorspronklik deur die asteroïde gewerk het (Zolensky en McSween, 1988). Die vloeibare water het ook met die primêre organiese materiaal gereageer, wat die oes van sekondêre organiese verbindings opgelewer het wat vandag in koolstofhoudende chondriete voorkom (Kerridge, 1993). Die duur van die waterige aktiwiteit is onbekend. Teoretiese ramings wissel tot 10 8 jaar, maar nie langer nie (Grimm en McSween, 1993).

Sommige C-asteroïede toon eintlik spektrale bewyse vir gehidreerde minerale op hul oppervlakke, terwyl ander nie. Dit is nie duidelik of diegene wat nie die water van hidrasie bevat nie, by matige hoë temperature gedehidreer het, of dat dit in die eerste plek nooit tot die smeltpunt van ys verhit is nie (of nooit water opgeneem is nie).

Ongedifferensieerde asteroïdes, veral die C-soorte wat ryk is aan organiese stowwe en wat bewyse toon van warm temperature wat aanleiding gee tot waterige veranderinge, is aanneemlike kandidaat-omgewings vir die oorsprong en lewensonderhoud. Soos hierbo verduidelik, het hierdie toestande egter net vir 'n kort tydjie geduur naby die begin van die planetêre geskiedenis. Ten spyte van talle vals alarms, is daar nooit oortuigende bewyse vir antieke organismes in koolstofagtige meteoriete gevind nie. Dit is egter duidelik dat die voorwaardes vir die oorsprong van die lewe, naamlik die aanwesigheid van vloeibare water, organiese materiaal, spoorelemente en 'n energiegradiënt, ten minste kortstondig voldoen is aan sommige, moontlik die meeste C-tipe asteroïdes. 2 Die mate waarin die populasie van meteoritiese organiese verbindings ooreenstem met die behoefte vir die produksie van 'n selfrepliserende stelsel, is tans nog nie bekend nie, maar met die voorbehoud blyk daar geen rede te wees om tot die gevolgtrekking te kom dat die opkoms van lewe in 'n C-asteroïde nie uitgesluit was. Die tydperk van vloeibare water moes egter meer as 4 Gyr gelede geëindig het (Grimm en McSween, 1993), en die vraag is dus of enige gevormde lewe kon oorleef al dan nie. Soos hierbo aangevoer vir alle asteroïdes, is dit duidelik dat gedeeltes van C-tipe asteroïdes beskerm word teen eksterne kosmiese bestraling. Binne die binnekant van sulke liggame is daar 'n lae vlak van radioaktiwiteit van langlewende radionukliede wat waarskynlik slapende entiteite sal steriliseer omring deur nie-vlugtige materiaal van kosmiese samestelling. But material embedded in pockets of ice, which may well exist within some C-, P-, and D-type asteroids, could have been protected from much of that radiation, and so there is the prospect that dormant life may have survived for the ensuing aeons.

Numerous carbonaceous chondrites have been found to contain organic compounds, some of which were apparently synthesized directly on the parent body during a period when aqueous conditions existed (Cronin and Chang, 1992). However, there is no evidence that prebiotic chemistry advanced beyond the synthesis of some of the monomeric molecules thought to be important in the origin of life. There is a rich history of claims of finding evidence of life in a variety of meteorites (see Appendix C), but these indications have all eventually been found to be either artifacts or the result of terrestrial contamination. Thus, although meteorites containing extraterrestrial organic compounds have fallen on Earth throughout its history, there is no evidence that this process has ever inoculated Earth with any extinct or extant organisms.

It is likely that Earth receives considerable cosmic dust from C-type asteroids, but this material has probably been sterilized during its transport time to Earth, although very rarely&mdashmuch more rarely than for comets&mdashdust might land on Earth very shortly after its liberation from an unsterilized portion of an Earth-crossing C-type asteroid. The dominant delivery mechanism of potentially unsterilized C-type material is by meteorites and by the infrequent impact of larger C-type projectiles.

In addition to C-types, there are P- and D-types located predominantly near and beyond the outer edge of the main asteroid belt (D-types predominate among the Trojans, at Jupiter's distance from the Sun). There appear to be no meteoritic analogs for these asteroid types, consistent with their virtual absence in the inner parts of the asteroid belt for which dynamical transport processes have been identified. On the other hand, it is probable that rare fragments of P- and D-type asteroids occasionally reach Earth. It is assumed that P- and D-types are even more primitive than C-types, although this concept is difficult to test. Conceivably their colors have more to do with the state of their surfaces (owing to greater distance from the Sun, lesser collisional environment) than to their interior compositions. For purposes of this report, it is reasonable to consider P- and D-type asteroids as being similar to C-types. But caution is warranted as their nature is truly only a matter of speculation. Given that it is also not proven that a significant portion of P- and D-type material reaches Earth as part of the apparently nonhazardous natural influx, uncontained sample return from such objects should proceed only after some of the unknowns have been resolved.

UNDIFFERENTIATED, METAMORPHOSED ASTEROIDS

Undifferentiated, metamorphosed asteroids are those that were heated to temperatures of less than 1,000 K so that minerals did not segregate in a macroscopic way, but are also dehydrated (if ever hydrated in the first place) and were probably subject to temperatures at which biological materials could not survive. The most common meteorites on Earth, the ordinary chondrites, are fragments of such asteroids. These meteorites are known to be undifferentiated because their bulk elemental compositions are similar to the nonvolatile elements in the solar system (e.g., Wasson, 1985).

There has been a long-standing dispute about which main-belt asteroids to associate with these common meteorites (Chapman, 1996). In all probability, some of the S-type asteroids (the most abundant asteroid type in the inner third of the asteroid belt) are undifferentiated but metamorphosed objects analogous to ordinary chondrites, although a few researchers deny this. Other S-types may be examples of differentiated asteroids of various kinds (see below). In general, the spectra of S-types show more diagnostic absorption features than do spectra of C-types. Their slightly reddish spectra show the clear presence of such silicate minerals as olivine and pyroxene. Relying solely on spectral reflectance data, however, it is not always possible to decide unambiguously whether an asteroid observed telescopically has experienced global differentiation or not. In addition to the minerals olivine and pyroxene, the reddish slope of the spectra suggests the presence of metallic iron together, these minerals constitute the dominant materials in undifferentiated ordinary chondrites. However, the same minerals are also found, in different proportions, in certain differentiated meteorites, and in addition the observed asteroid spectra do not exactly match those obtained in the laboratory from ordinary-chondrite specimens. Because the ordinary chondrites are the most abundant variety of meteorite falling on Earth today, which argues in favor of an abundant type of parent asteroid, it is commonly, though not universally, believed that the spectral differences between ordinary chondrites and some S-type asteroids are due to "space-weathering" of the asteroid surfaces, and that the ordinary chondrites are, in fact, derived from asteroids of spectral type-S (Wetherill and Chapman, 1988).

This issue will be further addressed by the Near Earth Asteroid Rendezvous (NEAR) mission, but in the interim it seems reasonable for present purposes to equate S-type asteroids with ordinary chondritic material, since those S-types that are actually differentiated asteroids have natures, and have undergone histories, even less hospitable to life. After all, the ordinary chondrites come from some main-belt asteroids, even if they are rare, and the mineralogy that the ordinary chondrites have in common imposes quite firm constraints on their prebiotic chemical history.

Based on their chemical and isotopic compositions, known chondrites, other than carbonaceous chondrites, are derived from at least a half dozen undifferentiated asteroids (Rubin, 1997). Even though they show no

evidence for igneous differentiation, most ordinary chondrites exhibit the effects of prolonged heating to temperatures sufficient to cause metamorphism and even, in some cases, incipient partial melting. It is generally believed (McSween et al., 1988) that this metamorphism was caused by internal heating of the asteroidal parent bodies, perhaps by decay of recently synthesized radionuclides such as 26 Al or 60 Fe, and that the most severely heated chondrites resided at the greatest depth within such an asteroid. Those ordinary chondrites that exhibit evidence for thermal metamorphism contain neither detectable organic matter nor hydrated minerals, so that two of the criteria for origin of life are not met. However, a small number of ordinary chondrites, known as unequilibrated ordinary chondrites (UOCs), show minimal evidence for metamorphism and in a few cases contain evidence for modest degrees of aqueous alteration (Alexander et al., 1989) and traces of organic matter (Yang and Epstein, 1983). Thus, some UOCs strictly satisfy the criteria for emergence of life, but the amount of water was apparently very limited (it may have been vapor rather than liquid), and there is no evidence for the kind of complex organic chemistry needed for development of self-replicating systems. Consequently, it seems highly unlikely that life could have originated on a UOC parent asteroid and, by extension, on any undifferentiated but metamorphosed, i.e., S-type, asteroid.

As mentioned above, the NEAR mission should help to resolve the question of whether ordinary chondrites are derived from S-type asteroids, thereby reducing that element of uncertainty, but otherwise, barring the fall of a UOC unusually rich in organics and hydrated minerals, it is unlikely that our understanding of the biological potential of undifferentiated asteroids is likely to improve in the foreseeable future, except through sample-return missions.

DIFFERENTIATED ASTEROIDS

Differentiated asteroids are inferred to be objects, or fragments of objects, that were once heated to the point of partial melting and geochemical segregation of materials. Vesta is a classic example of a largely intact differentiated body. As demonstrated by McCord et al. (1970), Vesta is covered with basalts (represented on Earth by the so-called HED basaltic achondritic meteorites). It is presumed (and, to some degree, observed [see Binzel et al., 1997]) that the basaltic crust overlies an olivine mantle on Vesta. Presumably Vesta has an iron core.

While Vesta is apparently unique as an intact, differentiated asteroid, many other asteroids look like pieces of a smashed-up Vesta, or fragments of smaller differentiated bodies. These include some so-called M-type bodies (the largest of which is 250-km-diameter 16 Psyche) that are apparently iron cores, or fragments of cores, from the interiors of preexisting bodies like Vesta ambiguous inferences of metallic composition from spectral reflectance studies are confirmed, in a few instances, by high (metallic) reflectances of radar echoes (Ostro, 1993). There are other classes of asteroids, including small objects (V- and J-class) that may be fragments of Vesta's crust (Binzel and Xu, 1993), the monominerallic (olivine-rich) A-type asteroids, and the E-type asteroids (iron-poor enstatite) that probably represent mantles or crusts of such differentiated bodies. Their spectra are certainly not compatible with being undifferentiated. In addition, as described in the previous section, some (or even most) S-types may be differentiated bodies, as well.

Generally speaking, the various kinds of metallic, stony-iron, and achondritic stony meteorites are believed to be derived from these, and analogous, kinds of asteroids, generally located in the inner to middle parts of the asteroid belt. In general, these materials have been subjected to long-term heating well above 1,000 K, and water has not been present. They seem to be even less likely to harbor biological materials than are the undifferentiated but metamorphosed asteroids discussed in the previous section.

Until recently, some meteorites classified as achondrites were known to have had a much more complex history, including young ages, and evidence of being derived from unusually large asteroids (not identified in space) where ongoing environments conducive to life could not be ruled out. These achondrites, the so-called SNC meteorites, are now understood to come from Mars&mdasha very large "asteroid," indeed, and beyond the purview of this report. As with Mars (and Earth), differentiation in and of itself does not preclude possible biological activity. It cannot be totally ruled out that there were other large asteroidal objects, not now being sampled by the ever-growing suite of collected meteorites, that might have had conditions leading to the origin and presence of life. However, remnants of any such objects are evidently very uncommon among meteorites striking Earth today.

POTENTIAL FOR A LIVING ENTITY TO BE IN OR ON SAMPLES RETURNED FROM ASTEROIDS

There are a few researchers who maintain that meteorites are only a very selective sample of the asteroids and, indeed, that the proportions of extraterrestrial materials striking Earth vary dramatically with time (Halliday et al., 1990). While these ideas are not widely supported, it is prudent to remain aware that generalities deduced from studies of meteorites and the likely associations of certain meteorite types with common asteroid types may not strictly apply to any particular asteroid. With this caveat, it can generally be stated that sample return from asteroids of the types sampled by the known meteorites evidently presents no known biological threat. Furthermore, both the differentiated and the undifferentiated-but-metamorphosed asteroids (e.g., M-, S-, V-, J-, and Q-types) have histories that appear to preclude the origin of life in the first place. For C-types to harbor dangerous biological materials (not so far identified in C-type meteorites), that material must have survived aeons since conditions suitable for replication ended.

While P- and D-type asteroids (and perhaps other rare, anomalous asteroid types) may be presumed to have histories similar to those of the C-types (and other types) discussed above, the asteroid population is evidently diverse and some mysteries remain. Thus, uncontained sample return from such unusual and/or unsampled bodies would have to await further investigation of their properties.

For many asteroids, the requirements for life to have emerged (presence of liquid water, organic matter, and a usable energy source) were probably met very early in their history. Although the known meteorites derived from such asteroids reveal no evidence of biological activity, those meteorites cannot be regarded as having sampled the entire population of such asteroids. Similarly, although the natural meteorite influx has apparently had no deleterious effect on terrestrial biology, it is not certain that samples of every asteroid type have fallen on Earth. Furthermore, although natural radioactivity present within the asteroidal/meteoritic material would have been adequate to sterilize any dormant organisms possibly present within the lithic fraction of such objects, if pockets of relatively pure water ice were to exist within an asteroid of this type, attenuation of the natural radiation field within that ice could in principle have permitted survival of putative dormant organisms.

Based on the task group's current knowledge of the origin and composition of asteroids, the answers to the assessment questions employed in this study are as follows:

Does the preponderance of scientific evidence 3 indicate that there was never liquid water in or on the target body?

There is unequivocal evidence for liquid water active within at least some C-type asteroids approximately 4.5 Gyr ago. A minor fraction of S-type asteroids may have experienced a transient episode of aqueous activity, but the great majority of S-types have never seen liquid water. Liquid water can also be ruled out for M-, V-, and E-type asteroids. For P- and D-type asteroids there is no evidence one way or the other regarding the presence of liquid water.

Does the preponderance of scientific evidence indicate that metabolically useful energy sources were never present?

There is no evidence one way or another regarding the presence of metabolically useful energy sources in other asteroid types.

Does the preponderance of scientific evidence indicate that there was never sufficient organic matter (or CO2 or carbonates en an appropriate source of reducing equivalents) 4 in or on the target body to support life?

In most asteroids (especially C-types), there was some (or even an abundance) of organic matter. In others, especially the metamorphosed and differentiated asteroids, there was not.

For the purposes of this report, the term "preponderance of scientific evidence" is not used in a legal sense but rather is intended to connote a nonquantitative level of evidence compelling enough to research scientists in the field to support an informed judgment.

For the purposes of this report, CO2 or carbonates en an appropriate source of reducing equivalents is equivalent to "organic matter" to accommodate chemolithoautotrophs.


Spectroscopy of K-complex asteroids: Parent bodies of carbonaceous meteorites?

This is the first focused study of non-Eos K asteroids. We have observed a total of 30 K-complex objects (12 K-2 Sk- and 13 Xk-type asteroids (from the Bus taxonomy), plus 3 K-candidates from previous work) and we present an analysis of their spectral properties from 0.4 to 2.5 μm. We targeted these asteroids because their previous observations are spectrally similar enough to suggest a possible compositional relationship. All objects have exhibited spectral redness in the visible wavelengths and minor absorptions near 1 micron. If, as suggested, K-complex asteroids (including K, Xk, and Sk) are the parent bodies of carbonaceous meteorites, knowledge of K-asteroid properties and distribution is essential to our understanding of the cosmochemical importance of some of the most primitive meteorite materials in our collection. This paper presents initial results of our analysis of telescopic data, with supporting analysis of laboratory measurements of meteorite analogs. Our results indicate that K-complex asteroids are distinct from other main belt asteroid types (S, B, C, F, and G). They do not appear to be a subset of these other types. K asteroids nearly span the range of band center positions and geometric albedos exhibited by the carbonaceous chondrites (CO, CM, CV, CH, CK, CR, and CI). We find that B-, C-, F- and G-type asteroids tend to be darker than meteorites, and can have band centers longer than any of the chondrites measured here. This could indicate that K-complex asteroids are better spectral analogues for the majority of our carbonaceous meteorites than the traditional B-, C-, F- and G-matches suggested in the literature. This paper present first results of our ongoing survey to determine K-type mineralogy, meteorite linkages, and significance to the geology of the asteroid regions.


Differentiation: chondrites & achondrites

When I first started learning about meteorites, I seemed to always be getting the terms "equilibrated" and "differentiated" mixed-up. Both terms can seem to describe a similar process at first glance, even while they are really quite different. If you've ever found it difficult to keep these terms straight, or just want to learn a little something about planetary science, this is pretty good place to start.

A good understanding of what differentiation actually is, will likely resolve the confusion. I'll get into the term 'equilibrated' in another post that tackles 'chondrites'. For this post though, we'll stick to the process of differentiation.

Keeping it simple, differentiation, also known as 'planetary differentiation' is the process of an asteroid accreting (growing) to the point that most of the metal distributed throughout its mass is pulled by gravity to its center to form a 'core'. This leaves the lighter rocky material to float above the iron-nickel core forming a 'mantle'. This little bit of information is a keyhole that allows us to glimpse just how intertwined planetary science and meteorites truly are.

So, what is a meteorite really? A meteorite is just a piece of an asteroid or planet that strikes another asteroid or planet, often ejecting bits of what it hit out into space. In a seemingly never ending cycle of cause-and-effect, one meteorite begets another meteorite and so on and so forth. This means a meteorite will either be ejected from a larger differentiated parent body or a smaller undifferentiated parent body.

Leaving out massive collisions that utterly destroy one or both of the bodies involved in a cosmic smashup, a meteorite will generally originate from the surface of an asteroid or planet it's ejected from. If the metal has been gravitationally pulled to the center to form a core, there will logically not be much metal in a meteorite originating from its surface.

Of all the types of meteorites known to us here on Earth, the most commonly found are known as 'chondrites'. There are many different kinds of chondrites, but the one thing nearly all of them share in common is something known as a 'chondrule'. All chondrites are essentially made of chondrules stuck together during the process of accretion and are considered to be undifferentiated. This is because their parent bodies have not undergone the process where metal separates (or differentiates) from silicates to form a core and mantle, a process that also destroys (or encrypts) the chondrules.

Any meteorite that is not a chondrite is technically an 'achondrite'. The prefix "a" will typically indicate "without" in scientific terms. So a meteorite with the designation of 'achondrite' signifies a meteorite that has no chondrules. This is important to note when trying to get a handle on differentiation because it is this very process of differentiation that creates the achondrites. And conversely if an asteroid never gets big enough to undergo the process of differentiation, it remains chondritic.

If we accept the idea that the entire Solar System was created or 'accreted' from the same giant molecular cloud of gas and dust particles, then we can make the leap to say that until any given accreted body in our Solar System grows large enough to undergo differentiation, it remains a potential chondritic meteorite parent body. When one of these same chondritic bodies in our Solar System grows large enough to undergo the process of differentiation, it can no longer produce chondritic meteorites, transforming instead into a potential achondrite parent body.

This is an extremely simplistic way of looking at planetary differentiation that does not take volatiles and gasses into account. It's also a very meteorite-centric way of looking at the Solar System and it's otherwise very diverse group of asteroids and planets. However, it is essentially a valid and useful distinction.

When ejecta is created by an impact event, some of which is ultimately destined to become a meteorite when it hits the Earth, if it is ejected from the surface of a smaller asteroid that hasn't undergone differentiation, it will be a chondrite. This means that as a rule all chondrites are undifferentiated because their parent bodies have not undergone the process of differentiation and their chondrules remain intact.

When ejecta from the surface of a larger asteroid or planet that has undergone differentiation hits the Earth, it will have had most if its metal removed and chondrules eliminated, making it an achondrite.

This, in a very simplified nutshell, is the process of differentiation and how it relates to the distinction between chondritic and achondritic meteorites.


Excess of l over d Structures

Researchers had previously reported small excesses of l over d structures in amino acids in carbonaceous chondrites. Glavin and colleagues confirm these excesses (see graph below). It appears that the percentage of l -isovaline is greater in the more aqueously altered chondrites. Orgueil, SCO 06043, GRO 95577, and Murchison all have l excesses well outside experimental uncertainties (shown by the bars in the figure). In the other, less altered samples, the l excess is zero within experimental uncertainty. A concern with an l excess is that it is caused by contamination once the meteorite landed on Earth, where amino acids are almost entirely l . However, in a 2009 paper, Daniel Glavin and Jason Dworkin address the analytical and contamination issues in detail. They conclude that contamination of the interiors of the meteorite samples is unlikely the l -isovaline excess is of extraterrestrial origin and not an analytical artifact. They also point out that the concentrations of isovaline on Earth are very small. Contamination of the other amino acids is thus more likely, and those that make up proteins have l / d of 1, indicating no contamination.

Glavin and coworkers conclude that the processes involved in aqueous alteration cause the l -isovaline excesses. Researchers had thought that the small excesses measured previously were caused by pre-accretion processes in the solar nebula, such as irradiation with ultraviolet light. While possible, the correlation between aqueous alteration and l excess indicates a role for alteration by water in the parent asteroids of carbonaceous chondrites.

This chart shows l excess  [ l /( d + l ) isovaline, expressed as percent]  for each chondrite studied, listed in decreasing amount of aqueous alteration from left to right. Experimental uncertainties are shown by the bars and are based on standard deviations of between 8 and 23 analyses for each chondrite. The four samples on the right are not enriched in l -isovaline within experimental uncertainties. The four on the left are clearly enriched.


Researchers support naming a new CY group of carbonaceous chondrite meteorites that may be similar to near-Earth asteroid Ryugu, the target of sample return in December 2020.

Ashley J. King (Planetary Materials Group, Natural History Museum, London) and colleagues analyzed six carbonaceous chondrites that share mineralogical, textural, and chemical similarities that are enough, they say, to warrant designation as a distinct, new chemical group, the CYs ("Yamato-type"). This was a conclusion also expressed in 1992 by Yukio Ikeda (Ibaraki University, Japan) who reported results from a consortium research effort on the inaugural three samples of the group, which were shown to have distinct oxygen isotopic compositions but some mineralogical and chemical similarities to CI and CM groups.

The meteorites were collected in Antarctica by the National Institute of Polar Research of Japan. Listed in the official Meteoritical Database currently as either CI or ungrouped hydrated (of petrologic type 1 or 2), these meteorites are among the most aqueously altered types. Data links from the Meteoritical Database:
Yamato 82162, Yamato 86029, Yamato 980115, Yamato 86720, Yamato 86789, and Belgica 7904.

One of the shared characteristics of this group of meteorites is the depletion of volatile elements relative to the concentrations in CI chondrites. This plot shows trace element concentrations in the six meteorites designated as the CY group (blue and green lines) compared to the average abundances for CM chondrites (red line) relative to CI chondrites.


The carbonaceous chondrite meteorites in this study record a period of intense aqueous alteration that was followed by at least one thermal metamorphic event at temperatures greater than 500 degrees Celsius. Moreover, King and coauthors say the short cosmic-ray exposure ages (≤ 1.3 million years) suggest these meteorites are from a near-Earth source. They point to a relationship of CY chondrites to C-type asteroids and suggest CY chondrites are good analogues for asteroid 162173 Ryugu. This idea will soon be tested! The Japan Aerospace Exploration Agency's Hayabusa2 spacecraft has finished sampling Ryugu and is en route to Earth, scheduled to release its re-entry capsule holding its collection of asteroid regolith to us in December 2020.

See Reference:
·   King, A. J., Bates, H. C., Krietsch, D., Busemann, H., Clay, P. L., Schofield, P. F., and Russell, S. S. (2019) The Yamato-type (CY) Carbonaceous Chondrite Group: Analogues for the Surface of Asteroid Ryugu? Geochemistry, doi: 10.1016/j.chemer.2019.08.003. [abstract]

See also:
·   Ikeda, Y. (1992) An Overview of the Research Consortium, "Antarctic Carbonaceous Chondrites with CI Affinities, Yamato-86720, Yamato-82162, and Belgica-7904," Procedings NIPR Symp. Antarctic Meteorites, v. 5, p. 49-73.
·   Jaumann, R. and 49 others (2019) Images from the Surface of Asteroid Ryugu Show Rocks Similar to Carbonaceous Chondrite Meteorites, Science, v. 365(6455), p. 817-820, doi: 10.1126/science.aaw8627. [abstract]
·   Kitazato, K. and 65 others (2019) The Surface composition of Asteroid 162173 Ryugu from Hayabusa2 Near-infrared Spectroscopy, Science, v. 364(6437), p. 272-275, doi: 10.1126/science.aav7432. [abstract]


Written by Linda Martel, Hawai'i Institute of Geophysics and Planetology, for PSRD.


Abstrak

The composition of asteroids and their connection to meteorites provide insight into geologic processes that occurred in the early Solar System. We present spectra of the Nightingale crater region on near-Earth asteroid Bennu with a distinct infrared absorption around 3.4 micrometers. Corresponding images of boulders show centimeters-thick, roughly meter-long bright veins. We interpret the veins as being composed of carbonates, similar to those found in aqueously altered carbonaceous chondrite meteorites. If the veins on Bennu are carbonates, fluid flow and hydrothermal deposition on Bennu’s parent body would have occurred on kilometer scales for thousands to millions of years. This suggests large-scale, open-system hydrothermal alteration of carbonaceous asteroids in the early Solar System.

This is an article distributed under the terms of the Science Journals Default License.


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