Astronomija

Če bi luna še vedno imela magnetno polje, kako dolgo bi bilo mogoče ohranjati ozračje?

Če bi luna še vedno imela magnetno polje, kako dolgo bi bilo mogoče ohranjati ozračje?


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Predpostavimo, da je imela Luna precejšnje magnetno polje, da je preprečilo razpršitev atmosfere, ki jo je povzročil sončni veter. Koliko let bi minilo, preden bi vsi plini (ozračja, podobnega Zemlji) ušli iz njenega telesa? Bi njegova gravitacija zadostovala za ulov težjih plinov, kot je CO2?


Odgovoril bom nekoliko drugače: Hitrost uhajanja na površju naše Lune je približno 2,38 km / s. Iz tega članka izhaja, da lahko ozračje preživi 4,5 milijarde let, če je njegova povprečna hitrost molekule manjša od 1/6 izhodne hitrosti planeta / lune. V primerjavi z našo Luno znaša 2380 m / s / 6 = 396,67 m / s.

Ogljikov dioksid ima molekulsko maso 44u. Primerna temperatura za povprečno hitrost molekule ogljikovega dioksida je torej $$ mbox {temperatura} = (v _ { mbox {plin}} / 157) ^ 2 cdot mbox {masa molekule} mbox = (396,67 / 157 ) ^ 2 cdot 44 mbox {K} = 280,87 mbox {K}. $$ Lunina srednja temperatura površine na ekvatorju je približno 220 K. To je precej pod dovoljenih 280,87 K. Ozračje ogljikovega dioksida bi lahko preživelo 4,5 milijarde let po teh poenostavljenih predpostavkah.

Ko pogledamo bližje ozračju, se izkaže, da lahko višje plasti termosfere in eksosfere dosežejo več kot 1000 K. Tudi Ogljikov dioksid lahko pri tej temperaturi sčasoma uide iz Lune: Povprečna hitrost molekule ogljikovega dioksida pri 1000 K je 157 USD $ cdot sqrt { frac {1000} {44}} mbox {m / s} = 748,5 mbox {m / s}. $$

Rezultat je jeanov pobegni parameter $$ lambda_0 = left ( frac {v _ { mbox {esc}}} {v _ { mbox {gas}}} right) ^ 2 = left ( frac { 2380} {748,5} desno) ^ 2 = 10,1, $$, kar ima za posledico stopnjo uhajanja približno 10 $ ^ {- 4} $ glede na prosti molekularni tok.

Iz tega še ni enostavno izračunati časovnega obdobja, saj bi Moon izgubil ozračje z ogljikovim dioksidom, saj je to funkcija odvisne od višine temperaturne krivulje ozračja. Ampak vsaj jasno je, da Luna ne bi nenadoma izgubila ogljikovega dioksida, saj je to verjetno le za $ lambda_0 <3 $.


Vaše vprašanje bi si lahko razlagali na dva načina: ali bi lahko močno magnetno polje zadostovalo za zadrževanje ozračja ali pa bi magnetno polje zagotovilo dovolj zaščite za ohranitev ozračja.

Prvič, magnetno polje nikoli ne bi moglo biti dovolj za ohranjanje ozračja na telesu.

Drugič, magnetno polje bi preprečilo močno obsevanje in odpiranje ozračja, vendar je Lunina gravitacija prenizka.


Magnetosfera Lune je bila včasih dvakrat močnejša kot Zemlja & # 8217s

Znanstveniki že desetletja trdijo, da je sistem Zemlja-Luna nastal kot trk med Zemljo in Marsovim objektom pred približno 4,5 milijardami let. Ta teorija, znana kot hipoteza o velikanskem vplivu, pojasnjuje, zakaj sta si Zemlja in Luna podobni po strukturi in sestavi. Zanimivo je, da so znanstveniki ugotovili tudi, da je imela Luna v svoji zgodnji zgodovini magnetosfero & # 8211 podobno kot danes Zemlja.

Vendar pa nova študija, ki so jo vodili raziskovalci z MIT (s podporo NASA), kaže, da je bilo magnetno polje Lune včasih dejansko močnejše od Zemlje. Prav tako so lahko postavili strožje omejitve, ko je to polje propadlo, in trdili, da bi se to zgodilo pred približno milijardo let. Te ugotovitve so pomagale razrešiti skrivnost, kakšen mehanizem je sčasoma poganjal magnetno polje Lune in # 8217.

Študija, ki se je nedavno pojavila v reviji Znanstveni napredek, je vodil Saied Mighani, eksperimentalni fizik kamnin z oddelka za zemeljske, atmosferske in planetarne znanosti MIT & # 8217s. Pridružili so se mu člani geokronološkega centra Berkeley na UC Berkeley in Kitajske univerze za geoznanosti, dodatno podporo pa je zagotovil sloviti profesor EAPS, dr. Benjamin Weiss.

Če povzamemo, je magnetno polje Zemlje bistvenega pomena za življenje, kakršno poznamo. Ko prihajajoči delci sončnega vetra dosežejo Zemljo, jih to polje odkloni in tvorijo premčni šok pred Zemljo in magnetni rep za njo. Preostali delci se odložijo na magnetnih polih, kjer sodelujejo z našo atmosfero, kar povzroča polarne svetlobe, ki jih vidimo na skrajni severni in južni polobli.

Če ne bi bilo tega magnetnega polja, bi Zemljino atmosfero skozi milijarde let počasi odstranil sončni veter in postalo hladno in suho mesto. Verjame se, da se je to zgodilo na Marsu, kjer je bila nekoč debelejša atmosfera pred 4,2 in 3,7 milijardami let izčrpana, zaradi česar se je vsa tekoča voda na njeni površini izgubila ali zamrznila.

V preteklih letih je skupina Weiss & # 8217 s preučevanjem lunin kamnin pomagala dokazati, da je imela Luna pred približno 4 milijardami let tudi močno magnetno polje z močjo približno 100 mikrotesl (medtem ko Zemlja danes približno 50 mikrotesl). Leta 2017 so preučevali vzorce, ki so jih zbrali astronavti Apollo in so bili datirani pred približno 2,5 milijardami let in so našli precej šibkejše polje (manj kot 10 mikrotesl).

Z drugimi besedami, magnetno polje Lune je oslabljeno s faktorjem pet pred 4 in 2,5 milijardami let, nato pa je v celoti izginilo pred približno 1 milijardo let. Takrat so Weiss in njegovi kolegi domnevali, da sta v notranjosti Lune morda obstajala dva dinamo mehanizma, ki sta bila odgovorna za to spremembo.

Meritve lunin kamnin so pokazale, da je starodavna luna v svojem tekočem kovinskem jedru (najbolj notranja rdeča lupina) ustvarjala dinamo magnetno polje. Zasluge: Hernán Cañellas / Benjamin Weiss

Skratka, trdili so, da bi prvi dinamo učinek lahko ustvaril veliko močnejše magnetno polje pred približno 4 milijardami let. Nato ga je pred 2,5 milijardami let nadomestil drugi dinamo, ki je bil bolj dolgotrajen, vendar je imel veliko šibkejše magnetno polje. Kot je dr. Weiss pojasnil v sporočilu za javnost MIT:

»Obstaja več idej o tem, kateri mehanizmi so poganjali Lunin dinamo, in vprašanje je, kako ugotoviti, kateri je to storil? Izkazalo se je, da imajo vsi ti viri energije različno življenjsko dobo. Torej, če bi lahko ugotovili, kdaj se je dinamo izklopil, potem bi lahko ločili med mehanizmi, ki so bili predlagani za lunin dinamo. To je bil namen tega novega prispevka. "

Do zdaj je bil pridobivanje lunin kamnin, starih manj kot 3 milijarde let, velik izziv. Razlog za to je v dejstvu, da je vulkanska dejavnost, ki je bila na Luni pogosta pred 4 milijardami let, prenehala pred približno tremi milijardami let. Na srečo je ekipa MIT lahko identificirala dva vzorca lunine kamnine, ki so ju dobili astronavti Apolla in sta bila ustvarjena z udarcem pred milijardo let.

Medtem ko so se te kamnine zaradi udarca stopile in nato znova strdile, s čimer so med tem izbrisali njihov magnetni zapis, je ekipa na njih lahko izvedla teste za rekonstrukcijo magnetnega podpisa. Najprej so analizirali usmerjenost elektronov kamnine & # 8217s, ki jih Weiss opisuje kot "majhne kompase", saj bi se bodisi poravnali v smeri obstoječega magnetnega polja bodisi bi se pojavili v naključnih usmeritvah, če ga ne bi bilo.

Lunine kamnine iz misije Apollo 11. Zasluge: NASA

V obeh vzorcih je ekipa opazila slednje, kar je nakazovalo, da so kamnine nastale v izredno šibkem magnetnem polju z največ 0,1 mikroteslami (morda sploh nobene). Sledila je tehnika radiometričnega datiranja, ki sta jo za to študijo prilagodila Weiss in David L. Shuster (raziskovalec geokrološkega centra Berkeley in soavtor študije). Ti rezultati so potrdili, da so kamnine res stare 1 milijardo let.

Nazadnje je skupina izvedla preskuse toplote na vzorcih, da bi ugotovila, ali lahko zagotovijo dober magnetni zapis v času udarca. To je vključevalo dajanje obeh vzorcev v pečico in njihovo izpostavljanje vrstam visokih temperatur, ki bi nastale ob trku. Ko so se ohladili, so jih v laboratoriju izpostavili umetno ustvarjenemu magnetnemu polju in potrdili, da so ga lahko zabeležili.

Ti rezultati potrjujejo, da je magnetna moč, ki jo je prvotno izmerila ekipa (0,1 mikrotesla), točna in da se je pred 1 milijardo let dinam, ki je poganjal magnetno polje Lune in # 8217s, verjetno končal. Kot je izrazil Weiss:

»Magnetno polje je ta meglica, ki prežema vesolje, kot nevidno polje sile. Pokazali smo, da je dinamo, ki je ustvaril lunino magnetno polje, umrlo pred približno 1,5 do 1 milijardo let in se zdi, da je poganjal Zemljo. "

Lunin odtis misij Apollo. Zasluge: NASA

Kot smo že omenili, ta študija pomaga tudi pri reševanju razprave o tem, kaj je poganjalo Lunin dinamo v njegovih poznejših fazah. Medtem ko je bilo predlaganih več teorij, so te nove ugotovitve skladne s teorijo, da je odgovorna kristalizacija jedra. V bistvu ta teorija navaja, da je Lunino notranje jedro sčasoma kristaliziralo, upočasnilo pretok električno napolnjene tekočine in zaustavilo dinamo.

Weiss domneva, da je bila pred tem morda precesija odgovorna za napajanje veliko močnejšega (vendar kratkotrajnega) dinamo, ki bi ustvaril močno magnetno polje. To se sklada z dejstvom, da naj bi Luna pred 4 milijardami let krožila veliko bližje Zemlji. To bi povzročilo, da bi gravitacija Zemlje imela bistveno večji učinek na Luno, kar bi povzročilo, da bi se njen plašč zibal in sprožil aktivnost v jedru.

Ko se je Luna počasi oddaljila od Zemlje, se je učinek precesije zmanjšal in dinamo, ki proizvaja magnetno polje, bi oslabelo. Pred približno 2,5 milijardami let je kristalizacija postala prevladujoči mehanizem, po katerem se je Lunin dinam nadaljeval, proizvajajoč šibkejše magnetno polje, ki je vztrajalo, dokler zunanje jedro dokončno ni kristaliziralo pred milijardo let.

Takšne študije bi lahko pomagale razrešiti tudi skrivnost, zakaj so planeti, kot sta Venera in Mars, izgubili magnetna polja (kar prispeva k kataklizmičnim podnebnim spremembam) in kako bi lahko Zemlja nekoč izgubila svoje. Glede na njegov pomen za bivalnost bi lahko boljše razumevanje dinamos in magnetnih polj pomagalo tudi pri iskanju bivalnih eksoplanetov.


Lunino magnetno polje je trajalo več kot dve milijardi let, zahvaljujoč Luninemu Dinamu

Luna, edini Zemljin naravni satelit, naj bi nastala kmalu po tem, ko je sam planet začel obstajati pred približno 4,5 milijardami let, potem ko se je na Zemljo zrušil predmet v velikosti Marsa in so se ostanki trka združili v luno. Povsem naravno je bilo, da obstajajo nekatere skupne lastnosti med Zemljo in Luno.

Magnetno polje je bilo ena taka stvar, a čeprav še danes obstaja okoli Zemlje - in velja za temeljni dejavnik bivalnosti -, je nekoč v preteklosti iz razlogov, ki še niso bili razumljeni, izginilo okoli Lune. Do nedavnega so mislili, da je Lunino magnetno polje pred približno 3,5 milijardami let izginilo ali vsaj znatno oslabelo. Toda nove raziskave so pokazale, da je bil lunin dinamo v luninem jedru - za katerega se domneva, da je odgovoren za magnetno polje - aktiven še pred 1 do 2,5 milijardami let.

Raziskovalci z Massachusettes Institute of Technology (MIT) in Rutgers University so analizirali vzorec lunine kamnine, ki jo je avgusta 1971 zbrala posadka Apolla 15. Relativno mlada kamnina, stara približno 1 do 2,5 milijarde let, je verjetno nastala kot rezultat udarca meteorja. In analiza je pokazala magnetno polje približno 5 mikrotesl v času nastanka.

V primerjavi z današnjim magnetnim poljem na Zemlji je 5 mikrotesla približno 10-krat šibkejše, vendar je še vedno približno 1000-krat močnejše od polj, ki danes obstajajo v medplanetarnem vesolju. Pred približno 4 milijardami let je imela luna magnetno polje, ki je merilo približno 100 mikroteslov, kar je skoraj dvakrat več od povprečne jakosti 50 mikrotesl današnjega magnetnega polja Zemlje.

Ta ugotovitev potiska časovni okvir, ko smo mislili, da je luna izgubila magnetno polje za 1 do 2,5 milijarde let. Po znatnem razpršitvi pred približno 3,5 milijardami let iz neznanih ali nerazumljenih razlogov nismo vedeli, ali se lunino magnetno polje še nekaj časa zadržuje. Do zdaj je tako.

To je pomembna ugotovitev z resnimi posledicami. Prvič, lahko nam pomaga razumeti, kako se je Lunin dinamo v luninem jedru - pojav, ki je odgovoren za ustvarjanje magnetnega polja - napajal in kako se je sčasoma obnašal.

Benjamin Weiss iz MIT-a, ki je bil soavtor raziskovalne naloge na to temo, objavljene v sredo v reviji Science Advances, je v izjavi z univerze pojasnil: "Koncept planetarnega magnetnega polja, ustvarjenega s premikanjem tekoče kovine, je ideja to je resnično le nekaj desetletij. Kaj poganja to gibanje na Zemlji in drugih telesih, zlasti na Luni, ni dobro razumljeno. To lahko ugotovimo tako, da poznamo življenjsko dobo Luninega dinamota. "

Potem je tu še vprašanje razmerja med bivalnostjo planetarnega telesa in magnetnim poljem, ki ga obkroža. Brez polja bodo nabiti delci, ki izvirajo iz gostiteljske zvezde telesa - v našem primeru sonca - verjetno privedli do izgube tekoče vode na površini telesa, če bi sploh obstajala, kot se je zgodilo na Marsu.

»Kadarkoli pogledamo eksoplanete ali lune eksoplanetov, ki bi lahko bili v bivalnem območju, lahko magnetno polje obravnavamo kot pomembnega dejavnika bivanja. Potem se postavlja vprašanje, kakšne velikosti planetov in lun bi morali obravnavati kot svetove, ki bi lahko bili vseljivi, "je povedala Sonia Tikoo iz Rutgersa, ki je bila glavna avtorica študije z naslovom" Dve milijardletni zgodovini za lunin dinamo ". ločena izjava.

»Nismo mislili, da bi lahko majhna planetarna telesa zelo dolgo ustvarjala magnetna polja, ker imajo manjša jedra, ki bi se hitro ohladila in kristalizirala v zgodnjem življenju. Ker je hitrost kristalizacije odvisna od sestave jedra, lahko naša ugotovitev izpodbija tisto, za kar mislimo, da je sestavljeno iz luninega jedra. Večinoma je iz železa, vendar je treba z njim mešati nekaj: žveplo, ogljik ali drug element, «je še pojasnila.

Težko je dobiti vzorce z lune, ki so beležili magnetno aktivnost in so bili tudi mlajši od 3,2 milijarde let, saj je Lunina vulkanska aktivnost v tem času večinoma prenehala. Na srečo raziskovalcev so nekateri vzorci, ki jih je zbrala posadka Apolla 15, nastali kasneje zaradi udarcev meteorjev. Specifični del, ki so ga analizirali, se imenuje Apollo 15 vzorec 15498, steklen material, ki zvari minerale in delce kamnine, pa zelo dobro beleži magnetne lastnosti.


Luna rja in raziskovalci želijo vedeti, zakaj

Medtem ko je naša Luna brezzračna, raziskave kažejo na prisotnost hematita, oblike rje, ki običajno zahteva kisik in vodo. To je znanstvenike zmedlo.

Mars je že dolgo znan po svoji rje. Železo na njegovi površini v kombinaciji z vodo in kisikom iz davne preteklosti daje Rdečemu planetu odtenek. A znanstveniki so bili nedavno presenečeni, ko so našli dokaze, da je tudi naša brezzračna Luna zarjavela.

Nov članek v reviji Science Advances pregleduje podatke indijske organizacije za vesoljske raziskave & # x27s Chandrayaan-1, ki je med raziskovanjem površine Lune leta 2008 odkrila vodni led in začrtala različne minerale. Vodilni avtor Shuai Li z univerze v Havaji so to vodo obsežno preučevali v podatkih instrumenta Chandrayaan-1 & # x27s Moon Mineralogy Mapper ali M3, ki ga je zgradil NASA-in laboratorij za reaktivni pogon v južni Kaliforniji. Voda sodeluje s kamninami, da ustvari raznolikost mineralov, M3 pa je zaznal spektre - ali svetlobo, ki se odbija od površin -, kar je razkrilo, da so polovi Lune & # x27s zelo drugačni kot ostali.

Zanimiv, Li se je podal na te polarne spektre. Medtem ko je površina Lune polna kamnin, bogatih z železom, je bil vseeno presenečen, ko je našel tesno ujemanje s spektralnim podpisom hematita. Mineral je oblika železovega oksida ali rje, ki nastane, ko je železo izpostavljeno kisiku in vodi. Toda Luna naj ne bi imela kisika ali tekoče vode, kako torej lahko rja?

Skrivnost se začne s sončnim vetrom, tokom napolnjenih delcev, ki teče iz Sonca in bombardira Zemljo in Luno z vodikom. Vodik otežuje tvorbo hematita. To je tisto, kar je znano kot reduktor, kar pomeni, da materialom, s katerimi sodeluje, doda elektrone. To je ravno nasprotno od tistega, kar je potrebno za izdelavo hematita: da železo rja, potrebuje oksidant, ki odstrani elektrone. In medtem ko ima Zemlja magnetno polje, ki jo ščiti pred tem vodikom, Luna nima.

& quot; Zelo zmedeno je, & quot; rekel je Li. & quotLuna je strašno okolje, v katerem lahko nastane hematit. & quot Tako se je obrnil na znanstvenike JPL Abigail Fraeman in Vivian Sun, da bi pomagali pri iskanju podatkov M3 & # x27s in potrditi svoje odkritje hematita.

& quot; Sprva nisem popolnoma verjel. Ne bi smel obstajati glede na razmere na Luni, "je dejal Fraeman. "Toda odkar smo odkrili vodo na Luni, ljudje ugibajo, da bi lahko bilo več različnih mineralov, kot se zavedamo, če bi ta voda reagirala s kamenjem."

Po natančnem ogledu sta se Fraeman in Sun prepričala, da podatki M3 & # x27s res kažejo na prisotnost hematita na luninih polih. & quot; Na koncu so bili spektri prepričljivo v hematitu, zato je bilo treba razložiti, zakaj se na Luni pojavljajo & quot ;, & quot; Sonce.

Tri ključne sestavine

Njihov prispevek ponuja trikraki model, ki pojasnjuje, kako lahko rja nastane v takem okolju. Za začetek, medtem ko Luni primanjkuje ozračja, je v resnici dom za sledenje količin kisika. Vir tega kisika: naš planet. Magnetno polje Zemlje se vleče za planetom kot vetrovka. Leta 2007 je japonski orbiter Kaguya odkril, da se lahko kisik iz zgornjih ozračja Zemlje pripelje na ta zadnji magnetni rep, kot je uradno znan, in prepotuje 385,00 kilometrov do Lune.

To odkritje se prilega podatkom M3, ki je našel več hematita na bližnji strani Lune, obrnjeni proti Zemlji, kot na njeni skrajni strani. "To nakazuje, da bi lahko kisik na Zemlji poganjal tvorbo hematita," je dejal Li. Luna se že milijarde let oddaljuje od Zemlje, zato je tudi možno, da je čez to razpoko skočilo več kisika, ko sta bila v starodavni preteklosti bližje.

Potem je tu še stvar vsega tega vodika, ki ga dovaja sončni veter. Kot reduktor mora vodik preprečevati oksidacijo. Toda magnetni rep Zemlje ima posredovalni učinek. Poleg prenosa kisika na Luno z našega domačega planeta blokira tudi več kot 99% sončnega vetra v določenih obdobjih kroženja Lune & # x27s (natančneje kadar koli v fazi polne Lune). To med luninim ciklom odpre občasna okna, ko lahko nastane rja.

Tretji del sestavljanke je voda. Medtem ko je večina Lune suha v kosteh, lahko vodni led najdemo v zasenčenih luninih kraterjih na oddaljeni strani Lune. Toda hematit je bil odkrit daleč od tega ledu. Članek se namesto tega osredotoča na molekule vode, ki jih najdemo na Lunini površini. Li predlaga, da bi lahko hitro premikajoči se prašni delci, ki redno krotijo ​​Luno, sprostili te površinske molekule vode in jih v luninem zemlji pomešali z železom. Toplota zaradi teh vplivov bi lahko povečala stopnjo oksidacije, saj lahko tudi sami delci prahu prenašajo molekule vode in jih vsadijo na površino, tako da se pomešajo z železom. V ravno pravih trenutkih - kadar je Luna zaščitena pred sončnim vetrom in je prisoten kisik - lahko pride do kemične reakcije, ki povzroča rjo.

Potrebno je več podatkov, da se natančno ugotovi, kako voda sodeluje s kamenjem. Ti podatki bi lahko pomagali razložiti še eno skrivnost: zakaj manjše količine hematita nastajajo tudi na skrajni strani Lune, kjer Zemlje & # x27s kisik ne bi smel doseči.

Fraeman je dejal, da ta model lahko razloži tudi hematit, ki ga najdemo na drugih zračnih telesih, kot so asteroidi. "Mogoče je, da majhni delci vode in vpliv prašnih delcev omogočajo rjavenje železa v teh telesih," je dejala.

Li je opozoril, da je za lunino znanost vznemirljiv čas. Skoraj 50 let od zadnjega pristanka Apolona je Luna spet glavni cilj. NASA načrtuje, da bo na začetku prihodnjega leta poslala na desetine novih instrumentov in tehnoloških eksperimentov za preučevanje Lune, čemur bodo sledile človeške misije, ki se bodo začele leta 2024, vse v okviru programa Artemis.

JPL gradi tudi novo različico M3 za orbiter z imenom Lunar Trailblazer. Eden od njegovih instrumentov, Map Map Hlapivih in mineralov z visoko ločljivostjo (HVM3), bo preslikal vodni led v trajno zasenčenih kraterjih na Luni, morda pa bo lahko razkril tudi nove podrobnosti o hematitu.

"Mislim, da ti rezultati kažejo, da se v našem sončnem sistemu dogajajo bolj zapleteni kemični procesi, kot so bili prej prepoznani," je dejal Sun. & quot; Lahko jih bolje razumemo s pošiljanjem prihodnjih misij na Luno, da preizkusimo te hipoteze. & quot


Za lunino zgodnje magnetno polje je lahko odgovoren ocean Magma

Pred približno štirimi milijardami let je imela Luna magnetno polje, ki je bilo približno tako močno, kot je danes magnetno polje Zemlje. Kako je lahko Luna z bistveno manjšim jedrom od Zemljinega imela tako močno magnetno polje, je bil v zgodovini Luninega razvoja nerešen problem.

Znanstvenik Aaron Scheinberg iz Princetona je s Kristo Soderlund z Inštituta za geofiziko Univerze v Teksasu in Lindo Elkins-Tanton z državne univerze v Arizoni določil, kaj je lahko napajalo to zgodnje lunino magnetno polje. Njihovi rezultati in nov model, kako se je to lahko zgodilo, so bili nedavno objavljeni v Zemeljska in planetarna znanstvena pisma.

Nov model

Zemeljsko magnetno polje ščiti naš planet tako, da odbija večino sončnega vetra, katerega nabiti delci bi sicer odstranili ozonski plašč, ki varuje Zemljo pred škodljivim ultravijoličnim sevanjem.

Medtem ko Zemljino magnetno polje ustvarjajo gibi njegovega konvekcijskega zunanjega jedra tekoče kovine, znanega kot dinamo, je Lunino jedro premajhno, da bi ustvarilo magnetno polje te velikosti.

Torej je raziskovalna skupina predlagala nov model, kako bi lahko magnetno polje doseglo Zemljino raven. V tem scenariju dinamo ne poganja Lunovo majhno kovinsko jedro, temveč težka plast staljene (tekoče) kamnine, ki sedi na njej.

V tem predlaganem modelu se spodnja plast Luninega plašča stopi in tvori kovinsko bogat "bazalni ocean magme", ki sedi na vrhu Luninega kovinskega jedra. Konvekcija v tej plasti nato poganja dinamo in ustvarja magnetno polje.

"Ideja o bazalnem oceanu magme je bila predlagana za magnetno polje zgodnje Zemlje in ugotovili smo, da je ta mehanizem lahko pomemben tudi za Luno," pravi soavtor Soderlund.

Soderlund nadalje pojasnjuje, da naj bi delno staljena plast še danes obstajala na dnu luninega plašča. "Močno magnetno polje je lažje doseči na Lunini površini, če je dinamo deloval v plašču in ne v jedru," pravi, "ker se moč magnetnega polja hitro zmanjšuje, kolikor bolj oddaljena je od dinamo regije."

V simulacijah jedrnega dinamovega luna, ki ga je izvedla ekipa, so ves čas ugotavljali, da se spodnja plast Luninega plašča pregreva in topi. Sprva so se poskušali osredotočiti na primere brez taljenja, ki jih je bilo lažje modelirati, na koncu pa so menili, da je postopek taljenja ključ njihovega novega modela.

"Ko smo o taljenju začeli razmišljati kot o značilnosti, namesto o napaki," pravi Scheinberg, "so se deli začeli prilagajati in spraševali smo se, ali lahko taljenje, ki smo ga videli na modelih, ustvari s kovino bogat magnetni ocean, ki bo poganjal močno zgodnje polje. "

Kasneje šibko magnetno polje

Nadalje v evoluciji Lune (pred približno 3,56 milijarde let) obstajajo tudi dokazi, da je močno magnetno polje, ki je obstajalo okoli Lune, sčasoma postalo šibko magnetno polje, ki se je nadaljevalo do relativno nedavnega. Tudi novi model ekipe lahko pomaga razložiti ta pojav.

"Naš model ponuja elegantno potencialno rešitev," pravi Scheinberg. "Ko se je Luna ohladila, bi se ocean magme utrdil, medtem ko bi jedro dinama še naprej ustvarjalo poznejše šibko polje."

"Ta rezultat nas navdušuje, ker pojasnjuje temeljna opazovanja o Luni - njeno zgodnje, močno magnetno polje in posledično oslabitev ter nato izginotje - s postopki prvega reda, ki jih že podpirajo druga opazovanja," dodaja soavtor Elkins -Tanton.

Poleg tega, da ponuja nove modele, iz katerih lahko gradimo, lahko te raziskave nudijo tudi boljše razumevanje ustvarjanja planetarnega magnetnega polja drugje v našem sončnem sistemu in zunaj njega.

"Oceanski dinamos iz bazalne magme, tako kot tisti v našem modelu, je bil verjetno pogost pojav na skalnatih planetih, kot sta Zemlja in Mars," pravi Scheinberg.


Magnetna polja na Luni so ostanek starodavnega jedra dinamo

Trenutno Luna nima notranjega magnetnega polja, kot ga lahko opazujemo na Zemlji. Vendar pa na njeni površini obstajajo lokalizirana območja do nekaj sto kilometrov, kjer prevladuje zelo močno magnetno polje. To so pokazale meritve na kamninah iz misij Apollo. Od takrat se raziskave sprašujejo o izvoru teh magnetnih peg. Ena od teorij je, da so na nek način ostanki magnetnega polja starodavnega jedra. Mogoče podoben tistemu, kar je še danes mogoče opaziti na Zemlji. Tu je jedro sestavljeno iz staljenega in trdnega železa, njegovo vrtenje pa ustvarja zemeljsko magnetno polje. Zakaj je notranje polje Lune v določenem trenutku ugasnilo, ostaja predmet raziskav.

Druga dolgo razpravljana teorija o lokalnih magnetnih pegah Lune kaže na to, da so rezultat magnetizacijskih procesov, ki jih povzročajo udarci masivnih teles na površino Lune. Študija, ki je bila nedavno objavljena v reviji Znanstveni napredek zdaj kaže, da je Luna v preteklosti že imela notranji jedrni dinam. Raziskovalci so prišli do zaključka z ovržitvijo te druge teorije s pomočjo zapletenih računalniških simulacij. Je rezultat velikega mednarodnega sodelovanja med MIT, GFZ-Potsdam, UCLA, Univerzo v Potsdamu, Univerzo v Michiganu in Avstralsko univerzo Curtin.

Drugo tezo je med drugim podprlo dejstvo, da so na drugi strani Lune, natanko nasproti velikih luninih kraterjev, našli velike in močne magnetne pege. Njihov izvor naj bi bil naslednji: Ker Luna - za razliko od Zemlje - nima atmosfere, ki bi jo zaščitila pred meteoriti in asteroidi, jo lahko tako masivna telesa udarijo s polno silo in na njeni površini prašijo in ionizirajo material. Tako ustvarjen oblak nabitih delcev, imenovan tudi plazma, teče okoli Lune, stisne magnetni sončni veter, prisoten v vesolju, in tako okrepi njegovo magnetno polje. Hkrati sončni veter inducira magnetno polje v sami Luni. Na površini nasproti udarca se vsa ta polja ojačijo in ustvarijo opaženi magnetizem v skorji skorje.

Z uporabo primerov nekaterih dobro znanih luninih kraterjev kot tistega, ki ga imamo za njegovo "desno oko", so raziskovalci zdaj simulirali vpliv, vključno s tvorbo plazme, širjenjem plazme okoli Lune in potekom polja, povzročenim v lunina notranjost. Z uporabo programske opreme, ki je bila prvotno razvita za vesoljsko fiziko in vesoljsko vreme, so simulirali zelo različne scenarije vplivov. Na ta način so znanstveniki lahko pokazali, da samo ojačanje magnetnih polj zaradi trkov in vrženega materiala ni zadostovalo za ustvarjanje velikih poljskih jakosti, kot so bile prvotno ocenjene in izmerjene na Luni: Nastalo magnetno polje je tisočkrat šibkejše, kot je potrebno za razlago opažanj. To pa ne pomeni, da ti učinki ne obstajajo, so le razmeroma šibki. Simulacije so zlasti pokazale, da je ojačanje polja s plazemskim oblakom na zadnji strani udarca bolj verjetno, da se bo zgodilo nad skorjo in da magnetno polje v Luni izgubi velik del energije zaradi razpada zaradi turbulence v plašč in skorja.

"Kako natančno so nastale magnetne pege, še vedno potrebujemo več raziskav. Zdaj pa je jasno, da je moralo biti v določenem trenutku prisotno notranje magnetno polje Lune, da se je to zgodilo," pravi Yuri Shprits, profesor na Univerzi v Potsdam in vodja Sekcije za fiziko magnetosfere pri GFZ-Potsdam. "Poleg tega nam lahko ta študija pomaga bolje razumeti naravo dinamo ustvarjenega magnetnega polja in dinamo procesa na Zemlji, zunanjih planetih in eksoplanetih."


Manjkajoči magnetizem antične Lune

Danes Luni primanjkuje globalnega magnetnega polja, vendar to ni bilo vedno tako. Meritve lunine skorje in lunin kamnin v vesoljskih plovilih, pridobljene z misijami Apollo, vsebujejo preostalo magnetizacijo, ki je nastala pred 4 do 3,5 milijardami let v magnetnem polju, ki je po moči primerljivo z zemeljskim. Znanstveniki trdijo, da je bil vir tega dinamo - magnetno polje, ki ga ustvarja luna, ki se raztaplja, stali, kovinsko jedro. Vendar raziskave kažejo, da sumljivo majhno jedro Lune morda ni moglo ustvariti dovolj energije za vzdrževanje starodavnega magnetnega polja, o katerem so planetarni znanstveniki sklepali v svojih kamninah.

V nedavnem Znanstveni napredek članek, raziskovalka Rona Oran in profesor planetarnih znanosti Ben Weiss z oddelka za zemeljske, atmosferske in planetarne znanosti MIT sta preučila verjetnost alternativne hipoteze, ki obstaja že od osemdesetih let prejšnjega stoletja in bi lahko povzročila preostalo magnetizacijo v Lunini skorji: prehodno plazme, ki jih povzročajo meteoroidni vplivi. Tu opisujejo nekatere svoje ugotovitve.

Modeli pretoka plazme (levo) in razvoja magnetnega polja po vplivu, ki tvori bazen. Zasluga: Slika avtorja avtorja.

V: Kakšna je hipoteza o "udarni plazmi" in zakaj jo še vedno obravnavamo kot potencialni mehanizem za razlago luninega starodavnega magnetizma?

Oran: There are two main hypotheses that have been put forward to explain the moon’s ancient magnetic field. One is that the moon once generated a dynamo. The primary challenge for this theory was that the moon is much smaller than the Earth, and it doesn’t have enough energy to generate a surface magnetic field with the high intensity inferred from the analyses of the Apollo samples and crust.

Weiss: A longstanding alternative hypothesis is that the source of the field was not the moon’s interior itself but rather meteoroid impacts on the surface. In particular, it was proposed that impact plasmas — highly conductive fluids produced by vaporization of the lunar surface — expanded around and engulfed the moon. As they did so, the plasmas would compress and amplify the interplanetary magnetic field, known as the solar wind. The fields would then be induced into the moon’s crust, and the enhanced field signal would then be seen in the soil on the other side of the moon. This hypothesis is supported, in part, by observations of four young, large craters that have strong and large magnetic signals on the opposite site of the moon.

Q: Looking at the impact plasmas model, how did you examine its plausibility, and why were you able to rule it out as a primary suspect?

Weiss: We tested this idea by conducting the first simulations of impact plasmas that self-consistently consider the physics governing the generation and decay of the magnetic field.

Oran: One of the reasons this hypothesis was not yet tested in this way was that the tools that we used belong to the discipline of space sciences nobody actually applied them to this problem before. Then, Ben, who researches paleomagnetism, and I joined forces to work on this together and showed that the impact plasmas hypothesis cannot work.

The evolution of magnetized plasmas is a complex process where the flow of plasma and the electromagnetic fields change in response to each other. It’s only by simultaneously simulating the plasmas and the magnetic field that you can get a realistic view of the process.

We found that whatever you do, however you play with it in terms of impact location, direction, and the direction of the initial field, you cannot create enough magnetic energy from these impact plasmas. That’s because we can think of the lunar body like this gigantic spherical resistor that basically kills off all the currents that these magnetic fields are trying to induce into it. Then, instead of having strong magnetic fields in the crust caused by the impact, we generate those fields, but they dissipate within minutes, so you end up heating the rock. So, we saw this completely opposite effect of what we originally set out to find.

Q: What does your finding tell us about the evolution of the moon, its magnetism and similar planetary bodies? And what questions remain?

Weiss: If the impact fields hypothesis were correct, it would mean that the remnant magnetization we find on the surface of the moon would essentially tell us nothing about the geophysical and thermal evolution of its interior. This would in turn have had profound implications for tracing out the magnetic history of the moon, and even for understanding the record of remnant magnetization found on other airless bodies like Mercury, which has cratering, and asteroids, which meteorites suggest could have crustal magnetization. Now that we have shown that the impact fields hypothesis is not likely to explain most of the lunar magnetism, this supports the core dynamo hypothesis for magnetism on the moon and other bodies.

Oran: Given that we now favor a lunar dynamo, the strong fields we see on the moon still demand an explanation, because a dynamo like the one we have on Earth, in which the core churns due to its own cooling, may not be sufficient. In recent years, some alternative dynamo theories were developed that might generate stronger fields, for example, stirring of the core by the wobbling of the overlying solid mantle.

Our most immediate followup study is to repeat the same type of simulations but, instead of a non-magnetically active body, we would allow the moon to generate its own core dynamo and then examine how impact plasmas would interact with such a field. Another issue to look at is if you can create an imprint at the impact site itself. One of those scenarios might give us a better match for the magnetizations that we see on the moon’s surface.

Also involved in the study were former MIT visiting professor Yuri Shprits of GFZ German Research Centre for Geosciences, former EAPS postdoc Katarina Miljković of Curtin University, and Gábor Tóth of the University of Michigan.

This research was funded, in part, by the NASA Solar System Workings Program, the NASA Solar System Exploration Virtual Institute, and the Skoltech Faculty Development Program for support.


Controversial New Paper Says The Moon May Have Once Been Able to Support Life

We may not need to travel far from our home planet to find a spot in our solar system that could once have supported life.

Long ago, Earth's Moon may have had conditions in which life could arise, according to a study published Monday in the journal Astrobiologija.

In fact, such conditions could have arisen on the Moon during two different periods, each tens of millions of years long, the study suggests.

The authors are not saying that life ever existed on the Moon – just that the conditions that make life as we know it possible seem to have been in place billions of years ago.

When we look for signs of life on other planets and Moons, clues that can indicate a climate supportive to life include liquid water, an atmosphere that would help keep water stable on the surface, a magnetic field offering protection from solar and cosmic radiation, and organic compounds that could make up life's building blocks.

According to the study's authors, at least some of those key conditions could have existed simultaneously on the Moon.

"If liquid water and a significant atmosphere were present on the early Moon for long periods of time, we think the lunar surface would have been at least transiently habitable," Dirk Schulze-Makuch, a Washington State University astrobiologist and co-author of the study, said in a statement.

But astronauts and rovers have never found any evidence of life on the Moon, and even if organic material did once exist on our planet's satellite, we don't know if any traces remain.

How the Moon could have supported life

The idea that the Moon could once have been habitable is based on a series of discoveries, mostly made within the past decade, that show the Moon isn't as dry as we thought.

There's probably still water ice in polar craters and water deposits trapped in the Moon's interior.

Billions of years ago, there could have been good amounts of liquid water on the surface, the new study says.

To understand why, a bit of lunar history is needed. Sometime around when our solar system settled into its current layout – about 4.5 billion years ago – a proto-Earth and another planetary body likely collided and were vaporized, according to a paper published earlier this year.

As this theory goes, the super-heated doughnut of molten, vaporized rock and liquid – called a synestia – cooled, then the Moon emerged, after which the remaining cloud of vapour condensed to form the Earth.

For a long time after its formation, the Moon was largely molten, with an ocean of magma spewing gases into its sky.

Those gases could have been enough to create an atmosphere. As that molten ocean finished solidifying (around 4 billion years ago), there could also have been deep pools of liquid water on the Moon's surface.

That time period, the new study suggests, was the first time conditions on the Moon could have supported life.

The second time was during a period of intense volcanic activity 500 million years later – 3.5 billion years ago. That activity could have created an even more dense atmosphere with more water on the lunar surface, the study says.

According to calculations cited in the paper, there could have been liquid water on the surface for 70 million years during that period, especially if there was a magnetic field protecting the Moon from solar winds.

Where early life could have come from

During both of these time windows, life may have already existed on Earth. We still don't know how organic material first appeared on our home planet.

It could have been delivered to Earth by tiny meteorites, or life could have been the result of a chemical transformation at volcanic vents in Earth's oceans. Scientists also still don't know how common it is throughout the universe for conditions that support life to exist.

Some of the oldest evidence of life we have on Earth comes from fossilized microbes known as cyanobacteria. Somehow, certain precursor molecules – the chemical building blocks for life – fused together to form organic materials, which evolved eventually into those cyanobacteria.

We don't know exactly how long that process took, but some researchers have estimated that it was less than 10 million years.

By that logic, there could have been enough time for something similar to happen on the Moon. If organic material was there, life could have emerged during these two windows.

Even if there was no organic material on the Moon during those years, they were periods of intense meteoric activity. Schulze-Makuch and co-author Ian Crawford wrote in the study that it's "expected that meteorites blasted off the surface of the Earth will have landed on the Moon."

So those meteorites could have brought microorganisms with them, which might have survived the crash if slowed down by an atmosphere.

But many doubts still linger

Although the idea of life on the Moon is intriguing, we don't know if the factors noted in the study ever came together to enable life on the Moon. Any efforts to find out more would involve an "aggressive future program of lunar exploration," the study authors wrote.

Plus, even if there were relevant evidence on the Moon, chances are it's been destroyed by billions of years of cosmic radiation, solar winds, and meteorite strikes.

Future missions to the Moon could, however, collect samples from layers of the Moon that might provide evidence about these periods of volcanic activity. Lunar explorers could also eventually collect samples from the craters that still might hold ice.

Further research could also involve simulation chambers that would mimic the Moon's conditions to see if life could have survived.

Regardless of any potential next steps, the researchers behind the study think their work at least shows life could have existed on the Moon during these two periods.

"It looks very much like the Moon was habitable at this time," Schulze-Makuch said.

"There could have actually been microbes thriving in water pools on the Moon until the surface became dry and dead."

This article was originally published by Business Insider.


Moon’s Magnetic Field May Have Lasted 2 Billion Years

New measurements of lunar rocks have demonstrated that the ancient moon generated a dynamo magnetic field in its liquid metallic core (innermost red shell). The results raise the possibility of two different mechanisms — one that may have driven an earlier, much stronger dynamo, and a second that kept the moon’s core simmering at a much slower boil toward the end of its lifetime.

New findings from MIT suggest two mechanisms may have powered the moon’s ancient churning, extending the lunar dynamo’s lifetime by at least 1 billion years.

New evidence from ancient lunar rocks suggests that an active dynamo once churned within the molten metallic core of the moon, generating a magnetic field that lasted at least 1 billion years longer than previously thought. Dynamos are natural generators of magnetic fields around terrestrial bodies, and are powered by the churning of conducting fluids within many stars and planets.

In a paper published in Science Advances, researchers from MIT and Rutgers University report that a lunar rock collected by NASA’s Apollo 15 mission exhibits signs that it formed 1 to 2.5 billion years ago in the presence of a relatively weak magnetic field of about 5 microtesla. That’s around 10 times weaker than Earth’s current magnetic field but still 1,000 times larger than fields in interplanetary space today.

Several years ago, the same researchers identified 4-billion-year-old lunar rocks that formed under a much stronger field of about 100 microtesla, and they determined that the strength of this field dropped off precipitously around 3 billion years ago. At the time, the researchers were unsure whether the moon’s dynamo — the related magnetic field — died out shortly thereafter or lingered in a weakened state before dissipating completely.

The results reported today support the latter scenario: After the moon’s magnetic field dwindled, it nonetheless persisted for at least another billion years, existing for a total of at least 2 billion years.

Study co-author Benjamin Weiss, professor of planetary sciences in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS), says this new extended lifetime helps to pinpoint the phenomena that powered the moon’s dynamo. Specifically, the results raise the possibility of two different mechanisms — one that may have driven an earlier, much stronger dynamo, and a second that kept the moon’s core simmering at a much slower boil toward the end of its lifetime.

The Apollo 15 moon rock sample, which was analyzed by MIT and Rutgers University researchers, consists of basalt fragments welded together by a dark glassy matrix that was produced by melting from a meteorite impact. The black scale cube is 1 centimeter across.

“The concept of a planetary magnetic field produced by moving liquid metal is an idea that is really only a few decades old,” Weiss says. “What powers this motion on Earth and other bodies, particularly on the moon, is not well-understood. We can figure this out by knowing the lifetime of the lunar dynamo.”

Weiss’ co-authors are lead author Sonia Tikoo, a former MIT graduate student who is now an assistant professor at Rutgers David Shuster of the University of California at Berkeley Clément Suavet and Huapei Wang of EAPS and Timothy Grove, the R.R. Schrock Professor of Geology and associate head of EAPS.

Apollo’s glassy recorders

Since NASA’s Apollo astronauts brought back samples from the lunar surface, scientists have found some of these rocks to be accurate “recorders” of the moon’s ancient magnetic field. Such rocks contain thousands of tiny grains that, like compass needles, aligned in the direction of ancient fields when the rocks crystallized eons ago. Such grains can give scientists a measure of the moon’s ancient field strength.

Until recently, Weiss and others had been unable to find samples much younger than 3.2 billion years old that could accurately record magnetic fields. As a result, they had only been able to gauge the strength of the moon’s magnetic field between 3.2 and 4.2 billion years ago.

“The problem is, there are very few lunar rocks that are younger than about 3 billion years old, because right around then, the moon cooled off, volcanism largely ceased and, along with it, formation of new igneous rocks on the lunar surface,” Weiss explains. “So there were no young samples we could measure to see if there was a field after 3 billion years.”

There is, however, a small class of rocks brought back from the Apollo missions that formed not from ancient lunar eruptions but from asteroid impacts later in the moon’s history. These rocks melted from the heat of such impacts and recrystallized in orientations determined by the moon’s magnetic field.

Weiss and his colleagues analyzed one such rock, known as Apollo 15 sample 15498, which was originally collected on August 1, 1971, from the southern rim of the moon’s Dune Crater. The sample is a mix of minerals and rock fragments, welded together by a glassy matrix, the grains of which preserve records of the moon’s magnetic field at the time the rock was assembled.

“We found that this glassy material that welds things together has excellent magnetic recording properties,” Weiss says.

Baking rocks

The team determined that the rock sample was about 1 to 2.5 billion years old — much younger than the samples they previously analyzed. They developed a technique to decipher the ancient magnetic field recorded in the rock’s glassy matrix by first measuring the rock’s natural magnetic properties using a very sensitive magnetometer.

They then exposed the rock to a known magnetic field in the lab, and heated the rock to close to the extreme temperatures in which it originally formed. They measured how the rock’s magnetization changed as they increased the surrounding temperature.

“You see how magnetized it gets from getting heated in that known magnetic field, then you compare that field to the natural magnetic field you measured beforehand, and from that you can figure out what the ancient field strength was,” Weiss explains.

The researchers did have to make one significant adjustment to the experiment to better simulate the original lunar environment, and in particular, its atmosphere. While the Earth’s atmosphere contains around 20 percent oxygen, the moon has only imperceptible traces of the gas. In collaboration with Grove, Suavet built a customized, oxygen-deprived oven in which to heat the rocks, preventing them from rusting while at the same time simulating the oxygen-free environment in which the rocks were originally magnetized.

“In this way, we finally have gotten an accurate measurement of the lunar field,” Weiss says.

From ice cream makers to lava lamps

From their experiments, the researchers determined that, around 1 to 2.5 billion years ago, the moon harbored a relatively weak magnetic field, with a strength of about 5 microtesla — two orders of magnitude weaker than the moon’s field around 3 to 4 billion years ago. Such a dramatic dip suggests to Weiss and his colleagues that the moon’s dynamo may have been driven by two distinct mechanisms.

Scientists have proposed that the moon’s dynamo may have been powered by the Earth’s gravitational pull. Early in its history, the moon orbited much closer to the Earth, and the Earth’s gravity, in such close proximity, may have been strong enough to pull on and rotate the rocky exterior of the moon. The moon’s liquid center may have been dragged along with the moon’s outer shell, generating a very strong magnetic field in the process.

It’s thought that the moon may have moved sufficiently far away from the Earth by about 3 billion years ago, such that the power available for the dynamo by this mechanism became insufficient. This happens to be right around the time the moon’s magnetic field strength dropped. A different mechanism may have then kicked in to sustain this weakened field. As the moon moved away from the Earth, its core likely sustained a low boil via a slow process of cooling over at least 1 billion years.

“As the moon cools, its core acts like a lava lamp — low-density stuff rises because it’s hot or because its composition is different from that of the surrounding fluid,” Weiss says. “That’s how we think the Earth’s dynamo works, and that’s what we suggest the late lunar dynamo was doing as well.”

The researchers are planning to analyze even younger lunar rocks to determine when the dynamo died off completely.

“Today the moon’s field is essentially zero,” Weiss says. “And we now know it turned off somewhere between the formation of this rock and today.”

This research was supported, in part, by NASA.

Publication: Sonia M. Tikoo, et al., “A two-billion-year history for the lunar dynamo,” Science Advances 09 Aug 2017: Vol. 3, no. 8, e1700207 DOI: 10.1126/sciadv.1700207


Earth And The Moon Once Shared A Magnetic Shield

Four-and-a-half billion years ago, Earth's surface was a menacing, hot mess. Long before the emergence of life, temperatures were scorching, and the air was toxic.

Plus, as a mere toddler, the Sun bombarded our planet with violent outbursts of radiation called flares and coronal mass ejections. Streams of charged particles called the solar wind threatened our atmosphere. Our planet was, in short, uninhabitable.

But a neighboring shield may have helped our planet retain its atmosphere and eventually go on to develop life and habitable conditions. That shield was the Moon, says a NASA-led study in the journal Science Advances.

"The Moon seems to have presented a substantial protective barrier against the solar wind for the Earth, which was critical to Earth's ability to maintain its atmosphere during this time," said Jim Green, NASA's chief scientist and lead author of the new study. "We look forward to following up on these findings when NASA sends astronauts to the Moon through the Artemis program, which will return critical samples of the lunar South Pole."

A brief history of the Moon

The Moon formed 4.5 billion years ago when a Mars-sized object called Theia slammed into the proto-Earth when our planet was less than 100 million years old, according to leading theories. Debris from the collision coalesced into the Moon, while other remnants reincorporated themselves into the Earth. Because of gravity, the presence of the Moon stabilized the Earth's spin axis. At that time, our planet was spinning much faster, with one day lasting only 5 hours.

And in the early days, the Moon was a lot closer, too. As the Moon's gravity pulls on our oceans, the water is slightly heated, and that energy gets dissipated. This results in the Moon moving away from Earth at a rate of 1.5 inches per year, or about the width of two adjacent dimes. Over time, that really adds up. By 4 billion years ago, the Moon was three times closer to Earth than it is today - about 80,000 miles away, compared to the current 238,000 miles. At some point, the Moon also became "tidally locked," meaning Earth sees only one side of it.

Scientists once thought that the Moon never had a long-lasting global magnetic field because it has such a small core. A magnetic field causes electrical charges to move along invisible lines, which bow down toward the Moon at the poles. Scientists have long known about Earth's magnetic field, which creates the beautifully colored aurorae in the Arctic and Antarctic regions.

A magnetic field serves as a shield causing electrical charges to move along its invisible lines. Scientists have long known about Earth's magnetic field, which causes the beautifully colored aurorae in the Arctic and Antarctic regions. The movement of liquid iron and nickel deep inside the Earth, still flowing because of the heat left over from Earth's formation, generates the magnetic fields that make up a protective bubble surrounding Earth, the magnetosphere.

But thanks to studies of samples of the lunar surface from the Apollo missions, scientists figured out that the Moon once had a magnetosphere, too. Evidence continues to mount from samples that were sealed for decades and recently analyzed with modern technology.

Like Earth, the heat from the Moon's formation would have kept iron flowing deep inside, although not for nearly as long because of its size.

"It's like baking a cake: You take it out of the oven, and it's still cooling off," Green said. "The bigger the mass, the longer it takes to cool off."

The new study simulates how the magnetic fields of the Earth and Moon behaved about 4 billion years ago. Scientists created a computer model to look at the behavior of the magnetic fields at two positions in their respective orbits.

At certain times, the Moon's magnetosphere would have served as a barrier to the harsh solar radiation raining down on the Earth-Moon system, scientists write. That's because, according to the model, the magnetospheres of the Moon and Earth would have been magnetically connected in the polar regions of each object. Importantly for the evolution of Earth, the high-energy solar wind particles could not completely penetrate the coupled magnetic field and strip away the atmosphere.

But there was some atmospheric exchange, too. The extreme ultraviolet light from the Sun would have stripped electrons from neutral particles in Earth's uppermost atmosphere, making those particles charged and enabling them to travel to the Moon along the lunar magnetic field lines. This may have contributed to the Moon maintaining a thin atmosphere at that time, too. The discovery of nitrogen in lunar rock samples support the idea that Earth's atmosphere, which is dominated by nitrogen, contributed to the Moon's ancient atmosphere and its crust.

Scientists calculate that this shared magnetic field situation, with Earth and Moon's magnetospheres joined, could have persisted from 4.1 to 3.5 billion years ago.

"Understanding the history of the Moon's magnetic field helps us understand not only possible early atmospheres, but how the lunar interior evolved," said David Draper, NASA's deputy chief scientist and study co-author. "It tells us about what the Moon's core could have been like -- probably a combination of both liquid and solid metal at some point in its history -- and that is a very important piece of the puzzle for how the Moon works on the inside."

Over time, as the Moon's interior cooled, our nearest neighbor lost its magnetosphere, and eventually its atmosphere. The field must have diminished significantly 3.2 billion years ago, and vanished by about 1.5 billion years ago. Without a magnetic field, the solar wind stripped the atmosphere away. This is also why Mars lost its atmosphere: Solar radiation stripped it away.

If our Moon played a role in shielding our planet from harmful radiation during a critical early time, then in a similar way, there may be other moons around terrestrial exoplanets in the galaxy that help preserve atmospheres for their host planets, and even contribute to habitable conditions, scientists say. This would be of interest to the field of astrobiology - the study of the origins of life and the search for life beyond Earth.

Human exploration can tell us more

This modeling study presents ideas for how the ancient histories of Earth and Moon contributed to the preservation of Earth's early atmosphere. The mysterious and complex processes are difficult to figure out, but new samples from the lunar surface will provide clues to the mysteries.

As NASA plans to establish a sustainable human presence on the Moon through the Artemis program, there may be multiple opportunities to test out these ideas. When astronauts return the first samples from the lunar South Pole, where the magnetic fields of the Earth and Moon connected most strongly, scientists can look for chemical signatures of Earth's ancient atmosphere, as well as the volatile substances like water that were delivered by impacting meteors and asteroids. Scientists are especially interested in areas of the lunar South Pole that have not seen any sunlight at all in billions of years -- the "permanently shadowed regions" - because the harsh solar particles would not have stripped away volatiles.

Nitrogen and oxygen, for example, may have traveled from Earth to Moon along the magnetic field lines and gotten trapped in those rocks.

"Significant samples from these permanently shadowed regions will be critical for us to be able to untangle this early evolution of the Earth's volatiles, testing our model assumptions," Green said.

The other co-authors on the paper are Scott Boardsen from the University of Maryland, Baltimore County and Chuanfei Dong from Princeton University in New Jersey.


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