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Toe ek dink aan uitsluitingsopsies waar dit nie die moeite werd is om na bewoonbare planete te soek nie, het die verlede by my opgekom.
Reg aan die begin van die heelal was daar geen moontlikheid om (koolstof gebaseerde) lewensondersteunende planete te vorm nie, bloot omdat daar nie genoeg materiaal was om dit te doen nie.
Bevolking III-sterre het binne miljoene jare uitgebrand om van hierdie elemente te verlaat, maar was dit genoeg om planete te vorm? Ek het 'n paar probleme om dinge in perspektief te kry, in ag genome dit
- daar is soveel prosesse wat verskillende soorte en hoeveelhede isotope produseer
- hierdie isotope word op verskillende maniere versprei (en dus ook verskillende konsentrasies van die manier waarop dit geskep is) deur die interstellêre medium
- dit neem waarskynlik baie tyd om swaartekrag genoeg metale in 'n planeet te kondenseer sodra dit uitgespuit is van bv. 'n super nova
- die meerderheid sterre is kleiner, dus dit sal waarskynlik 'n samevoeging van swaarder sterre verg om die toestande vroeër in die heelal te bied.
Ek sou 'n soortgelyke verspreiding van metale soos in die aarde as voldoende vir die lewe beskou, en sou slegs konsentreer op die wat in DNA (N, H, C, P, O), vette (C, H, O) en proteïene (C, O, H, N) sowel as die meeste van die essensiële minerale (Ca, Cl, K, Na, Mg, P, S) en miskien ten minste 'n klomp (moontlik noodsaaklike) spoorelemente (Co, Fe, I, Cu , Mn, Mo, Se, Zn, As, B, Cr, F, Rb, Te, V, Sn, Ni).
Dus, wat is die beste benadering waarop ons tans kan voorsien toe hierdie elemente volop genoeg was om soos rotse met die aarde soos elementêre verspreiding in die aarde te kondenseer?
In dele van die vroeë heelal kan jy redelik vinnig 'n relatiewe hoë metaalvermoë kry - veral in sommige bolvormige trosse en die middelpunte van massiewe sterrestelsels - omdat die vorming van sterre op daardie plekke baie hoog was, wat baie massiewe sterre en 'n aantal " massiewe sterre vorm, gaan supernova en saad rondom gas met metale, nuwe massiewe sterre vorm uit verrykte gas "- die tydskaal van formasie tot supernova kan tot regtig 2 miljoen jaar vir regtig massiewe sterre wees.
Hoe groot metaalagtigheid het u dan nodig om planete te vorm, en hoe vroeg het (dele van) die heelal daar gekom?
Enersyds, ons weet van 'n ster met slegs 'n kwart van die son ([Fe / H] = -0,63) met 'n metaalagtigheid (ystervloed) en 'n planeet van die Neptunus-massa, en Kepler het ongeveer Aardgrootte planete gevind sterre amper soos metaalarm. U het dus nie baie hoë metaalwerke nodig om planete te vorm nie.
Aan die ander kant is daar bewyse van gas rondom kwasars wat verryk word hierbo sonmetaal deur rooi verskuiwings van 6 tot 7 (ongeveer 800 miljoen jaar na die oerknal). Daar is ook taamlik ou sterre in ons sterrestelsel met 'n hoë metaalvermoë: hierdie artikel skat byvoorbeeld dat die sterre in die bolvormige tros NGC 6258 10-12 miljard jaar oud is met byna sonkragmetaal.
Die kort antwoord is dus dat jy binne die eerste 1-2 miljard jaar na die oerknal planete kan laat vorm, en heel moontlik binne 'n paar honderd miljoen jaar nadat die eerste sterre gevorm het. Omdat die metale meestal uit kern-ineenstorting-supernova kom (in teenstelling met die yster-gedomineerde produkte van tipe Ia-supernovas), is daar 'n oormaat sogenaamde "alfa-elemente" - veral suurstof, magnesium, neon, silikon, sulfer, argon en kalsium - relatief tot yster; Ek dink nie jy hoef jou te bekommer oor te veel lewens-kritieke elemente nie.
Hierdie planete sal nie almal baie goeie plekke wees nie leweomdat hulle in digte gebiede met baie sterre en baie sterrevorming sou wees, wat beteken dat daar baie supernovas in die omgewing sal afgaan, en moontlik ook 'n aktiewe kwasar as ons praat oor 'n sterrestelsel.
Hoeveel aardagtige planete bestaan daar in die heelal?
NASA & # x27s Kepler-ruimteteleskoop het nege jaar in die diep ruimte deurgebring om data te versamel wat aan die lig gebring het dat ons naghemel gevul was met miljarde verborge planete
'N Nuwe studie van navorsers aan die Brigham Young University en Pennsylvania State University bied die akkuraatste skatting van die aantal Aardagtige planete in die heelal. Die span het gekyk na die frekwensie van planete wat soortgelyk is aan die Aarde in grootte en in afstand van hul gasheerster, sterre soortgelyk aan ons Son. Om te weet hoe vinnig hierdie potensieel bewoonbare planete voorkom, sal belangrik wees vir die ontwerp van toekomstige astronomiese missies om nabygeleë rotsagtige planete rondom sonagtige sterre te kenmerk wat die lewe kan ondersteun.
Duisende planete is ontdek deur die Kepler-ruimteteleskoop van NASA. Kepler, wat in 2009 van stapel gestuur is en in 2018 deur NASA afgetree is toe die brandstofvoorraad uitgeput is, het honderdduisende sterre waargeneem en planete buite ons sonnestelsel - eksoplanete - geïdentifiseer deur transito-gebeure te dokumenteer. Transitgebeurtenisse vind plaas wanneer 'n planeet se baan tussen sy ster en die teleskoop beweeg, en sommige van die ster se lig blokkeer sodat dit lyk of dit verdof. Deur die hoeveelheid verdof en die duur tussen deurgange te meet en inligting oor die ster se eienskappe te gebruik, tipeer sterrekundiges die grootte van die planeet en die afstand tussen die planeet en sy gasheerster.
"Ons wou planete van ander sterre verstaan, spesifiek die hoeveelheid, grootte en of hulle naby hul onderskeie sterre is," het Darin Ragozzine, 'n BYU-astronomieprofessor en mede-outeur van die studie, gesê. 'Ons kan egter nie net die inligting wat uit die teleskoop gevind is, neem nie. Kepler het eintlik meer groter planete gevind, terwyl daar eintlik meer klein planete is, dit is net moeiliker om te sien. '
Om die hindernis te oorkom, het navorsers 'n nuwe metode ontwerp om die voorkoms van planete oor 'n wye verskeidenheid groottes en wentelafstand af te lei. Die nuwe model simuleer 'heelalle' van sterre en planete en 'waarneem' dan hierdie gesimuleerde heelalle om vas te stel hoeveel van die planete deur Kepler in elke 'heelal' sou ontdek word.
"Sodra ons weet hoe goed ons 'n planeet kan opspoor, kan ons voorspel hoeveel ander planete daar is," het Ragozzine gesê. 'Dit was die hoofdoel van die Kepler-missie om die frekwensies van hierdie planete te bepaal.'
Die resultate van hierdie studie is veral relevant vir die beplanning van toekomstige ruimtemissies om potensieel aardagtige planete te kenmerk. Terwyl die Kepler-missie duisende klein planete ontdek het, is die meeste so ver weg dat dit moeilik is vir sterrekundiges om besonderhede oor hul samestelling en atmosfeer te leer.
'Ons het gevind dat ongeveer tien persent van die sterre 'n planeet het wat ongeveer dieselfde grootte as die aarde het en ongeveer dieselfde hoeveelheid sonlig as die aarde kry. Daar is al lank hieroor gepraat, maar tot Kepler was daar geen data om dit te ondersteun nie, 'het Ragozzine gesê. "Ons het nou regte getalle wat ons regtig help om planete soos die aarde baie algemeen te verstaan."
'N Vraestel wat die model beskryf, verskyn in Die Astronomiese Tydskrif, wat mede-outeur was van Ragozzine en BYU-student Keir Ashby, asook navorsers van Penn State.
Hoe vroeg kon lewensondersteunende planete gevorm word? - Sterrekunde
Is daar lewe in ander wêrelde? As ander planete die lewe chemies kan ondersteun soos ons dit hier op aarde ken, hoe hou dit verband met die oorsprong van die lewe self?
Wetenskaplikes bespiegel al lank oor die teorie dat lewe in sy primitiefste vorm die volgende stap in die kosmiese evolusie kan wees na die vorming van planete. Alhoewel dit slegs 'n teorie is, het nuwe idees oor planetêre oorsprong en onlangse ontdekkings in die chemie dit ondersteun.
By hierdie skrywe is byvoorbeeld veertig miljoen kilometer van die aarde af, 'n planeet kouer as die aarde, met geen suurstof in sy atmosfeer nie en min water op die oppervlak. 'N Man wat na Mars vervoer is, sou snak en sterf - en die meeste ander bekende organismes sou ook vergaan.
Tog het sterrekundiges al meer as 'n halwe eeu geringe seisoenale kleurvariasies op die planeetvariasies waargeneem wat blykbaar saamval met die beskikbaarheid van water. Dit is geïnterpreteer as bewyse vir die plantlewe op Mars, wat spesifiek aangepas is vir die strengheid van die Mars-omgewing. As die aangemelde kleurveranderings werklik is, blyk daar geen ander redelike interpretasie te wees nie.
Verder dui marginale spektroskopiese waarnemings deur W. M. Sinton daarop dat daar molekules met C-H-bindings op die oppervlak van Mars kan wees. Koolstof en waterstof is fundamentele elemente vir alle landorganismes, en die chemiese binding wat dit kombineer, is noodsaaklik vir die struktuur van proteïene, nukleïensure en ander biologiese boustene. Is dit dan moontlik dat dieselfde soort lewe, soortgelyk aan sy basiese chemiese samestelling, twee keer in dieselfde sonnestelsel ontstaan het? Alhoewel dit bespiegelend is in sommige van die besonderhede, is die algemene patroon van kosmiese evolusie redelik goed gevestig.
Kosmiese evolusie begin met 'n enorme kosmiese stofwolk, soos dit vandag tussen die sterre bestaan. So 'n wolk het 'n 'kosmiese' oorvloed van die elemente, wat hoofsaaklik bestaan uit waterstof en helium, met slegs 'n klein mengsel van swaarder elemente. Hier en daar sal die saak ietwat digter wees as in nabygeleë streke. Die meer diffuse streke sal swaartekrag aangetrokke wees tot die digter gebied, wat gevolglik in grootte en massa sal groei. Namate materie na die kondenserende sentrale kern stroom, sal die behoud van die hoekmomentum die hele streek, die kern en die stromende materie, al hoe vinniger laat draai.
Aangesien groot hoeveelhede materie steeds met die kern bots, sal die temperatuur daarvan geleidelik styg. Na miskien honderd miljoen jaar sal die temperatuur in die middel van die wolk tot ongeveer vyftien miljoen grade gestyg het. Dit is die ontstekingstemperatuur vir termonukleêre reaksies (soos die omskakeling van waterstof na helium in die waterstofbom). Op hierdie stadium word die kern van die wolk 'n ster, wat "aanskakel" en lig en hitte uitstraal na die nabygeleë ruimte. As die rotasie voldoende vinnig is, sal die vormende ster onder sekere omstandighede in kleiner dele skei en 'n dubbel- of meervoudige sterstelsel lewer.
Terwyl die ster nou vorm, is daar nog 'n groot stofwolk wat die ster omring en daarmee draai. In hierdie wolk begin die klein newel, klein, digter streke die materie in die omgewing aantrek, soos in stervorming. Die protoplanete wat uit hierdie streke groei (in die swaartekragveld van die nabygeleë ster), styg egter nooit deur botsing tot die termonukleêre ontstekingstemperatuur nie, en word dus planete en nie sterre nie.
Gerard P. Kuiper, professor in sterrekunde aan die Yerkes-sterrewag, het die afgelope paar jaar beskryf hoe planete so gevorm word. In die vormende protoplanete is die neiging dat die swaarder elemente na die middelpunt sak, wat die veel meer waterstof en helium as hoofbestanddele van die atmosfeer rondom die nuwe planete laat. Wanneer die nuutgevormde ster "aanskakel", sal die stralingsdruk hierdie atmosfeer wegwaai.
As die protoplanet egter baie massief of ver van die son af is, kan die aantrekkingskrag van die protoplanet vir 'n gasmolekule groter wees as die bestralingskrag wat dit probeer wegwaai, en die protoplanet kan 'n atmosfeer behou. Hierdie atmosfeer kan oorbly van die prototmosfeer, of dit kan te wyte wees aan uitasemings uit die planetêre binneland. Die aarde se huidige atmosfeer is byvoorbeeld te wyte aan uitasemings Jupiter se huidige atmosfeer is oorblywend.
Op so 'n manier kan 'n mens die atmosfeer van die planete in hierdie sonnestelsel oor die algemeen verstaan:
- Mercurius: Nie massief nie, naby die son, behou die weglaatbare atmosfeer.
- Venus: Massiewer as Mercurius, verder van die son af, behou slegs die swaar gas, koolstofdioksied.
- Aarde: Behou die ligter gasse, stikstof, suurstof en waterdamp, maar het byna alle waterstof en helium verloor.
- Mars: Alhoewel dit verder van die son af is, is dit minder massief as die aarde of Venus, en behou dit dus hoofsaaklik net die swaar gas, koolstofdioksied.
- Jupiter, Saturnus, Uranus, Neptunus: Baie verder van die son af en baie massief, behou hulle baie waterstof en helium, terwyl die ander planete s'n verloor het.
Een feit van ons sonnestelsel wat die doodsklok van baie kosmogonieë laat ontstaan het, is die feit dat alhoewel meer as 99 persent van die massa van die sonnestelsel in die son is, meer as 98 persent van die hoekmomentum van die stelsel in die planete. Dit is asof die rotasie traagheid van die son na die planete oorgedra is. H. Alfven het dit verduidelik as 'n magnetiese rem van die son se rotasie, as gevolg van die interaksie van “sy magnetiese veld met die geïoniseerde sonnevel. Op grond hiervan sal die bestaan van 'n sonnevel waaruit planetêre stelsels vorm, die sentrale ster al hoe stadiger laat draai.
Die oorsprong van planete moet nou afhang van die temperatuur van die sentrale ster. As dit te koud is, sal die atmosfeer van die protoplanete nie weggewaai word nie, wat moontlik die vorming van 'n soort planete soos Jupiter tot gevolg het, maar selfs groter en massiewer. Aan die ander kant, as die ster te warm is, sal die stralingsdruk die sonnevel vinnig versprei en, indien enigiets, klein atmosfeerlose planete agterlaat, of 'n stelsel van miljoene klein asteroïdes. Om planete te vorm, moet die temperatuur van die ster tussen hierdie uiterstes wees.
Daar is nog 'n rede om te glo dat warm sterre nie planete het nie. As die vorming van planetêre stelsels en die verlangsaming van sterre-rotasie voortvloei uit die bestaan van sonnebels, moet ons verwag dat die warm sterre wat hul sonnebusse verdryf en nie planete vorm, vinniger sal draai nie. Dit is presies wat waargeneem word! Hoe warmer die ster, hoe vinniger draai dit. Koeler sterre draai stadiger as wat anders verwag sou word.
By 'n temperatuur van ongeveer 7,000 grade, kenmerkend van wat F-sterre genoem word, is daar 'n skielike afname in die gemiddelde rotasiesnelhede, en dit is moontlik dat alle sterre onder hierdie temperatuur genoeg van hul sonnebusse behou om planete te vorm, (met dien verstande dat hulle nie hul sonnewel gebruik het om dubbele of meerdere sonstelsels te vorm nie).
Die aantal sulke sterre is tussen een en tien persent van die totale aantal sterre, wat daarop dui dat daar net tien miljard sonnestelsels in ons sterrestelsel alleen is. Hiervan het miskien een persent of 100 miljoen planete soos die aarde. Wat is die waarskynlikheid van lewe in hierdie wêrelde?
Aangesien waterstof die kosbaarste element kosmies is, moet die atmosfeer van die vroeë protoplanete van enige stelsel baie waterstof- en waterstofverbindings bevat. Die waterstofverbindings van koolstof, stikstof en suurstof is waarskynlik die meeste waterstofverbindings in die prototmosfeer. Dit is onderskeidelik metaan, CH4, ammoniak, NH3 en waterdamp, H20.
In 1953 het Stanley Miller, PhD'54, toe 'n afgestudeerde student wat onder professor Harold C. Urey gewerk het, getoon dat wanneer waterstof, metaan, ammoniak en waterdamp saamgevoeg word en van energie voorsien word, enkele fundamentele organiese verbindings geproduseer word. (Die energiebron in proto-atmosfeer is waarskynlik ultraviolet lig van die son waaroor die protoplanet draai.)
Hierdie verbindings is byna almal aminosure, die biochemiese boustene waaruit proteïene vervaardig word. Daar is ook rede om te glo dat aminosure lei tot die vorming van puriene en pirimidiene, wat op hul beurt boustene vir nukleïensure vorm. Proteïene en nukleïensure is die twee fundamentele bestanddele van die lewe soos ons dit ken op aarde oorerflike materiale soos gene en chromosome bestaan miskien uitsluitlik uit nukleïensure en proteïene. Daarbenewens is ensieme, wat stadige chemiese reaksies kataliseer en daardeur komplekse lewensvorme moontlik maak, altyd proteïene.
Eksperimente van dieselfde belang as dié van Miller is uitgevoer deur S. W. Fox. Fox het hitte toegedien, tussen 100 en 200 grade Celsius, op eenvoudige molekules, soos dié wat deur Miller gesintetiseer is. Hierdie eenvoudige prosedure het klein hoeveelhede komplekse organiese molekules opgelewer wat wyd versprei word in alle aardorganismes. In die besonder het Fox ureidosuccinic acid vervaardig, 'n belangrike tussenganger in die sintese van nukleïensure. Die temperatuur wat Fox benodig, kan maklik voorsien word deur radioaktiewe verhitting van die aardkors. Daar is bewyse dat sulke radioaktiewe verwarming 'n normale deel van die vroeë evolusie van alle planete is.
Dit is nou opvallend dat die molekules wat deur Miller en Fox geproduseer word, juis die molekules is wat nodig is om lewe te vorm soos ons dit ken. Byna geen molekules is vervaardig wat nie fundamenteel betrokke is by moderne landorganismes nie.
Die prosesse wat deur Miller en Fox beskryf word, sal waarskynlik op ten minste een planeet van elke ster met matige temperatuur plaasvind. Al wat benodig word, is 'n manier om die molekules wat deur hierdie prosesse geproduseer word, op een plek te versamel waar hulle interaksie kan hê. 'N Vloeibare medium op die oppervlak van die planeet dien hierdie doel uitstekend. Molekules wat in die atmosfeer geproduseer word, sal in hierdie vloeistowwe val, en molekules wat op die land geproduseer word deur hitte toe te pas, sal ook daarin gewas word. Alhoewel die seë van vloeibare ammoniak of fluoorwaterstofsuur dien, kan aangetoon word dat die seë van water die doeltreffendste is om die bio-molekules te versamel en te bewaar.
Die een planeet in elke stelsel wat ons oorweeg, het waarskynlik vroeg in sy geskiedenis vloeibare waters, en daarom kan die produksie van proteïene en nukleïensure op sulke planete verwag word.
Nou het proteïene en nukleïensure 'n paar ongewone eienskappe, sover ons weet, nie in enige ander molekule nie. Hulle kan 'n nuwe molekule vorm wat nie net ander identiese molekules kan konstrueer uit die materie wat in die see rondom dit dryf nie, maar wat ook op 'n manier verander kan word, en ook kopieë van die veranderde struktuur kan konstrueer. So 'n muterende, selfreproduserende molekule of versameling van molekules moet natuurlik gekies word. Om hierdie redes moet dit geïdentifiseer word as die eerste lewende wese op die betrokke planeet.
Daar kan dus 100 miljoen planete alleen in hierdie sterrestelsel wees waarop organismes floreer, ten minste biochemies soos ons self. Aan die ander kant, as gevolg van natuurlike seleksie, moet hierdie organismes goed aangepas wees, elkeen na sy eie omgewing. Aangesien selfs geringe verskille in die omgewing uiteindelik ekstreme verskille in die struktuur van organismes veroorsaak, moet ons nie lewensvorms van buite die buiteland aanvaar as iets wat bekend is nie. Maar daar is rede om te glo dat hulle daar is.
NASA-klimaatmodellering stel voor dat Venus moontlik bewoonbaar was
Venus het moontlik tot 2 miljard jaar van sy vroeë geskiedenis 'n vlak oseaan met vloeibare water en bewoonbare oppervlaktemperature gehad, volgens wetenskaplikes aan die NASA se Goddard Institute for Space Studies in New York deur rekenaarmodellering van die antieke klimaat van die planeet.
Die bevindings, wat vandeesweek in die tydskrif Geophysical Research Letters gepubliseer is, is verkry met 'n model soortgelyk aan die soort wat gebruik word om toekomstige klimaatsverandering op aarde te voorspel.
"Baie van dieselfde instrumente wat ons gebruik om klimaatsverandering op aarde te modelleer, kan aangepas word om klimaatstudies op ander planete te bestudeer, beide in die verlede," het Michael Way, 'n navorser van GISS en die hoofskrywer van die artikel, gesê. "Hierdie resultate toon dat die ou Venus dalk 'n heel ander plek was as vandag."
Venus vandag is 'n helse wêreld. Dit het 'n verpletterende koolstofdioksiedatmosfeer 90 keer so dik soos die aarde s'n. Daar is amper geen waterdamp nie. Temperature bereik op die oppervlak 864 grade Fahrenheit (462 grade Celsius).
Wetenskaplikes het lank geredeneer dat Venus gevorm het uit bestanddele soortgelyk aan die aarde s'n, maar 'n ander evolusionêre weg gevolg het. Metings deur NASA se Pioneer-missie na Venus in die 1980's het voorgestel dat Venus oorspronklik 'n oseaan gehad het. Venus is egter nader aan die son as die aarde en ontvang baie meer sonlig. As gevolg hiervan het die planeet se vroeë oseaan verdamp, waterdampmolekules is deur ultravioletstraling uitmekaar gebreek en waterstof het na die ruimte ontsnap. Met geen water meer op die oppervlak het koolstofdioksied in die atmosfeer opgebou, wat gelei het tot 'n sogenaamde wegholkweekeffek wat die huidige omstandighede geskep het.
Vorige studies het getoon dat hoe vinnig 'n planeet op sy as draai, beïnvloed of dit 'n bewoonbare klimaat het. 117 dae op Venus is 117 dae. Tot onlangs toe is aanvaar dat 'n dik atmosfeer soos dié van die moderne Venus nodig is om die planeet se huidige stadige rotasiesnelheid te hê. Nuwer navorsing het egter getoon dat 'n dun atmosfeer soos dié van die moderne aarde dieselfde resultaat kon lewer. Dit beteken dat 'n ou Venus met 'n aardagtige atmosfeer dieselfde rotasietempo kon gehad het as vandag.
'N Ander faktor wat die klimaat van 'n planeet beïnvloed, is topografie. Die GISS-span het die antieke Venus gepostuleer dat dit oor die algemeen meer droë grond gehad het as die aarde, veral in die trope. Dit beperk die hoeveelheid water wat uit die oseane verdamp is en gevolglik die kweekhuiseffek deur waterdamp. Hierdie tipe oppervlak lyk ideaal om 'n planeet bewoonbaar te maak. Dit lyk asof daar genoeg water was om oorvloedige lewe te onderhou, met voldoende land om die sensitiwiteit van die planeet vir veranderinge deur inkomende sonlig te verminder.
Way en sy GISS-kollegas het toestande gesimuleer van 'n hipotetiese vroeë Venus met 'n atmosfeer soortgelyk aan die aarde, 'n dag so lank soos Venus se huidige dag, en 'n vlak oseaan wat ooreenstem met vroeë data van die Pioneer-ruimtetuig. Die navorsers het inligting oor Venus se topografie bygevoeg uit radarmetings wat deur die NASA se Magellan-missie in die negentigerjare geneem is, en die laaglande met water gevul het, en die hooglande blootgestel as Venusiese vastelande. Die studie is ook verreken in 'n antieke son wat tot 30 persent dowwer was. Nietemin het antieke Venus nog ongeveer 40 persent meer sonlig gekry as wat die aarde vandag kry.
"In die simulasie van die GISS-model stel Venus se stadige draai sy dagkant vir byna twee maande op 'n slag bloot," het mede-outeur en mede-GISS-wetenskaplike Anthony Del Genio gesê. 'Dit maak die oppervlak warm en produseer reën wat 'n dik wolklaag skep, wat soos 'n sambreel optree om die oppervlak teen die grootste deel van die sonverwarming te beskerm. Die resultaat is gemiddelde klimaatstemperature wat eintlik 'n paar grade koeler is as vandag se aarde. '
Die navorsing is gedoen as deel van NASA se Planetary Science Astrobiology-program deur die Nexus for Exoplanet System Science (NExSS) -program, wat poog om die soeke na lewe op planete wat om ander sterre of eksoplanete wentel, te versnel deur insigte uit die velde van astrofisika te kombineer, planetêre wetenskap, heliofisika en aardwetenskap. Die bevindings het direkte gevolge vir toekomstige NASA-missies, soos die Transiting Exoplanet Survey Satellite en James Webb Space Telescope, wat probeer om moontlike bewoonbare planete op te spoor en hul atmosfeer te karakteriseer.
Meld u aan om die nuutste nuus, gebeure en geleenthede van die NASA Astrobiologie-program te kry.
Kernfragmente kan help om die oorsprong van lewensondersteunende planete te ontdek
Nuwe navorsing wat vandag in die tydskrif gepubliseer is Fisiese oorsigbriewe beskryf hoe herskepping van isotope wat plaasvind wanneer 'n ster ontplof, fisici kan help om te verstaan waar lewensondersteunende elemente in die ruimte voorkom.
Vir die eerste keer kon 'n navorsingspan onder leiding van die RIKEN Nishina-sentrum van die Universiteit van Surrey in Japan en die Universiteit van Beihang, die isotope van sekere elementêre chemikalieë waaruit 'n ster ontplof, waarneem. Die isotope van hierdie elemente (samarium en gadolinium) is sensitiewe spoorsnyers van die manier waarop sterre ontplof, en help dus om die oorsprong van die swaar elemente wat nodig is om die lewe in die heelal te ondersteun, te verstaan.
Zena Patel, PhD-student aan die Universiteit van Surrey, wat die leiding geneem het met die ontleding van die data, het gesê: "Die belangrike en opwindende kernfisika wat ons uit hierdie eksperimente leer, sal ons baie leer oor die heelal wat ons vandag sien."
Professor Phil Walker, medeskrywer van die Universiteit van Surrey, het gesê: "Ons werk het behels dat sommige van die isotope wat gevorm word as 'n ster ontplof, herskep word. Dit is gedoen deur uraan tot 70 persent van die ligsnelheid te versnel en dit in te bots deur die fragmente te analiseer met behulp van 'n gammastraalmikroskoop, het ons ontdek dat hierdie reaksie gelei het tot die skep van eksotiese isotope waarvan die struktuur nog nooit tevore bestudeer is nie. Dit help om die weg te karteer vir die skepping van noodsaaklike elemente om die lewe te ondersteun.
"Ons studie demonstreer in wese hoe sterstof - die oorblyfsels van ontplofte sterre - 'n rol speel in die vorming van lewensondersteunende planete. Dit is net een ontdekking in 'n lang proses, maar dit sal die weg baan vir verdere werk om die begrip te begryp. die omstandighede wat nodig is vir lewe in die heelal. '
Biomerkers
Ons waarnemings suggereer toenemend dat planete op aarde wat in die bewoonbare sone wentel, algemeen in die Melkweg voorkom - huidige beramings dui daarop dat meer as 40% van die sterre ten minste een het. Maar is daar een van hulle bewoon? As u geen probes daarheen kan stuur om monsters te neem nie, sal ons die antwoord moet kry uit die lig en ander straling wat deur hierdie verre stelsels na ons toe kom (Figuur 6). Watter tipe waarnemings kan goeie lewensgetuienis wees?
Figuur 6: Aarde, soos gesien deur NASA se Voyager 1. In hierdie beeld, geneem van ongeveer 4 miljard myl weg, verskyn die aarde as 'n & # 8220pale blou punt & # 8221 wat minder as 'n pixel se lig voorstel. Sou hierdie lig Aarde openbaar as 'n bewoonbare en bewoonde wêreld? Ons soeke na lewe op eksoplanete sal afhang van die vermoë om inligting oor die lewe uit die flou lig van verre wêrelde te haal. (krediet: wysiging van werk deur NASA / JPL-Caltech)
Om seker te wees, moet ons op soek na robuuste biosfere (atmosfeer, oppervlaktes en / of oseane) wat verandering op die planeet kan skep. Die aarde is gasheer vir so 'n biosfeer: die samestelling van ons atmosfeer en die ligspektrum wat van ons planeet weerkaats word, verskil aansienlik van wat verwag sou word in die afwesigheid van lewe. Tans is die aarde die enigste liggaam in ons sonnestelsel waarvoor dit waar is, ondanks die moontlikheid dat bewoonbare toestande in die ondergrond van Mars of binne die ysige mane van die buitenste sonnestelsel kan heers. Al bestaan daar lewe in hierdie wêrelde, is dit baie onwaarskynlik dat dit veranderinge op die planeet kan oplewer wat beide teleskopies waarneembaar en duidelik biologies van oorsprong is.
Wat die Aarde & # 8220 spesiaal & # 8221 maak onder die potensieel bewoonbare wêrelde in ons sonnestelsel, is dat dit 'n fotosintetiese biosfeer het. Dit vereis die aanwesigheid van vloeibare water aan die oppervlak van die planeet, waar organismes direkte toegang tot sonlig het. Die bewoonbare sone-konsep fokus op hierdie vereiste vir vloeibare water op die oppervlak - alhoewel ons weet dat bewoonbare toestande op die oppervlak op die omliggende bane kan heers - presies omdat hierdie wêrelde biosfere op 'n afstand opspoorbaar sal wees.
Inderdaad, plante en fotosintetiese mikro-organismes is so volop aan die Aarde se oppervlak dat dit die kleur van die lig beïnvloed dat ons planeet na die ruimte weerkaats - ons lyk groener in sigbare golflengtes en weerkaats meer naby-infrarooi lig as wat ons andersins sou doen. Daarbenewens het fotosintese die aarde se atmosfeer op groot skaal verander - meer as 20% van ons atmosfeer is afkomstig van die fotosintetiese afvalproduk, suurstof. Sulke hoë vlakke sal baie moeilik verklaar kan word in die afwesigheid van lewe. Ander gasse, soos stikstofoksied en metaan, word gelyktydig met suurstof aangetref as moontlike lewensaanwysers. As dit genoeg is in 'n atmosfeer, kan sulke gasse bespeur word deur hul effek op die ligspektrum wat 'n planeet uitstraal of weerkaats. (Soos ons in die hoofstuk oor eksoplanete gesien het, begin sterrekundiges vandag die vermoë om die spektrum van die atmosfeer van sommige planete op te spoor wat om ander sterre wentel.)
Sterrekundiges het dus tot die gevolgtrekking gekom dat 'n soeke na lewe buite ons sonnestelsel, ten minste aanvanklik, moet konsentreer op eksoplanete wat soveel soos die Aarde lyk - ongeveer aarde-grootte planete wat in die bewoonbare sone wentel - en soek na die teenwoordigheid van gasse in die atmosfeer of kleure in die sigbare spektrum wat moeilik verklaarbaar is, behalwe deur die aanwesigheid van biologie. Eenvoudig, of hoe? In werklikheid stel die soeke na die lewe van eksoplanet baie uitdagings.
Soos u kan dink, is hierdie taak meer uitdagend vir planeetstelsels wat verder weg is, en dit sal ons soeke prakties beperk tot die bewoonbaarste wêrelde wat die naaste aan ons eie is. As ons beperk word tot 'n baie klein aantal nabygeleë teikens, sal dit ook belangrik wees om die bewoonbaarheid van planete in ag te neem wat wentel om die M-dwerge wat ons hierbo bespreek het.
As ons daarin slaag om 'n skoon sein van die planeet af te skei en 'n paar funksies in die ligspektrum te vind wat 'n aanduiding van die lewe kan wees, sal ons hard moet werk om te dink aan enige nie-biologiese proses wat hulle kan verantwoord. & # 8220Life is die hipotese van die laaste uitweg, & # 8221 opgemerk sterrekundige Carl Sagan — wat beteken dat ons alle ander verklarings moet uitput vir wat ons sien voordat ons beweer dat ons bewyse van buiteaardse biologie gevind het. Dit vereis 'n mate van begrip van watter prosesse in werelde kan werk, en wat ons relatief min sal weet oor wat ons op aarde vind, kan dien as 'n riglyn, maar ook om ons te mislei (Figuur 7).
Onthou byvoorbeeld dat dit uiters moeilik sou wees om die oorvloed suurstof in die Aarde se atmosfeer te verreken, behalwe deur die teenwoordigheid van biologie. Maar daar is vermoed dat suurstof tot aansienlike vlakke kan opbou op planete wat om M-dwergsterre wentel deur die werking van ultravioletstraling op die atmosfeer - sonder dat dit biologies nodig is. Dit is van kritieke belang om te verstaan waar sulke & # 8220valse positiewe & # 8221 kan bestaan in die soektog.
Ons moet verstaan dat ons miskien nie biosfere sal kan opspoor nie, al is dit ook daar. Die lewe floreer miskien 3,5 miljard jaar op aarde, maar die atmosferiese & # 8220biosignatures & # 8221 wat vandag goeie bewyse vir die lewe aan verre sterrekundiges sou lewer, was nog nie al die tyd aanwesig nie. Suurstof het byvoorbeeld net iets meer as 2 miljard jaar gelede tot waarneembare vlakke in ons atmosfeer opgehoop. Kon die lewe op aarde voor daardie tyd bespeur word? Wetenskaplikes werk aktief om te verstaan watter addisionele kenmerke bewyse kon lewer van die lewe op aarde gedurende daardie vroeë geskiedenis, en sodoende ons kanse om 'n lewe verder te vind, help.
Figuur 7: Ligspektrum wat deur die aarde se atmosfeer oorgedra word. This graph shows wavelengths ranging from ultraviolet (far left) to infrared. The many downward “spikes” come from absorption of particular wavelengths by molecules in Earth’s atmosphere. Some of these compounds, like water and the combination oxygen/ozone and methane, might reveal Earth as both habitable and inhabited. We will have to rely on this sort of information to seek life on exoplanets, but our spectra will be of much poorer quality than this one, in part because we will receive so little light from the planet. (credit: modification of work by NASA)
Sleutelkonsepte en samevatting
The search for life beyond Earth offers several intriguing targets. Mars appears to have been more similar to Earth during its early history than it is now, with evidence for liquid water on its ancient surface and perhaps even now below ground. The accessibility of the martian surface to our spacecraft offers the exciting potential to directly examine ancient and modern samples for evidence of life. In the outer solar system, the moons Europa and Enceladus likely host vast sub-ice oceans that may directly contact the underlying rocks—a good start in providing habitable conditions—while Titan offers a fascinating laboratory for understanding the sorts of organic chemistry that might ultimately provide materials for life. And the last decade of research on exoplanets leads us to believe that there may be billions of habitable planets in the Milky Way Galaxy. Study of these worlds offers the potential to find biomarkers indicating the presence of life.
Woordelys
biomarker: evidence of the presence of life, especially a global indication of life on a planet that could be detected remotely (such as an unusual atmospheric composition)
habitable zone: the region around a star in which liquid water could exist on the surface of terrestrial-sized planets, hence the most probable place to look for life in a star’s planetary system
The planet hunters
Observing from Earth, and from orbit
The very first planets detected around other stars were wild, extreme worlds. Some orbited a spinning stellar corpse &ndash the core of an exploded star &ndash called a pulsar, and were regularly raked by pulses of radiation. Another, a scorching gas giant with about half the heft of our own planet Jupiter, hugged its star so tightly that a year, once around the star, took only four days.
Their extreme nature, however, also made them easier to find with the early planet-hunting technology of the 1980s and &rsquo90s. Ground-based observatories took the reins, providing the historic first burst of exoplanet discovery. The technology got better and the planet count ran into the hundreds. Still, Earth&rsquos thick atmosphere and its rippling interference kept even the best ground-based telescopes from seeing more clearly.
Lifting our telescopes above the veil of Earth's atmosphere revealed a dazzling universe across the light spectrum. It also extended our reach in the search for planets around other stars. Now we count these confirmed distant worlds &ndash exoplanets &ndash in the thousands, many of them about the size of Earth and orbiting in their stars&rsquo "habitable zones." The next generation of space telescopes will open new windows in the search for life as we peer into the atmospheres of these planets, and taste their skies.
Historic timeline
Legacy of light: Hubble
NASA's Hubble Space Telescope, marking its 30th anniversary in orbit in 2020, was a pioneer in the search for planets around other stars Hubble even has been used to make some of the earliest profiles of exoplanet atmospheres.
Kepler and K2
Another space explorer, NASA's Kepler Space Telescope, made history with its discovery of thousands of exoplanets, searching for tiny dips in starlight as the planets crossed the faces of their stars. In its first mission, from 2009 to 2013, Kepler monitored more than 150,000 stars, watching for tiny dips in starlight as planets crossed in front of their stars. The first mission ended in 2013 when technical problems caused the spacecraft to lose much of its pointing ability. In 2014, it began its second mission, dubbed K2, and continued discovering exoplanets despite its diminished directional capability. Decommissioned in 2018, Kepler remains credited with discovering the most exoplanets of any mission so far &ndash more than 2,600. Researchers are steeds finding planets in Kepler&rsquos data and will continue to for years.
Spitzer
Spitzer probed the heavens in the infrared portion of the spectrum, capturing images of newborn stars nestled inside thick clouds of dust along with millions of other images. The space telescope was retired in 2020 &ndash although, like Kepler, the data it gathered will be mined by scientists for years to come, likely yielding a continuing stream of discovery.
One of NASA's four Great Observatories, a distinction it shares with Hubble, Chandra and the Compton Gamma Ray Observatory, Spitzer proved a powerful contributor to the hunt for exoplanets and analysis of their atmospheres. Among its most celebrated work is the detection of seven planets roughly the size of Earth orbiting a star called TRAPPIST-1 Spitzer was able to determine both the masses and densities of these worlds. It ended its 16-year observing run in January 2020.
Taking the baton: TESS
The Transiting Exoplanet Survey Satellite (TESS) picked up where Kepler and K2 left off, again conducting a grand survey of the sky. But while Kepler in a sense drilled core-samples into the heavens &ndash taking deep, penetrating looks into small patches &ndash TESS's star pictures are painted in broad strokes. TESS is conducting a nearly all-sky survey in sequential segments, first the dome of stars that would be seen from the Southern Hemisphere, then the Northern. Its mission is to find planets around brighter, closer stars, again by searching for shadows: the incredibly tiny subtraction of light from a star when a planet crosses in front of it.
During its 4-year prime mission, Kepler was a statistical transit survey designed to determine the frequency of Earth-sized planets around other stars. Kepler revealed thousands of exoplanets orbiting stars in its 115 square degree field-of view, which covered about 0.25 percent of the sky. While Kepler was revolutionary in its finding that Earth-to-Neptune-sized planets are common, the bulk of the stars in the Kepler field lie at distances of hundreds to thousands of light-years, making it difficult to obtain ground-based follow-up observations for many systems.
TESS is designed to survey more than 85% of the sky (an area of sky 400 times larger than covered by Kepler) to search for planets around nearby stars (within about 200 light-years). TESS stars are typically 30-100 times brighter than those surveyed by Kepler. Planets detected around these stars are therefore far easier to characterize with follow-up observations, resulting in refined measurements of planet masses, sizes, densities, and atmospheric properties.
Partnerships &ndash from the ground up
NASA works with partners across the country and around the world to investigate exoplanets &ndash whether studying them from space or from the ground.
Collaborating with ground-based telescope teams is essential. When the TESS space telescope captures evidence of a new exoplanet, observations from the ground not only can confirm its existence but tell us more about the planet itself. Measurements of the planet's "mass," or heft, can be combined with TESS' measurement of its diameter, yielding its density. That, in turn, can tell us whether it's a gas planet, like Neptune, or a more dense, rocky world like ours.
Ground-based telescopes that have helped confirm and characterize exoplanets, or will soon, include the Magellan II at Las Campanas Observatory in Chile, the NEID instrument on the WIYN telescope at Kitt Peak, Arizona, the Keck Observatory on Mauna Kea, Hawaii, and the Hale Telescope at the Palomar Observatory in Southern California to name just a few among dozens. They will work with space-based telescopes &ndash TESS and, soon, the James Webb Space Telescope &ndash to provide details of exoplanet atmospheres, composition and other vital statistics.
Missions to come
Powerful next-generation instruments will bring us closer to what would be a long-anticipated, profound discovery: a small, rocky, habitable world somewhere in the galaxy with an atmosphere that reminds us of our own.
James Webb Space Telescope
This giant spacecraft could cover a typical tennis court with its sunshield fully deployed. It's set to launch from French Guiana in 2021. Atop the sunshield will be the largest primary mirror ever sent into space &ndash some 6.5 meters (21 feet, 4 inches) across. Seeing the universe in infrared light, the Webb telescope is expected to become the premiere observatory of the decade, studying billions of years of the universe's history and reaching back nearly to the Big Bang. It will reveal details of the formation of planetary systems like our own, and even sample (via the rainbow spectrum of captured light) the composition of exoplanet atmospheres.
A space-based platform: the Roman telescope
A space-based platform: the Roman telescope
A telescope powerhouse now under development could open new windows of knowledge when it launches, as soon as the mid-2020s. And the Nancy Grace Roman Space Telescope &ndash formerly known as WFIRST &ndash will have a wide window indeed, about 100 times the field of view of the Hubble Space Telescope.
The Roman telescope, named for a NASA pioneer, will probe the depths of dark matter and dark energy &ndash mysterious, mostly unknown phenomena that make up most of the universe &ndash as well as making direct images and other observations of exoplanets as part of a technology demonstration. At the heart of its mission: the star-dense interior of the Milky Way galaxy, where the telescope could find thousands of exoplanets through gravitational microlensing.
Ask Ethan: Were Mars And Venus Ever Living Planets?
While Mars is known as a frozen, red planet today, it has all the evidence we could ask for of a . [+] watery past, lasting for approximately the first 1.5 billion years of the Solar System. Could it have been Earth-like, even to the point of having had life on it, for the first third of our Solar System's history?
One of the most elusive questions in all of science is the question of life in the Universe. We know that it exists on Earth, that every extant living organism on Earth descended from the same common ancestor going back billions of years, and that life has been on Earth continuously for over 4 billion years: at least 90% of our planet’s existence. But we don’t know how ubiquitous life is at all. We have no information about life on other worlds in our Solar System, on life in other Solar Systems, or on intelligent life anywhere else in the Universe. All we have are constraints on what could be out there.
Every planet that could have had life on it, at any point, represents a chance for life to develop. We know Earth was one of those chances that panned out, but at least two other worlds in our young Solar System — Mars and Venus — represented potential chances as well. Could they have had life on them, if not now, than in our distant past? That’s what Carol Lake wants to know, writing in to ask:
“Could it be possible that Mars and Venus were living worlds? Like Earth climate change is killing it so climate change is going to kill all living things and then Earth will become just another planet that the new life wonders about the possibility of us?”
It’s an interesting question to explore, as both Mars and Venus did suffer catastrophic climate events billions of years ago. Here’s what remains possible based on what we know.
Although we now believe we understand how the Sun and our solar system formed, this early view is an . [+] illustration only. When it comes to what we see today, all we have left are the survivors. What was around in the early stages was far more plentiful than what survives today.
JOHNS HOPKINS UNIVERSITY APPLIED PHYSICS LABORATORY/SOUTHWEST RESEARCH INSTITUTE (JHUAPL/SWRI)
Let’s go way, way back some 4.6 billion years: back to the earliest days of our Solar System’s formation. When Solar Systems like our own first form, there are a number of things that must occur in a particular order. In the case of what gave rise to our Solar System, we believe this is what had to occur:
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Uitgelê: Waarom hierdie week se 'Strawberry Moon' so laag, so laat en so helder sal wees
- a molecular cloud of gas contracts under its own gravity,
- the regions with the greatest concentrations of matter collapse more quickly,
- leading to the formation of new stars and star systems in the regions of greatest collapse,
- where the largest mass clumps grow fastest, becoming the most massive stars,
- but smaller clumps grow slower, becoming lower-mass stars,
- and that one of those smaller clumps, with only one large initial (central) mass, became the proto-star that would grow into our Sun.
That central mass will continue to grow, emitting copious amounts of radiation and slowly heat up in its core. As material continues to gently fall onto the central proto-star, a circumstellar disk emerges around it. Gravitational instabilities will form in that disk, leading to planetesimals: the seeds of what will eventually become planets.
What happens next is not an easy process to predict, as planet formation is a chaotic process. There’s are basically three “zones” with respect to the star or proto-star that’s forming in the center, which defines what types of elements you wind up with.
- In the innermost region, closest to the star, is what’s known as the “soot line.” Interior to this zone, many of the carbon-based molecules that are thought to be precursors to life, like polycyclic aromatic hydrocarbons, are destroyed. Only heavy elements, like metals, can survive in this innermost region.
- Beyond that, exterior to the soot line, you can have these complex compounds, but no ices: water-ice, ammonia ice, dry ice, nitrogen ice, etc. As long as you’re still inside the frost line, those volatile compounds will be vaporized. A young Venus, Earth, and Mars were all outside the soot line but inside the frost line.
- And exterior to the frost line, you can have all the volatile compounds there are. Various ices are fine large amounts of hydrogen and helium can easily survive when bound to a gas giant asteroid-like and comet-like bodies are common.
Over time, the planetesimals that form will gravitationally interact, grow, merge, and chaotically influence one another. Some bodies get flung into the Sun others out of the Solar System others accrete onto larger masses. Eventually, a stable planetary configuration is reached.
The early Solar System was filled with comets, asteroids, and small clumps of matter that struck . [+] practically every world around. This period, known as the late heavy bombardment, may be the mechanism responsible for bringing the majority of the water found on the inner solar system worlds to those worlds, including Earth.
In these latter stages, the volatile compounds bound onto the objects located beyond the frost line suffer two fates: they either wind up bombarding one of the surviving planets, or they wind up getting scattered elsewhere. (It’s thought that this is likely where the water found on Earth and the other inner planets comes from.) Typically, there are only two locations, long-term, where those objects wind up: exterior to the initial frost line but interior to the orbit of the next planet out, and beyond the orbit of the final planet in the solar system. These locations, in our own Solar System, correspond to the asteroid belt and the Kuiper belt/Oort cloud, respectively.
At last, we come to about 4.5 billion years ago, where in our Solar System, we had three worlds that we suspect were relatively similar. Venus, Earth, and Mars all were rocky planets, with thin-but-substantial atmospheres, water on their surfaces, some of which was likely in liquid form, and they were all extremely rich in organic compounds: the precursor molecules to life.
Earth, at left, and Venus, as seen in infrared at right, have nearly identical radii, with Venus . [+] being approximately
90-95% the physical size of Earth. However, due to its close proximity to the Sun, Venus suffered a tremendously different fate earlier on. It's possible that, about a billion years from now, Earth will finally follow suit.
Arie Wilson Passwaters/Rice University
The big question we have to ask ourselves is: what happened?
What happened, on Venus, to turn it into the hellhole of an inferno that it is today? When did it occur, how did it happen, and could there have been life thriving and surviving on that planet prior to this catastrophic event?
What happened, on Mars, to cause it to lose its atmosphere, to dry up, and to freeze, rendering the biological processes that we associate with life either impossible or so rare that we have yet to detect them?
And what’s happening now, on Earth, and does that have the potential to lead to a similar fate to either Venus or Mars: where a once habitable (or, at least, potentially habitable) planet is now totally inhospitable to life as we know it?
One thing is certain: despite all the uncertainties surrounding the origin of life on Earth, we know that once it took hold on our planet — an event that occurred more than 4 billion years ago — it survived and thrived in an unbroken chain of events that have occurred ever since. While there were many mass extinction events, they only served to make way for the surviving species to reproduce and fill the then-vacant ecological niches. Our planet remains a living one.
The Mars Orbiter Laser Altimeter (MOLA) instrument, part of Mars Global Surveyor, collected over 200 . [+] million laser altimeter measurements in constructing this topographic map of Mars. Everywhere that appears with a dark or light blue color, as well as some of the greener areas, was likely covered in water long ago.
Mars Global Surveyor MOLA team
In the early stages of our Solar System, however, Earth wasn't necessarily the only living planet. All three worlds — Venus, Earth, and Mars — experienced external impact events and had to deal with internal geologic processes. There were magnetic events in the core, continental uplifting and erosion, and the eventual presence of mountain ranges and basins. All of these worlds experienced extensive volcanic activity, which added volatile compounds and copious amounts of carbon dioxide to the atmosphere, while also creating relatively smooth ocean bottoms. All three worlds, very likely, had a watery past.
But there are three major differences between these planets that likely led to their vastly differing fates.
- One is their differing orbital distances from the Sun, with Venus orbiting at just
72% of the Earth-Sun distance and Mars orbiting much farther out, at around
This four-panel illustration shows a possible pathway for the eventual terraforming of Mars to be . [+] more Earth-like. What very likely happened in the past, however, was a reversal of this process: where a once watery, wet, and possibly life-rich Mars lost its protective magnetic field, which led to its atmosphere being stripped away. Today, liquid water is largely impossible on the Martian surface.
English Wikipedia user Ittiz
Life on a world is generally regarded as a stabilizing force, the same way that a buffer solution in chemistry prevents the addition of an acid or base from making the entire solution too acidic or too basic. Life reaches a sort of equilibrium state with its environment, where any major changes in temperature — either in the positive or negative direction — will lead to life processes working to counteract that change. Only if a major change occurs to fundamentally alter the equilibrium state, like the great oxygenation event did on Earth, what yeast cells do in an unlimited-nutrient environment, or what humans are doing with fossil fuels today, can a runaway event take place.
But on Venus and Mars, even if life was once present on those worlds, its presence was insufficient to stop the runaway processes that were very likely initiated by astrophysical and geological factors. Venus may have been a thriving world for hundreds of millions of years, possibly even as many as 2 billion, according to some. Its conditions may have been Earth-like, with liquid water on the surface and possibly a whole lot more. Similarly, Mars once had oceans, rivers, formed sedimentary rocks and hematite spherules, and was temperate and wet for at least 1.5 billion years.
This iconic photograph of the Martian blueberries, or hematite spheres, was taken by Opportunity in . [+] the lowlands of Mars. It is thought that a watery past led to the formation of these spherules, with very strong evidence coming from the fact that many of the spherules are found attached together, which ought to occur only if they had a watery origin.
JPL / NASA / Cornell University
The big question, of course, is “what happened?”
On Venus, the factor that doomed it is likely very simple: its proximity to the Sun. Given how close it is, it receives about double the amount of incident energy on every square meter of its surface compared to Earth. With even a small amount of water vapor in the atmosphere of early Venus, a large greenhouse effect would ensue, raising the temperature of Venus further. At higher temperatures, the water vapor concentration in the atmosphere increases further, which raises the temperature further as well.
Unfortunately for Venus, this process cannot simply gradually increase forever. At some critical moment, the surface temperatures on Venus will reach a critical value: about 100 °C (212 °F), or maybe a little higher depending on the atmospheric pressure at the time. When that occurs, the liquid water on the surface of Venus will begin boiling away, launching an enormous amount of water vapor — basically, the sum of all of the Venusian oceans — into the atmosphere, and that leads to a runaway greenhouse effect. All of a sudden, Venus’s atmosphere is far too hot to admit life on the surface the only place where it could theoretically have persisted is in the upper atmosphere of Venus,
60 km up or so. Whenever this occurred, any life that previously existed on Venus would likely meet its end.
NASA's hypothetical HAVOC mission: High-Altitude Venus Operational Concept. This balloon-borne . [+] mission could look for life in the cloudtops of our nearest neighbor, as the condition on Venus that are
60 km above the surface are surprisingly Earth-like in terms of pressure and temperature. As this would be above the layers of sulfuric acid, life may have persisted up here for billions of years.
NASA Langley Research Center
Meanwhile, on Mars, it receives only
43% of the energy Earth receives (from the Sun) on every square meter. In order for Mars to have been watery and wet — which there’s an overwhelming amount of geological evidence for — there must have been a substantial, thick atmosphere on Mars long ago. Only a strong greenhouse effect could have kept both the temperatures and pressures where they needed to be for liquid water to exist on the Martian surface.
The only thing that could have kept Mars’s atmosphere intact was the protection of a planet-wide magnetic field, similar to what Earth has today. Without it, Mars’s atmosphere would get stripped away by the solar wind: something that NASA’s MAVEN mission has measured directly. Due to the much smaller size of Mars compared to Earth, its core cooled much more rapidly, eventually leading to the death of the internal magnetic dynamo that actively diverts those solar particles away. Without a protective magnetic field — which we estimate died after about
1.5 billion years — practically the entire Martian atmosphere would have been stripped away in only
0.01 billion years: a cosmic blink-of-an-eye.
Without that atmosphere, the liquid water either froze or sublimated, any life either went dormant or died out, and Mars has been cold and (largely) lifeless for the
3 billion years that have passed ever since.
Mars, the red planet, has no magnetic field to protect it from the solar wind, meaning that it loses . [+] its atmosphere in a way that Earth doesn't. The timescale over which Mars will lose an Earth-like atmosphere is on the order of
10 million years only, but Earth's magnetic field should remain intact for many billions of years this mechanism will not result in the inhabitability of Earth.
Will humanity wind up destroying all life on Earth? It’s an unlikely prospect. It’s not impossible, as we’ve already entered what scientists have classified as the 6th great mass extinction. The climate is changing our wild places are disappearing (less than one-third of the Earth’s surface is now wilderness) the oceans are acidifying the CO2 concentration in the atmosphere is higher than it’s been in millions of years, and continues to increase at a record rate owing to human activities. If we’re not careful, the possibility of ecological collapse is very real, and could very well result in humanity’s eradication and possibly even the fall of mammals entirely.
But life, in some form, should still persist on our planet. Just as was the case on Venus and Mars, the “game over” moment for life on Earth will likely arise from the influence of the Sun. As time goes on and the Sun continues to burn through its nuclear fuel, it will heat up and get more luminous. After approximately another
1 billion years, give or take, its energy output will boil the Earth’s oceans as well, bringing an end to life-as-we-know-it here on our planet. While human-caused climate change might bring about our own demise, life on Earth is far more resilient. If we can survive our technological infancy, we’ll have at least many hundreds of millions of years until a planet-threatening crisis arrives. May we continue to rise to the challenge of finding a balance with nature. It’s our only hope of long-term survival.
Could Life Be 12 Billion Years Old?
Much of thesearch for life outside of Earth's biological oasis has focused on examiningthe conditions on the other planets in our solar system and probing the cosmosfor other Earth-like planets in distant planetary systems.
But oneteam of astronomers is approaching the question of lifeelsewhere in the universe by looking for life'spotential beginning.
AparnaVenkatesan, of the University of San Francisco, and Lynn Rothschild, of NASA'sAmes Research Center in Moffett Field, Calif., are using models of starformation and destruction to determine when in the roughly 13.7 billion-year historyof the universe the biogenic elements ? those essential to life as we know it ?might have been pervasive enough to allow life to form.
We can pindown the emergence of life on Earth to somewhere around 3.5 billion years ago. Venkatesanand Rothschild want to find out what happens when you broaden the question tolife throughout the universe.
"Canyou blast that open? Could you really start really talking about life in theuniverse at 12 billion years? And that's the question that we're talking about,"Rothschild said.
With basicestimates of the elements produced by the first several generations of stars,the pair has so far found that "most of [the essential elements] can becreated fairly quickly in the early universe," Venkatesan said.
Venkatesanpresented their first findings last week at the 214th meeting of the AmericanAstronomical Society in Pasadena, Calif.
For life aswe know it to form and thrive, four conditions must be met: sufficient amountsof the so-called biogenic elements, a solvent (on Earth, that solvent is liquidwater), a source of energy, and time "for the elements to build up andcreate a home and conditions for life to thrive," Venkatesan explained.
The biogenicelements include carbon, nitrogen, oxygen, phosphorous, sulfur, iron, andmagnesium.
"Carbonin particular is very interesting," Venkatesan said. Carbon is "ubiquitousin the solar system and beyond" and "is extremely versatilechemically."
Theseelements, like all elements present in the universe today, are forged in thefurnaces of stars. But not all stars make each element, and some produceelements much faster than others.
Low-massstars create all the elements on the periodic table through carbon,but because these stars live long lives, they produce the elements slowly.Intermediate mass stars tack on nitrogen through oxygen. Finally, the mostmassive stars, with their intense ovens, make all the elements up to iron and someother heavy metals. And because these stellar beasts lead such short,violent lives, they can churn out elements faster than smaller stars.
Theexplosions that end these stars' lives can vary though, and their differentsignatures indicate the amounts of metals, such as iron and nickel, involved,Venkatesan said.
It isthought that the first stars to form in the early universe were very massive.These stars would have characteristic compositions that in turn imply that theywould have specific elemental abundances "that they create in their deaththroes."
The twoscientists came up with the idea for applying the study of the first stars toastrobiology when Rothschild came to Venkatesan's department for a talk. Whiletalking at dinner that night, "we began to realize it might be really funto look at just when the first building blocks for life could be outthere," Venkatesan said. "To the best of our knowledge, we didn'tknow anyone else out there who was at the time talking about it or thinkingabout it."
Rothschilddrew up what she calls her "wish list" of elements that she considersabsolutely essential to life as we know it. Venkatesan then used currenttheories of star formation, from the first very massive stars to the stars thatformed later from the seeds sown by the first stars, to model the build up ofeach of the biogenic elements.
"Thenumber one element is carbon," Rothschild said. "And you come up withthat because they're really only two elements that have any real versatility interms of being able to create a bunch of compounds that could then form a life,and one is silicon and one is carbon."
But silicongets ruled out because it isn't as prevalent in the universe, nor as chemicallyversatile.
"Thereality check is that we're sitting on a big silicate rock, and we're not madeof silicon," Rothschild said.
Roundingout the list of must-haves are hydrogen, oxygen and nitrogen.
"Nitrogenseems to be critical. It's found in so many compounds, and that really addshuge versatility then to the suite," Rothschild said. Nitrogen, forexample, is the backbone of amino acids, which in turn are the building blocksof proteins and have been detected in interstellar space.
Secondaryand tertiary lists include phosphorus, sulfur, iron and magnesium, "andall sorts of funky things which are used a lot, but I could more easilyconceive of a system without it," Rothschild said.
They foundthat "nitrogen can actually build up very quickly," Venkatesan said.But not right at the beginning, because those first massive stars "woefullyunder-produce nitrogen." It takes later-generation stars to boost levelshigh enough to what scientists think might be needed to make the elementpervasive enough.
Carbon also"takes a little while to build up," because it needs low- andintermediate- mass stars, Venkatesan said.
While thoseearly massive stars would have had trouble producing nitrogen, they "arefairly efficient at producing iron early on. That is because they completelyblow apart," Venkatesan said.
Overall,the modeling effort found that iron and magnesium levels would have surgedearly on, with carbon taking at least 100 million years to build up.
Though thecritical masses of biogenic elements needed to allow life to form aren't known,"these amounts will be more than enough," Venkatesan said.
So byperhaps around 100 million after the universe began, many of these elements would be found insubstantial enough numbers, though the timescale may be more around 500 millionyears for carbon and the jury is still out with nitrogen.
Bettermodels and improved knowledge of the physics at work in early stars couldchange the picture somewhat, changing the timescales for the buildups of theelements and the interstellar environment they are born into.
Of course, knowingwhich elements need to be present and whether or not they are won't answer thequestion of when life might have been able to spring forth. The elements mustalso collect in pools in significant enough amounts.
"Thatfinal question is not only which elements, but what concentration do you buildup locally?" Rothschild said.
OnceRothschild comes up with estimates of the amounts of different elements likelyrequired, she and Venkatesan can use models that estimate concentrations ingalaxies and solar systems over time and see if they find any likely-lookingspots for life to form.
"Allwe need is one place in the universe that has the conditions, the prerequisites,"Rothschild said.
Solvents,such as liquid water or methane, will also have to be factored in. Venkatesansaid that in the long term, they hope to use the same methods to figure outwhen water might have existed in sufficient quantities.
There isalso the question of whether life could have thrived in the harsh,ultraviolet-dominated environments of the earlystars. Ultraviolet light is thought to have both beneficial and detrimentaleffects on life, but which might have won out in the early universe isn'tknown.
Ultimatelythe question will become, "can we build up the building blocks" earlyon, Venkatesan said. Though answering that question will take some time, itcould have a substantial impact on studies of the early universe, exoplanetresearch, and the expectations of how far along alien life might have evolved,not to mention our view of our place in the universe.
"It'snot going to cure cancer," Rothschild said. "But I think in a way,it's a very profound question: when can you start talking about life in ouruniverse?"
Bio-Markers
While it may be possible for life to exist on a planet or moon below its surface, we will not be able to detect its presence from a great distance away (e.g., if it is in another star system beyond our solar system). In our fastest rocket-propelled spacecraft, it would take us over 70,000 years to travel to the next star system (Alpha Centauri). The type of inhabited planet we will be able to detect outside of our solar system is life that has changed the chemistry of the planet's atmosphere, i.e., the life will have to be on the surface. By analyzing the spectrum of the planet's atmosphere, we may be able to detect bio-markers---spectral signatures of certain compounds in certain proportions that could not be produced by non-biological processes. Bio-markers are also "biosignatures"
Spectral lines from water would say that a planet has a vital ingredient for life but it does not mean that life is present. If oxygen, particularly ozone (a molecule of three oxygen atoms), is found in the atmosphere, then it would be very likely that life is indeed on the planet. Recall from the solar system chapter that molecular oxygen quickly disappears if it is not continually replenished by the photosynthesis process of plants and cyanobacteria. However, it is conceivably possible for a few non-biological processes to create an atmosphere rich in molecular oxygen and ozone. For example, on a planet with a runaway greenhouse effect, ultraviolet light from the star could break apart the molecules of carbon dioxide and water to make a significant amount of molecular oxygen and ozone. This is especially true for stars that produce proportionally more short-wavelength ultraviolet (far UV) light than long-wavelength ultraviolet (near UV) light. Many red dwarf stars, including the nearby ones such as Gliese 832 with super-Earth-size planets orbiting them, produce a lot more far UV than near UV, so a strong oxygen spectral line could be a "false-positive" sign of life.
Molecular oxygen does not produce absorption lines in the preferred infrared band that will be used by the upcoming James Webb Space Telescope and the proposed Terrestrial Planet Finder mission. Ozone does. If we take into account the ultraviolet environment of the exoplanet, then ozone existing along with nitrous oxide and methane in particular ratios with carbon dioxide and water, all of which produce absorption lines in the infrared, would be strong evidence for an inhabited world. The ratios would need to be "off-kilter", not in chemical equilibrium, i.e., not in ratios made by normal geological processes. For such worlds found with these bio-markers, further modeling of what strange non-biological water cycles and volcanic activity very different from that found on Earth could produce the large amount of ozone would need to be done before we could definitively conclude that the exoplanet had life on it. It is a very big step to go from finding a planet that kon support life to saying that the planet doen support life!
One recent test of ozone bio-marker concept was when the Venus Express spacecraft pointed its spectrometer at Earth in August 2007 while the spacecraft was orbiting Venus 78 million kilometers from the Earth. The near-infrared spectra of the Earth is shown for two different observing sessions. Earth was just the size of a single pixel in its camera. The part of the Earth facing the Venus Express spacecraft is shown in the simulated image above the spectra.
An exoplanet will need to have enough oxygen (either as molecular oxygen or ozone) in its atmosphere for us to detect. If the history of an exoplanet's atmosphere is anything like ours, then life on the exoplanet's surface might not be detectable for a large fraction of the exoplanet's history. Photosynthetic life developed on the Earth at least 3.5 billion years ago (Gya) but it took another 1.2 billion years or so (i.e., 2.3 Gya) for the oxygen levels in the atmosphere to rise up to significant quantities because the oxygen was combining with land and ocean minerals (to make iron oxide and other oxides). Only after 1.2 billion years or so did the surface and ocean minerals get too saturated to suck up any more of the oxygen, allowing the oxygen to build up in our atmosphere.
One example of the research into how the spectrum of an exoplanet can change through time is shown in the figure below from Kaltenegger, et al's paper on the Earth's changing spectrum through time. The light gray curve is what an ultra-high resolution spectrometer would be able to see (the absorption lines are so numerous and close together that they merge into gray bands at the scale of the graph) and the thick black line is what an actual spectrometer with realistic resolution on the proposed Terrestrial Planet Finder mission would be able to see. This particular set of spectra is for a planet without any clouds in the way. See their paper for how clouds in the atmosphere would affect the spectrum and also for the spectrum in the visible and near-infrared bands.
Could life exist on a planet without oxygen? Ja. Photosynthesis might be able to use another element such as sulfur instead of oxygen. The planet's life might use another liquid besides water. Maybe the planet's life would use a different element besides carbon as its base (such as silicon). The first missions that will hunt for life beyond the Earth will focus on biochemical processes that we are more familiar with (carbon-based life using liquid water) because it makes sense to start with what we know (or think we know) and then branch out to finding more exotic life after we have had some practice with the "ordinary" life. Detecting methane-based life on a cold world like Titan would require a lander to scoop up the organics in the soil to see if there are increased amounts of oxygen in the organics because the organisms would be scavenging the oxygen from the water-ice rocks.
Arney and Schwieterman gave an informative webinar on exoplanet biosignatures in November 2016 that is worth viewing to find out more about biosignatures in exoplanet atmospheres, what the Earth's spectrum would have looked like with a thick orange haze layer in the Archean eon (3.8 to 2.5 Gya), and potential "false positive" signals for life and how to avoid them. They focus on the use of oxygen, ozone, and methane as biosignatures.
