Accretion and early history of planetesimals and planets: the noble gas record Rainer Wieler, ETH Zürich Origin and Evolution of Planets 2008 Ascona, June 29 - July 4, 2008

Noble gas geochemistry often seems to non-practitioners to have much the air of the secret society and its dark art M.Ozima & F. A. Podosek Noble gases in 20 minutes Ne-A Ne-B Ne-C Ne-E(L) Ne-E(H) Ne-Q Ne-HL Ne-P3 Ne-P6 Ne-G Ne-S SW-Ne Ne atm He Ar Kr Xe….. trapped cosmogenic nucleogenic radiogenic fissiogenic in-situ primordial exotic normal

The noble gas record Where has a particular noble gas "component" been established (and by what process) presolar?molecular cloud?solar nebula? planetesimals? accretion? metamorphism? W. K. Hartmann

Exotic noble gases exotic = very different from solar isotopic composition Xe mass number normalized to solar Xe carriers: circumstellar grains G (SiC): s-process nucleosynthesis in AGB stars (Asymptotic Giant Branch) HL (diamonds): p- & r-process in supernovae? SiC 2  m exotic noble gases:  carrier identification  important contributors to meteorite bulk inventory exotic isotopes in general: test and advance theories of nuclear astrophysics

exotic = very different from solar isotopic composition Xe mass number normalized to solar Xe SiC 2  m Gallino et al Kr has two branching points for s-process ( 80,86 Kr/ 82 Kr diagnostic for neutron densities) Kr-G in Murchison SiC grains as expected for s-process in AGB stars of ~ M sol and metallicities slightly less than solar (from He- shell) Exotic noble gases theory data

He and Ne in single circumstellar grains Presolar ages of presolar grains: no classical age determinations possible, due to ubiqituous isotope anomalies  cosmic ray exposure ages ( 3 He and 21 Ne) Expected lifetime of interstellar dust ~500 Ma (e. g. Jones et al., 1997) (destruction by supernova shock waves, sputtering by stellar winds, UV evaporation..) However: interstellar SiC grains gently recovered from meteorites look pristine (no erosion pits etc., as expected for old grains)

He and Ne in single circumstellar grains Presolar ages of presolar grains: no classical age determinations possible, due to ubiqituous isotope anomalies  cosmic ray exposure ages ( 3 He and 21 Ne) T3=T21 exposure ages of single large! (  m) interstellar SiC mostly lower (< 150 Ma) than expected lifetimes (Heck et al.)

He and Ne in single circumstellar grains Presolar ages of presolar grains: no classical age determinations possible, due to ubiqituous isotope anomalies  cosmic ray exposure ages ( 3 He and 21 Ne) T3=T21 exposure ages of single large! (  m) interstellar SiC mostly lower (< 150 Ma) than expected lifetimes stellar source: planetary nebulae (ABG stars) stellar source: supernovae molecular cloud  grains do not originate from molecular cloud (lifetime too short to form AGB stars of 1-3 M sol )  Si isotopes in SiC: star burst due to galaxy merger 1-2 Ga before solar system (Clayton, 1993).  SiC grains from first AGB stars from this burst??

"Normal" noble gases of uncertain origin "Phase Q" (for Quintessence): main carrier (90%) of Ar, Kr, Xe, minor carrier (10%) of He & Ne in bulk meteorites ill-defined oxidizable, almost mass-less carbonaceous carrier light noble gases strongly depleted relative to solar comp. Origin?: molecular cloud? solar nebula? planetesimals? W. K. Hartmann

"Normal" noble gases of uncertain origin The most abundant noble gas (Ar-Kr-Xe) component in meteorites must reflect important early solar system processes. But what?? Molecular cloud origin of phase Q?: Q present in all primitive meteorite classes: implies good mixing Trapping of Ar, Kr, Xe in icy mantles? Q intimately mixed with presolar diamonds?

"Normal" noble gases of uncertain origin The most abundant noble gas (Ar-Kr-Xe) component in meteorites must reflect important early solar system processes. But what?? Solar nebula origin of phase Q?: Q present in all primitive meteorite classes: a problem for the nebula hypothesis? Trapping in plasma into presolar diamonds? Fractionation by hydrodynamic escape during dissipation of accretion disk (Pepin)?

"Normal" noble gases of uncertain origin The most abundant noble gas (Ar-Kr-Xe) component in meteorites must reflect important early solar system processes. But what?? Planetesimal origin of phase Q?: Fractionation by hydrodynamic escape during loss of early atmosphere (Pepin)? (Hydrodynamic escape induced, e. g., by impacts) How was thorough later mixing achieved? However, Q (and other) noble gases are important tracers to study metamorphic history of parent bodies

Microdistribution of noble gases in meteorites exposure age (Ma) 2% of the olivine grains in Murchison contain solar flare tracks (radiation damage induced by solar energetic particles). Some of these grains contain much more cosmic-ray produced 21 Ne than expected (Hohenberg et al.) Does this testify of an early active sun? Young stars are prodigious emitters of energetic particles (strong magnetic activity seen in x-rays implies strong flaring activity) However: do the gas- and track-rich grains not simply represent a mature portion of a "regolith" (dust layer) of a parent body? see also poster by Roth et al. on chondrule pre-irradiation Hohenberg et al. (~1990) x-ray emitting YSOs in Chamaeleon I (Feigelson & Montmerle 1999)

Noble gases in planets atmosphere crust mantle atmosphere (Viking & met.) mantle (met.) atmosphere potential information on: source planetesimal (meteorite) types? accretion processes & timing degassing (early and late) atmosphere formation and modification mantle structure and evolution crustal processes......

Sources of noble gases in planets planetesimals (meteorites)?  elemental abundances in terrestrial planets very similar to each other and to meteorite values However: He and Ne isotopes (but not Xe) in terrestrial and martian mantle are "solar-like" Different processes can strongly deplete light noble gases. Are the similar elemental patterns a coincidence? Solar noble gases in interiors of Mars and Earth:  trapped from nebula (e. g. in magma ocean)?  trapped from solar wind (e. g. irradiated dust in nebula accreted later to Earth)?  trapped from comets? R. O. Pepin D. Graham

Noble gases in giant planets He abundance in giant planets is ~solar  primary atmospheres (plus some helium migration towards interior of Jupiter and Saturn) He, Ne, Ar & Xe isotopic composition ~solar/protosolar Ne depleted in Jupiter Ar-Kr-Xe all enriched by same factor (as are C, S & N)  Ne preferentially segregated with He droplets  highly volatile elements supplied by icy planetesimals (in solar proportion) (Comets from Kuiper belt, 30K; Owen et al., 1999)

Degassing of terrestrial planets Main tool: radiogenic noble gas isotopes with long-lived or short-lived (extinct) precursors 40 Ar ( 40 K, 1.27 Ga) 129 Xe ( 129 I, 15.7 Ma) Xe ( 244 Pu, 82 Ma; 238 U) 40 Ar in atmosphere of Venus 4 times lower than in terrestrial atmosphere  Earth degassed to about 60% (over geologic time), Venus less (consistent with dry mantle of Venus, Kaula 1999) Early loss: Xe closure age of the Earth only ~0.8% of 129 Xe ever produced is now in the atmosphere  requires early loss  formal closure age ~100 Ma (also from 136 Xe/ 129 Xe) what does this age mean? giant impact? planetesimal degassing? core formation?…. time (Ma) since start of solar system

Microdistribution of noble gases in meteorites Vogel et al., 2004 chondrules are gas-poor (gas loss upon heating) chondrule rims are gas-rich: rims were aquired in nebula, not on parent body continuous dilution of noble gas carriers with gas- free matter during accretion

Only neutral atoms from the ISM shutters open only in direction of neutral flux closed when directed along velocity of spacecraft no terrestrial contamination grids at 6 keVno ions of magnetospheric origin Komza collectors on “Spektr” modul

Galactic Chemical “Evolution” of 3 He

Primordial 3 He abundance Bania et al. 2002all stars in total 3 He net producers 3 He addition to ISM small but positive gradient with time simple HII region far from galactic centre: upper limit for primordial 3 He ( 3 He/ 4 He) primordial ≤ (1.16  0.24) x 10 -4