Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.517h&link_type=abstract
Meteoritics, vol. 30, no. 5, page 517
Computer Science
Accretion, Comets, Earth, Atmospheres, Cosmochemistry, Mars, Isotopes, Ar, He, Kr, Ne, Xe
Scientific paper
The deep Earth is the key to understanding the primordial evolution of the Earth's atmosphere. However the atmosphere was not derived by degassing of the Earth, as widely held. Isotopic characterization of mantle noble gases and modeling based on this information [1] suggests the atmosphere experienced a 3-stage early history. This follows from 5 basic observations: (i) Ne in the mantle is solar-like, with light (high) 20Ne/22Ne relative to the atmosphere [2]; (ii) mantle Xe has higher 128Xe/130Xe than the atmosphere [3], which carries an extreme heavy isotope enriched mass fractionation signature of >3%/amu (iii) most of the radiogenic Xe from l29I and 244Pu decay in the Earth is not present either in the mantle or in the atmosphere; (iv) the inferred abundances of noble gases in the deep Earth "plume source" are insufficient to generate the present atmospheric abundances, even for whole mantle degassing; and (v) mantle noble gases indicate a 2 component structure, with solar light gases (He and Ne) and planetary heavy gases [4]. The present day noble gas budgets (and likely also N2) must derive from late accretion of a volatile-rich "veneer." This is stage III. Stage II is a naked (no atmosphere) epoch indicated by evidence for Hadean degassing of 244Pu (T1/2 = 80 Ma) fission Xe from the whole mantle, which was not retained in the present atmosphere. The naked stage must have lasted for more than ~200 Ma, and was supported by the early intense solar EUV luminosity. Stage I, a massive solar-composition protoatmosphere, occurred during the Earth's early accretion phase. Its existence is indicated by the presence of the solar gas component in the Earth. This is not attributable to subduction of solar wind rich cosmic dust, or solar wind irradiation of coagulating objects. It is best explained by accretion of a solar composition atmosphere from the nebula. This provided a thermal blanket supporting a magma ocean in which solar gases dissolved. Under these conditions the H2-rich protoatmosphere was oxidized by reaction with the surface magma to yield H20(g) and gained further opacity, supporting a reducing high temperature condition which controlled liquid-liquid siderophile segregation near the magma ocean surface. According to this scenario, the planetary gas component in the mantle would have been accreted subsequently from planetesimals infalling after blowoff of the protoatmosphere. The impact-degassed portion was lost to space. Modeling of Xe isotope fractionahon by hydrodynamic escape is problematic because 84Kr/130Xe in the atmosphere is very close to solar. Hydrodynamic escape fraction (HEF) of a planetary-pattern degassed atmosphere cannot produce this, and HEF of a solar-pattern primordial protoatmosphere cannot preserve it. Comets have been suggested as a potential source of atmospheric 36Ar-84Kr-130Xe, based on ice trapping expenments at 50 degrees K [5]. However, 22Ne-36Ar-84Kr has a planetary-like pattem, whereas Ne does not condense in ice at 50 degrees K. As cometary ice may contain degassed icy molecular cloud (MC) grains formed at lower temperatures [6], experiments with analogs may hold the key to the conundrum of planetary-like 22Ne-36Ar-84Kr and solar-like 84Kr-130Xe, common to both Earth and Mars. References: [1] Jacobsen S. B. (1995) Eos Trans. AGU, 76, S42; O'Nions R. K. and Tolstikhin I. N. (1995) Eos Trans. AGU, 76, S43; Wasserburg G. J. and Porcelli D. (1995) Eos Trans. AGU, 76, S41. [2] E.g., Honda M. et al. (1993) GCA, 57, 859-874. [3] Caffee M. W. et al. (1988) LPS XIX, 154-155; Jacobsen S. B. and Harper C. L. Jr. (1995) AGU Monograph, in press. [4] Harper C. L. Jr. and Jacobsen S. B. (1995) Eos Trans. AGU, 76, S42, and Nature, submitted. [5] Owen T. et al. (1992) Nature, 358, 43-45. [6] Lunine J. I. et al. (1991) Icarus, 94, 333-344.
Harper Charles L. Jr.
Jacobsen Stein B.
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