Origin of the Earth and Moon Conference 4102.pdf

MODELING THE FORMATION OF THE EARTH’S ATMOSPHERE BY HYDRODYNAMIC ESCAPE AND PLANETARY OUTGASSING. R. O. Pepin, School of Physics and Astronomy, University of Minnesota, Minneapolis MN 55455, USA.

Accretion of lunar- to Mars-sized terrestrial Models of this kind have had some success in planet embryos is believed to have occurred on accounting for the details of terrestrial noble gas timescales of »105 years in the presence of nebular mass distributions [3,6]. However, they are not with- gas [1,2]. Mechanisms for trapping nebular (“solar”) out problems. “Solar” isotopic distributions in the noble gases in these embryos include occlusion of initial terrestrial reservoirs are taken to be those ambient gases in the planetesimals accreted to form measured in the solar wind. Although current esti- them, and, probably more important, efficient ad- mates for the isotopic compositions of solar-wind Ne, sorption of gravitationally condensed nebular gases Ar, and Kr are compatible with those required in the on embryo surfaces once they had grown to modeling for Earth’s primordial noble gas invento- ~Mercury size [3]. During the following ~100–200 ries, this assumption that the wind correctly repre- m.y. of growth through the “giant-impact” stage sents the composition of the nebular source supply- [1,4] to a fully assembled planet, a coaccreting pri- ing these gases to the early Earth is not strictly valid mordial atmosphere is likely to develop by impact- for Xe. Generating the isotope ratios of terrestrial degassing of colliding embryos and inward-scattered nonradiogenic Xe by fractionation in hydrodynamic icy planetesimals, and by gravitational capture of escape requires an initial composition called U-Xe, nebular gases if the gas phase of the nebula had not which appears to be isotopically identical to meas- yet fully dissipated. A planet accreting in this way ured solar-wind Xe except for sharply lower abun- acquires external (atmospheric) and interior volatile dances of the two heaviest isotopes [10]. Originally reservoirs, the latter probably sited primarily in its an inferred component [3], U-Xe-like compositions deeply buried embryo since increasing collisional have since been found in basaltic achondrites [11]. energies as the planet grows beyond the embryo But even though the existence of U-Xe in the early stage promote efficient impact-degassing of projec- solar nebula now seems confirmed, we are still left tile volatiles to the atmospheric reservoir [5]. Both with the puzzling question of why it differs isotopi- reservoirs are arguably dominated by isotopically cally from the Xe composition in the solar wind and solar noble gases. presumably also in the Sun. The observation that nonradiogenic Ne, Ar, Kr, Another unresolved issue has to do with Xe de- and Xe in Earth’s current atmosphere are all isotopi- gassing from the Earth’s deep interior reservoir. In cally heavier than their solar counterparts is an im- the models discussed here, escape fractionation of portant clue to the nature of the processes that subse- the planet’s primordial atmosphere generates the quently acted on these primordial planetary reser- abundance and nonradiogenic isotope ratios of pres- voirs, and is central to evolutionary models in which ent-day atmospheric Xe, and also leaves behind the atmosphere was driven from solar progenitors to highly fractionated residual abundances of the its present compositional state by energetic hydrody- lighter noble gases. Mixing of these residuals with namic escape of the coaccreted atmosphere, followed degassed solar-composition Kr, Ar, and Ne repli- by mixing of its isotopically fractionated residue with cates their present elemental and isotopic composi- gases evolved from the interior [3,6]. Hydrodynamic tions. But only minor amounts of degassed Xe could escape of a hydrogen-rich atmosphere [7] is powered have been added to the post-escape atmosphere with- in these models by absorption of intense extreme- out perturbing its isotopic distribution, and so one of ultraviolet (EUV) radiation from the young evolving two assumptions must be made about the Xe initially Sun [3,8] or by energy deposited in a giant Moon- present in the planetary-embryo reservoir. Either it forming impact [6,9]. Required H2 inventories range was preferentially outgassed very early by fractional from the equivalent of ~1–2 wt% water in terrestrial degassing [12,13], well before the bulk of the lighter accretional materials, for escape driven solely by noble gases, and therefore most of it was already solar EUV [3], to roughly a factor of 10 less for im- present in the primary atmosphere prior to hydrody- pact-powered escape [6]. The atmospheric H2 is namic escape, or it was stabilized and segregated in plausibly produced by reduction of accreted water on some way deep in the planetary interior and thus was metallic Fe during planetary growth. never transported to the atmosphere. The suggestion that deeply sited Xe might have metallized and al- Origin of the Earth and Moon Conference 4102.pdf

MODELING FORMATION OF EARTH’S ATMOSPHERE: R. O. Pepin

loyed with iron [3] is not supported by recent high- ment of the Ar/Ne elemental ratio, by a factor of ~20 pressure measurements on the Xe-Fe system [14]. relative to the solar-wind ratio, in the EPR source The possibility that the missing Xe ultimately wound reservoir [20]. This is close to the mean fractionation up in the Earth’s metallic core is not entirely dead, factor seen in laboratory adsorption experiments however: Jephcoat [15] has proposed a novel (data summarized and referenced in [3]), suggesting mechanism in which microclusters of dense crystal- that the interior noble gas reservoir was indeed line Xe — a solid throughout much of the mantle populated by adsorption of nebular gases on Earth’s where its melting temperature is calculated to lie accretional embryo rather than by solar-wind im- above the geotherm — may have been entrained by plantation and retention in proto-Earth materials percolating iron melts near the deep embryo reser- (where the Ar/Ne fractionation factor would be ex- voir and transported into the core during core for- pected to be ~1). It is prudent to note that this con- mation. clusion assumes that noble gases are not elementally One consequence of this modeling approach is fractionated during their transport from a deep inte- that noble gases presently evolving from the unde- rior reservoir to ocean basalt source regions. pleted mantle have their principal source in whatever References: [1] Wetherill G. W. (1990) Annu. is left of the primordial accretional embryo of the Rev. Earth Planet. Sci., 18, 205. [2] Wetherill G. W. planet after core formation had penetrated, fractured, and Stewart G. R. (1993) Icarus, 106, 190. [3] Pepin and overturned it, and thus they should display solar R. O. (1991) Icarus, 92, 2. [4] Wetherill G. W. isotopic compositions. A central question for atmos- (1992) Icarus, 100, 307. [5] Ahrens T. J. (1990) in pheric formation and evolution, as well as for the Origin of the Earth, p.†211, Oxford Univ. [6] Pepin source, incorporation mechanism, and transport of R. O. (1997) Icarus, 126, 148. [7] Hunten D. M. et noble gases in the Earth, is therefore whether solar- al. (1987) Icarus, 69, 532. [8] Walter F. M. et al. like noble gases are present in the mantle. There are (1988) Astron. J., 96, 297. [9] Benz W. and Cam- strong indications in the MORB database that this is eron A. G. W. (1990) in Origin of the Earth, p. 61, indeed the case for He and Ne [16,17]. But the ob- Oxford Univ. [10] Pepin R. O. et al. (1995) GCA, servational problem has been that solar isotopic sig- 59, 4997. [11] Eugster O. et al. (1994) in Noble Gas natures, which for Ar and Xe differ significantly Geochemistry and Cosmochemistry, p. 1, Terra Sci., from atmospheric compositions, have appeared only Tokyo. [12] Zhang Y. and Zindler A. (1989) JGR, subtly if at all in analyses of the heavier noble gases 94, 13719. [13] Tolstikhin I. N. and O’Nions R. K. in mantle-derived samples; their nonradiogenic iso- (1994) Chem. Geol., 115, 1. [14] Caldwell W. A. et tope ratios have generally been found to be indistin- al. (1997) Science, 227, 930. [15] Jephcoat A. P. guishable or only slightly offset from those in air — (1998) Nature, 393, 355. [16] Honda M. et al. (1993) a consequence, one now suspects, of massive air- GCA, 57, 859. [17] Farley K. A. and Poreda R. J. derived contamination by post-collection adsorption, (1993) EPSL, 114, 325. [18] Valbracht P. J. et al. seawater-magma interaction, subduction, or combi- (1997) EPSL, 150, 399. [19] Niedermann S. et al. nations of all of these. The first promising isotopic (1997) GCA, 61, 2697. [20] Pepin R. O. (1998) Na- evidence for a solar-like Ar component appeared in ture, 394, 664. new analyses of basalt glasses from the Loihi Seamount [18]. Shortly thereafter, Niedermann et al. [19] published results of measurements of Ne and Ar isotopes in a suite of MORB samples from the south- ern East Pacific Rise (EPR). Their data, when plot- ted on a 38Ar/36Ar vs. 20Ne/22Ne diagram, define a remarkably coherent mixing curve between air- derived Ne and Ar and a second component isotopi- cally consistent with current estimates for the solar wind [20]. A substantial fraction of the Loihi data [18] also fall along this mixing curve, and the rest are consistent with contamination by variably frac- tionated air. These observations greatly strengthen the case for the presence of solar Ar, and by infer- ence solar Kr and Xe as well, in the Earth’s mantle. Moreover, all two-component mixing models that fit the EPR data trends require a substantial enhance-