We report N isotope analysis of a gold plate from the Genesis spacecraft concentrator. We did not find evidence for a light N component in the solar wind

Allton, J. H.

Burnard, P. G.

Burnett, D. S.

Franchi, I. A.

Heber, V. S.

Marty, B.

Sestak, S.

Wieler, R.

Wiens, R. C.

Zimmermann, L.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsIn Search of the Solar Wind Nitrogen IsotopeComposition: Analysis of a Gold Plate from theGenesis Spacecraft ConcentratorConference or Workshop ItemHow to cite:Marty, B.; Zimmermann, L.; Burnard, P. G.; Burnett, D. S.; Allton, J. H.; Heber, V. S.; Wieler, R.; Wiens,R. C.; Sestak, S. and Franchi, I. A. (2008). In Search of the Solar Wind Nitrogen Isotope Composition: Analysis of aGold Plate from the Genesis Spacecraft Concentrator. In: 39th Lunar and Planetary Science Conference (Lunar andPlanetary Science XXXIX), 10-14 Mar 2008, League City, Texas, USA, p. 1314.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2008/pdf/1314.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukIN SEARCH OF THE SOLAR WIND NITROGEN ISOTOPE COMPOSITION : ANALYSIS OF A GOLD PLATE FROM THE GENESIS SPACECRAFT CONCENTRATOR B. Marty1, L. Zimmermann1, P.G. Bur-nard1, D. Burnett2,  J.H. Allton3,  V. Heber4, R. Wieler4, R.C. Wiens5, S. Sestak6 and I.A. Franchi6.1CRPG-CNRS, Université de Nancy, BP 20, 54501 Vandoeuvre Lès Nancy Cedex, France, bmarty@crpg.cnrs-nancy.fr. 2Department of Geological Sciences, California Institute of Technology, Pasadena, CA 91125, USA. 3Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058, USA. 4Isotope Geology and Mineral Resources, ETH Zürich, Switzerland. 5Los Alamos National Laboratory, Space and Atmospheric Science, Los Alamos, NM 87544, USA, 6Planetary and Space Sciences Research Institute, Open University, MK7 6AA, Milton Keynes,. UK.  Introduction:  Nitrogen displays the largest (after H) isotope variations in the solar system materials (ex-cluding pre-solar grains). The causes of these varia-tions are mostly unknown and discussed in term of mixing between different solar, possibly presolar, res-ervoirs. Hence knowledge of the N isotopic composi-tion of the Sun is important as this would presumably shed light on the composition of a major reservoir, the  protosolar nebula. From the analysis of lunar soils ex-posed at different periods, an allegedly secular varia-tion of the 15N/14N ratio, was proposed, from a 15N depletion of ≥300 ‰ (relative to terrestrial N, e.g., ATM) in soils exposed ~2 Ga ago to 15N enrichments of up to 150 ‰ in  ≤ 200 Ma soils (e.g., [1]).  Recently several studies concluded that protosolar nebula N was depleted by ~400 ‰, from (i) the com-bined ion probe analysis of H and N isotopes in lunar soil grains (δ15N ≤240 ‰, [2], (ii) the analysis of the Jupiter atmosphere by either infra-red spectroscopy [3] or in-situ by the Galileo probe [4], and (iii) δ15N values of -400 ‰ for osbornite (TiN) embedded in a CAI [5].  The Genesis solar wind exposure experiment: The Genesis mission [6] sampled solar wind ions dur-ing 27 months in space at the Lagrange point L1, by implantation of SW ions in several targets made of pure material and during different SW regimes. De-spite a hard landing of the sample capsule, target mate-rial could be recovered, giving new insights into the noble gas composition of the different solar wind re-gimes [7, 8]. Determining the nitrogen isotopic com-position of the solar wind, which is a main scientific goal of the mission, is complicated by the very low amount of implanted solar wind N (fluence of 2 x 10-12 mol/cm2 14N) compared to potential terrestrial con-tamination. The latter was exacerbated by the capsule crash since nitrogen is a major constituent of the at-mosphere, of organics, and of sediments. Decontami-nation procedures are crucial and those used for sam-ples analyzed for N include washing, UV-ozone clean-ing and handling in class 100 rooms. We have devel-oped at CRPG Nancy, France, a ultra high vacuum line and static mass spectrometry, where N is extracted by laser (wl : 193 nm) ablation with a spot size of approx. 50x150 μm rastered over surfaces up to 1 cm2. By modulating either the power of the laser beam, or the number of pulses per area, we are able to remove se-quentially layers of matter with a depth resolution of a few nm. The release of SW gases is monitored by the simultaneous analysis of noble gas  (He, Ne, Ar) abundances and isotopic ratios. Gold-plated targets were chosen because of the presumed purity of Au layers and of the good extraction efficiency of nitrogen from gold, checked using artificially implanted targets with known 15N fluence. We analyzed recently gold-over-sapphire (AuoS) target fragments [9] and could not find evidence for a light (15N-depleted), Jupiter-like, nitrogen component. The comparatively large data uncertainties due to low amounts and large corrections for terrestrial contami-nation did not permit to derive a firm value for the SW N isotopic ratio. We have analyzed further samples, focusing our effort on target material exposed in the Genesis SW ion concentrator and more enriched in SW ions. Sample and analytical:  The Genesis concentra-tor is a ion-focusing experiment designed to enrich solar wind ions [10] by up to a factor of 30 [11]. It was originally developed for the 3-isotope analysis of SW oxygen in different targets (Fig. 1). The latter were mounted in a gold-plated stainless steel frame, the gold cross. N together with He, Ne and Ar were extracted by laser ablation of one gold cross arm (GCA) (Fig. 1). Five areas of 4-6 mm2 were ablated from the border to the center of the GCA. Released gases were split into 2 fractions for nitrogen purification and cryogenic sepa-ration/purification of noble gases, respectively. The concentration and isotopic composition of SW neon were analyzed previously at ETH Zürich, Switzerland, along the same arm (laser ablation, spot size of 100 μm), giving us a monitor of SW abundance and iso-topic fractionation as a function of location along the arm [11]. Our Ne contents were found to be similar to those determined by [11] within 20 %, despite the large differences in the ablated surfaces. The proce-dural blank for nitrogen was 1.1±0.1 x 10-12 mol 14N, with δ15N = -22±15‰. Together with the flown gold cross arm, we analyzed a spare gold cross arm, which yielded on average 2.64±0.11 x 10-12 mol/mm2 with Lunar and Planetary Science XXXIX (2008) 1314.pdfδ15N =14±9‰. Blanks were negligible for noble gases, which isotopic compositions showed an almost pure SW origin.  Fig.1: The central target holder ("gold cross") of the Genesis concentrator.The "12 à'clock" arm was analysed.  Results:  For evaluating the fraction of SW N in the total N, we use a mixing diagram: δ15N = f[(20Ne/14N)Sample/(20Ne/14N)SW] (SW 20Ne/14N = 0.50±0.09 from [12]). In this format, mixing between two different end-members yields a unique straight line. Data have been corrected for isotopic fractiona-tion of SW N, from Ne isotope data from [11] and assuming mass dependant functionality for both Ne and N isotope fractionations. Only the fraction of ni-trogen that was estimated to be of solar wind origin from the 20Ne/14N ratio was isotopically corrected. Based on Ne isotopic fractionation, the potential iso-topic shift for nitrogen is ~40 ‰ at most, which is small compared to for example dthe 400 ‰ difference between terrestrial N and Jovian atmosphere N.  Fig. 2: δ15N isotopic composition vs. the Ne/N ratio normalized to the SW value. Dots represent total data for each area along the GCA arm. Data (either stepwise release ones or those for total release, Fig. 2), define a 2 end-member correlation. One of these has no neon and a δ15N value of +10±5 ‰, which is consistent with the GCA blank value.  Discussion: The other N end-member has an ex-trapolated δ15N value of -40±94 ‰ (2σ) when all data (previous AuoS targets together with the present re-sults; in Fig. 2 only "total" data for GCA are taken for computing the extrapolated SW value) are considered, which is clearly different from the composition of the Jupiter atmosphere and from the inferred protosolar nebula value. It is, however, in agreement with a δ15N of +40±8‰ for recent SW from the analysis of soils exposed in the last few Ma [13]. The present value encompasses also N isotope compositions found in most primitive meteorites (except the CR clan), the Earth, Mars (interior) and Venus. The difference between modern SW and Jupiter atmosphere is particularly striking because both reser-voirs are gigantic and should have averaged local het-erogeneities. Thus if the Jovian composition is not representative of the protosolar nebula one, then proc-esses of N isotope fractionation specific to this atmos-phere, or having acted during trapping of N in parent icy planetesimals, needs to be explored. Another pos-sibility is a large scale N isotopic heterogeneity in the nascent solar system originating from an N speciation gradient  (e.g., [14]).  If the presumed SW N isotope ratio determined in this study is representative of the solar wind composi-tion through time, it is also necessary to find not only one, but two other components trapped in lunar soil, one enriched in 15N which can be accounted by IDPs [2], and the other one depleted in 15N, such as terres-trial atmosphere ions implanted in the past [15].  We are presently working on further concentrator samples cleaned differently with the aim of improving precision on the N isotopic composition of the modern solar wind. Study funded by CNES, CNRS and Région Lorraine. References: [1] Kerridge J.F. (1993) Rev. Geo-phys., 31; 423-437. [2]  Hashizume K. et al. (2000) Science, 290; 1142-1145. [3] Fouchet T. et al.,(2000) Icarus,143; 223-243. [4] Owen T. et al. (2001) Ap. J. 553; L77-L79. [5] Meibom A. et al.(2007) Ap J. 656; L33-L36. [6] Burnett D.S. et al.(2003) Space Sci. Rev., 105; 509-534. [7] Grimberg A. et al. (2006) Science, 314; 1133-1135. [8] Meshik A. et al. (2007) Science 318; 433-435. [9] Marty B. et al. (2007) LPS XXXVIII. Abstract # 1706. [10] Nordholt J.E. et al. (2003) Space Sci Rev, 105; 561-599. [11] Heber V.S. et al., (2007) Space Science Rev. 130; 309-316. [12] Gloecker G. and J. Geiss J., Space Sci. Rev., (2007), 130, 139-152. [13] Kim J.S. et al. (1995) Nature, 375; 383-385. [14] Maret S. et al. (2006) Nature, 442; 425-427. [15] Ozima M. et al. (2005) Nature 436; 655-659.  Lunar and Planetary Science XXXIX (2008) 1314.pdf

In Search of the Solar Wind Nitrogen Isotope Composition: Analysis of a Gold Plate from the Genesis Spacecraft Concentrator

In Search of the Solar Wind Nitrogen Isotope Composition: Analysis of a Gold Plate from the Genesis Spacecraft Concentrator

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