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Solar composition from the Genesis Discovery Mission

  1. Genesis Science Team2
  1. Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125
  1. Edited by Mark H. Thiemens, University of California, La Jolla, CA, and approved March 18, 2011 (received for review October 8, 2010)

  1. Fig. 1.

    Overview of Genesis collector materials. (A) Canister as opened at L1. Materials were 10?cm hexagons mounted in five arrays as shown in (B). One of the lower three arrays in stack was exposed to the solar wind to separately sample solar wind regimes. Reentry crash broke hexagons, but pieces down to 3?mm size were sorted (C). (D) Concentrator: a focusing ion telescope designed to give ×?20 enhanced signal to noise for O and N isotopic measurements (2).

  2. Fig. 2.

    (A) The MegaSIMs, an accelerator and secondary ion mass spectrometer hybrid at UCLA (4). The instrument fills a whole room and weighs more than the Genesis spacecraft. McKeegan research group for scale. (B) SARISA, a laser resonance ionization mass spectrometer at ANL (5). Emil Tripa and Igor Veryovkin for scale. (A) and (B) were developed specifically for Genesis. (C) Pits in a mm-sized Genesis Si sample made by rastering a UV laser, releasing He and Ne to be analyzed by noble gas mass spectrometry at the Eidgen?sssische Technische Hochschule, Zurich (6). (D) Some instruments used for Genesis sample analysis are larger than NASA centers. Here the Advanced Photon Source at ANL (7). Small dots at bottom of image are automobiles.

  3. Fig. 3.

    A mass spectrometer measures separately the ion currents for the different isotopes of an element. Oxygen has three isotopes, allowing two isotopic abundance ratios to be calculated. Intersample variations are calculated as the fractional deviation δ of the measured 18O/16O or 17O/16O from terrestrial ocean water (‰ units are permil, parts in 1,000). Terrestrial geochemical processes produce a wide range in O isotope fractionations, but these variations lie almost entirely on the line labeled TF (terrestrial fractionation). The Genesis solar wind composition (8) is very different from the major inner solar system objects (Earth, Moon, Mars) and most asteroidal (meteoritic) materials, but lies close to the trend set by unique high temperature Ca-Al-rich inclusions (CAI). Theoretically, solar wind acceleration fractionates isotopes by the amount shown by the line between the solar wind and the Sun? point (9).

  4. Fig. 4.

    N isotopic compositions in solar system objects, [modified from Marty et al., (13)] vary widely, showing no simple heliocentric distance trend. Genesis data (13–16) show that the Sun is like Jupiter, whereas all inner solar system samples show much larger amounts of 15N. The TiN analysis refers to a mineral from an unusual meteorite (17). The origins of these variations are unknown.

  5. Fig. 5.

    Comparison of various data for solar wind Ar isotopic composition compared to that of terrestrial atmosphere (air). The Genesis data (6, 19) have error bars smaller than the symbol size and provide a precise measure of the Ar fractionation of the terrestrial atmosphere that could not be obtained from Apollo SWC data. The necessity of sample return to measure relatively small but important differences is illustrated by comparison with the Soho spacecraft instrument analysis (22). The Genesis results match the lunar regolith value of ref.?20 but not ref.?21.

  6. Fig. 6.

    Permil deviations of Genesis solar wind Xe relative to the terrestrial atmosphere (24) have a more complex fractionation pattern than the other noble gases. Relative to an assumed mass fractionation trend from masses 128 through 131, the 129Xe anomaly reflects contributions from the decay of 129I to initial terrestrial materials. The deviations at 134 and 136 are less clear (23). These deviations were recognized from the first measurements of solar wind in Apollo lunar soil samples. Genesis and lunar data agree well.

  7. Fig. 7.

    Relative to bulk solar wind collectors, the Low speed (IS), High speed (CH), and Coronal Mass Ejection (CME) regime samples show precisely defined differences in the He, Ne, and Ar isotopic compositions (27).

  8. Fig. 8.

    Depth profile of solar wind Mg. A SIMS depth profile (Jurewicz, Guan, ASU) measures the ion counting ratio Mg/C from a diamond-like-C collector as a function of depth. Crash-derived surface contamination is cleanly separated from the implanted solar wind.

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