Comparison of the oxidation state of Fe in comet 81P/Wild 2 and chondritic-porous interplanetary dust particles

Comparison of the oxidation state of Fe in comet 81P/Wild 2 and chondritic-porous interplanetary dust particles R.C. Ogliore a, ⁎ , A.L. Butterworth a , S.C. Fakra b , Z. Gainsforth a , M.A. Marcus b , A.J. Westphal a a b Space Sciences Laboratory, University of California at Berkeley, Berkeley, CA 94720, USA Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA a r t i c l e i n f o a b s t r a c t The fragile structure of chondritic-porous interplanetary dust particles (CP-IDPs) and their minimal parent- body alteration have led researchers to believe these particles originate in comets rather than asteroids where aqueous and thermal alterations have occurred. The solar elemental abundances and atmospheric entry speed of CP-IDPs also suggest a cometary origin. With the return of the Stardust samples from Jupiter- family comet 81P/Wild 2, this hypothesis can be tested. We have measured the Fe oxidation state of 15 CP- IDPs and 194 Stardust fragments using a synchrotron-based x-ray microprobe. We analyzed ∼ 300 ng of Wild 2 material — three orders of magnitude more material than other analyses comparing Wild 2 and CP-IDPs. The Fe oxidation state of these two samples of material are N2σ different: the CP-IDPs are more oxidized than the Wild 2 grains. We conclude that comet Wild 2 contains material that formed at a lower oxygen fugacity than the parent-body, or parent bodies, of CP-IDPs. If all Jupiter-family comets are similar, they do not appear to be consistent with the origin of CP-IDPs. However, comets that formed from a different mix of nebular material and are more oxidized than Wild 2 could be the source of CP-IDPs. Keywords: comet 81P/Wild 2 interplanetary dust particles XANES Jupiter-family comets 1. Introduction Chondritic-porous interplanetary dust particles (CP-IDPs), collect- ed in the stratosphere by high-altitude aircraft, are widely thought to originate in comets (Schramm et al., 1989; Bradley, 1994). The collisional history of asteroids makes it unlikely that these particles (fragile, porous aggregates of small grains) are asteroidal (Brownlee, 1985). The atmospheric entry speed of CP-IDPs, as deduced from thermal release profiles of solar wind He (Nier and Schlutter, 1992), is consistent with cometary, rather than asteroidal, orbits (Brownlee et al., 1995). Infrared and electron beam studies of IDPs show that those consisting of mostly anhydrous minerals are usually chondritic- porous, whereas IDPs rich in phyllosilicates are usually chondritic- smooth (Sandford and Walker, 1985; Bradley and Brownlee, 1986). The presence of GEMS (glass with embedded metal and sulfides) in IDPs indicates a lack of thermal alteration (Bradley, 1994). CP-IDPs, therefore, show very little parent-body alteration and must originate from either anhydrous objects or hydrous objects that have been kept at very low temperature — again, consistent with cometary origin. Nevertheless, a cometary origin of CP-IDPs is not universally accepted (Flynn, 1992; Thomas et al., 1995). With the return of samples from comet 81P/Wild 2 by NASA's Stardust mission (Brownlee et al., 2006), it is now possible to compare CP-IDPs with material from this Jupiter- ⁎ Corresponding author. E-mail address: ogliore@ssl.berkeley.edu (R.C. Ogliore). family comet. We seek to prove or disprove, with a known level of confidence, the hypothesis that CP-IDPs originate from parent bodies with the composition of comet Wild 2. The oxidation state of Fe is a clear mineralogic indicator of the oxidation state of meteorites (Rubin et al., 1988). Meteorite groups are in fact distinguished from each other by their differing Fe oxidation states, as Urey and Craig first reported more than fifty years ago (Urey and Craig, 1953). Parent-body processing can drastically change the Fe oxidation state of some of the meteorite groups, obscuring information about the original oxidation state the material acquired in the solar nebula (Rubin et al., 1988). Carbonates and Fe-bearing crystalline silicates, possible products of aqueous alteration on asteroids (Krot et al., 1995), are found alongside anhydrous minerals in the same Stardust track (Flynn, 2008), and therefore co-existed in one large aggregate in comet Wild 2. No phyllosilicates have been unambiguously identified in the Stardust samples (Zolensky et al., 2008). With these and other pieces of evidence, Wooden (2008) argues that instead of selective aqueous alteration on submicron scales in the comet itself, grains which formed in different regions of the solar nebula under varying reduction–oxidation conditions (e.g. Mg- and Fe-rich crystalline silicates) migrated, aggregated, and formed comet Wild 2. Likewise, the oxidation state of anhydrous CP-IDPs was unaffected by parent- body processing (Schramm et al., 1989; Zolensky and Thomas, 1995), so a comparison of the Fe oxidation state of comet Wild 2 with anhydrous CP-IDPs yields insight into the nebular environment in which they formed.