Isotopically Anomalous Organic Globules from Comet 81p/wild 2. B

Introduction: Comets preserve some of the most “primitive” material in the solar system. Proto-nebular and/or interstellar organic matter can be identified by anomalous δD and δN values well outside the range of solar values (eg. [1]). Since carbonaceous nanoglobules are often the carriers of anomalous δD and δN in carbonaceous chondrites [2] and interplanetary dust [3], organic globules should be present in cometary samples. Organic matter is present in several particles from comet 81P/Wild 2 captured in silica aerogel by the Stardust spacecraft [4, 5]. One terminal particle contains a sulfide grain surrounded by a carbonaceous rim [6], but due to the limited number of carbonaceous samples analyzed by TEM, hollow organic globules were not observed during preliminary examination. Here we describe two organic globules from comet 81P/Wild 2 which also contain anomalous δD or δN, likely signifying a presolar origin. Materials and Methods: Stardust samples C2092,6,80,43,2 (Track 80, Particle 43) and FC3,0,2,4,5 (Track 2, Particle 4) were S-embedded, ultramicrotomed, and placed on SiO-coated Cu TEM grids (Track 2 sections prepared by K. NakamuraMessenger). Sections were first analyzed by X-ray absorption near-edge structure spectroscopy (XANES) at beamline X1A1 at the National Synchrotron Light Source (NSLS) or at beamline 5.3.2 at the Advanced Light Source (ALS) to determine the local C, N, and O bonding environments. Sample morphology was observed using a JEOL 2200FEG TEM at the Naval Research Laboratory (NRL). After XANES and TEM analyses, δD and δN were measured using a Cameca NanoSIMS 50L at the Carnegie Institution of Washington (CIW). Results: Each organic globule is near aerogel from the Stardust sample collector, and both globules show evidence of a hollow interior. The Track 80 globule has a 1.4 μm diameter with a wall thickness of 450 nm, while the Track 2 globule has a 2.3 μm diameter with a wall thickness of 700 nm (Fig. 1). Isotopic Anomalies. The Track 80 globule is highly enriched in N and has an average δN value of +1120 ± 30‰ (Fig. 1A), above the upper limit observed in cometary comae [7] and at the upper end of previously observed values for “hotspots” in Wild 2 samples [4, 8]. A nearby section containing cometary organic matter and aerogel also shows a small N enrichment (δN ~ +200‰). The support film sputtered away before a reliable D/H measurement could be made, but the partial data limit the magnitude of any D enrichment in the globule to < 500‰. The globule also shows a small C depletion, with δC = -77 ± 13‰. In contrast, the Track 2 globule has an average terrestrial δN = -7 ± 5‰ but a modest D enrichment of +1000 ±170‰ (Fig. 1B). Carbon isotopes in this globule are less depleted, with δC = -35 ± 3‰, well within the range of meteoritic organic matter [9]. Organic Chemistry. The two globules have fundamentally different organic bonding. C-XANES spectra of cometary organic matter from Track 80 show peaks at 285.0 eV due to carbon-carbon double bonds and polyaromatic domains, at 286.7 eV most likely due to enol, phenol, and vinyl ketone functional groups, and at 288.5 eV due to carboxyl functional groups (Fig. 2A). However, the Track 80 globule is dominated by aromatic carbon with only small contributions from carbon-oxygen bonding (Fig. 2B). In contrast, C-XANES spectra of the Track 2 globule do not show any aromatic carbon but rather intense 286.7 eV and 288.5 eV (carboxyl) photoabsorptions (Fig. 2C). This organic matter is beam sensitive and changed dramatically after TEM imaging, although the