Abstract
Abstract Geochemical indicators in meteorites imply that most formed under relatively oxidizing conditions. However, some planetary materials, such as the enstatite chondrites, aubrite achondrites, and Mercury, were produced in reduced nebular environments. Because of large-scale radial nebular mixing, comets and other Kuiper Belt objects likely contain some primitive material related to these reduced planetary bodies. Here, we describe an unusual assemblage in a dust particle from comet 81P/Wild 2 captured in silica aerogel by the NASA Stardust spacecraft. The bulk of this ∼20 μm particle is comprised of an aggregate of nanoparticulate Cr-rich magnetite, containing opaque sub-domains composed of poorly graphitized carbon (PGC). The PGC forms conformal shells around tiny 5–15 nm core grains of Fe carbide. The C, N, and O isotopic compositions of these components are identical within errors to terrestrial standards, indicating a formation inside the solar system. Magnetite compositions are consistent with oxidation of reduced metal, similar to that seen in enstatite chondrites. Similarly, the core–shell structure of the carbide + PGC inclusions suggests a formation via FTT reactions on the surface of metal or carbide grains in warm, reduced regions of the solar nebula. Together, the nanoscale assemblage in the cometary particle is most consistent with the alteration of primary solids condensed from a C-rich, reduced nebular gas. The nanoparticulate components in the cometary particle provide the first direct evidence from comets of reduced, carbon-rich regions that were present in the solar nebula.
Highlights
Comets are relatively pristine reservoirs of early solar system materials
This is supported by measurements of comet Wild 2 samples and chondritic porous interplanetary dust particles (IDPs), thought to originate largely in comets, both of which show a range of oxidation states (Ogliore et al 2010)
Independent of the local C/O conditions, if efficient nebular mixing occurred during the accretion of comet Wild 2, it is possible that reduced and C-rich mineral assemblages reminiscent of those of enstatite chondrites, in addition to oxidized phases from other source regions, might be present in the Stardust sample collection
Summary
Comets are relatively pristine reservoirs of early solar system materials. Among the dust particles collected from the Jupiterfamily comet 81P/Wild 2 by the NASA Stardust mission are early nebular condensates (Brownlee et al 2006, 2012), chondrule-like materials (Nakamura et al 2008; Ogliore et al 2012; Joswiak et al 2014), presolar grains (McKeegan et al 2006; Stadermann et al 2008; Leitner et al 2010; Floss et al 2013; Brownlee 2014), and solid carbonaceous matter (Sandford et al 2006; Cody et al 2008a; Matrajt et al 2008; Wirick et al 2009; De Gregorio et al 2011). Meteoritic and planetary evidence indicates that the solar nebula experienced large variations in redox conditions This is supported by measurements of comet Wild 2 samples and chondritic porous interplanetary dust particles (IDPs), thought to originate largely in comets, both of which show a range of oxidation states (Ogliore et al 2010). Independent of the local C/O conditions, if efficient nebular mixing occurred during the accretion of comet Wild 2, it is possible that reduced and C-rich mineral assemblages reminiscent of those of enstatite chondrites, in addition to oxidized phases from other source regions, might be present in the Stardust sample collection. The particle described represents the first evidence from comets of materials condensed in highly reducing environments in the early solar nebula
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