In January 2004, the NASA Stardust spacecraft encountered the coma of comet 81P/Wild 2 at a relative velocity of 6.1 km s−1. Cometary particles were captured in the spacecraft’s gradient density silica aerogel collectors, and the Stardust mission successfully returned the samples to Earth in January 2006 (Brownlee et al. 2003, 2006). Individual microscale cometary dust fragments produced impact tracks of varying morphology in the collector (Hörz et al. 2006; Burchell et al. 2008; Ebel et al. 2009). The incident particles decelerated over ∼0.3 μs and, because aerogel is highly insulating, kinetic energy conversion during impact caused intense heat pulses lasting approximately a microsecond, peak temperatures in excess of 2000 K, thermal gradients of ∼2500 K/μm near grain surfaces, and sub-millisecond cooling times (Noguchi et al. 2007; Roskosz et al. 2008; Trigo-Rodríguez et al. 2008; Dominguez 2009).
Preliminary examination (PE) of material returned from comet 81P/Wild 2 revealed that approximately half of the first-characterized largest captured grains along and at the termini of the capture tracks show only limited signs of alteration and consist of olivine, pyroxene, Fe-Ni sulfide, and other minerals largely unmodified by hypervelocity capture in a silica aerogel capture medium (Zolensky et al. 2006). Subsequent research has established multiple similarities between such Stardust “microrocks” (Velbel and Harvey 2009; after Burnett 2006; a.k.a. “nanorocks”) and primitive solar-system materials such as chondritic porous (CP) interplanetary dust particles (IDPs) and a number of anhydrous components of chondritic meteorites (Hörz et al. 2006; Zolensky et al. 2006, 2008; Leroux et al. 2008; Nakamura et al. 2008; Tomeoka et al. 2008; Jacob et al. 2009; Joswiak et al. 2009).
Melted grains, which constituted the other half of the grains characterized during PE, are a vesicular amorphous mixture of quench products of melted cometary silicates mixed with molten aerogel, and include an emulsion of spherical nanoscale Fe-Ni-S droplets (Flynn et al. 2006; Hörz et al. 2006; Zolensky et al. 2006; Ishii et al. 2008; Leroux et al. 2008, 2009; Nakamura et al. 2008; Rietmeijer et al. 2008; Roskosz et al. 2008; Tomeoka et al. 2008; Burchell et al. 2009; Fries et al. 2009; Rietmeijer 2009; Stodolna et al. 2009; Velbel and Harvey 2009). Stardust melted grains quenched from an ensemble of multiple immiscible fluids, including Fe-Ni metal melt, Fe(±Ni)-S melt, Mg-rich silicate melt (dominantly melted Mg-silicates), Si-rich melt (dominantly melted silica aerogel), and a vapor phase that produced the vesicles (Leroux et al. 2008; Roskosz et al. 2008; Velbel and Harvey 2009; Rietmeijer 2009).
On the basis of research to date (references in previous paragraphs), it is generally accepted that incident particles consisted of two categories of material that responded differently to aerogel capture, one yielding the observed material that largely survived aerogel capture as “microrocks,” and the other yielding the extensively or completely melted grains. However, the specific properties of the category that yielded the melted grains and how those fragments differed (e.g., finer grain size, fluffier incident-particle structure, composition including volatile elements) from the population that survived as “microrocks” remain to be firmly established.
We note that the nomenclature and allocation numbering system used in this article will follow that described by Velbel and Harvey (2009). “Particle” is used to refer to pre-capture projectiles, while “grain” is reserved for captured, extracted solids. Ultramicrotome sections of grains prepared by the Stardust curatorial facility are called “allocations,” with identifiers designated by the Stardust Cometary Sample Catalog (Bastien et al. 2006). The terms “bead” and “droplet” are used specifically to indicate nanometer-sized Fe-Ni-S inclusions identified within allocations and inferred to be solidified from formerly molten liquid.
Modification of Incident Particles During Aerogel Capture
The characterization of aerogel capture modification processes has emerged as an area of research supporting interpretation of Stardust sample analyses (e.g., Bradley et al. 2009). Laboratory experiments using analog materials to simulate Stardust incident particles have indicated several models of track formation (recent examples include Burchell et al. 2008; Trigo-Rodríguez et al. 2008; Burchell et al. 2009; Dominguez 2009; Fries et al. 2009; Kearsley et al. 2009; Tsuchiyama et al. 2009; Wozniakiewicz et al. 2009; Ida et al. 2010; also see citations to earlier papers in Velbel and Harvey 2009). These studies supplement a large body of existing literature on hypervelocity impact experiments, performed in preparation for the Stardust mission and for other applications (for example, Anderson and Ahrens 1994; Burchell et al. 2006, and references therein). Moreover, detailed studies of individual Stardust particles and tracks have been performed using a wide array of analytical techniques (for example, Zolensky et al. 2006; Ebel et al. 2009; Jacob et al. 2009; Stodolna et al. 2009; Velbel and Harvey 2009).
The characteristics of the ubiquitous nanoscale Fe-Ni-S droplets in Stardust melted grains have been investigated to infer the thermal modification history of the incident cometary particles during aerogel capture (Zolensky et al. 2006; Ishii et al. 2008; Leroux et al. 2008, 2009; Rietmeijer et al. 2008; Tomeoka et al. 2008; Velbel and Harvey 2009). These inclusions range in size from just a few nm in diameter to greater than 100 nm, with the largest examples exhibiting a Fe-Ni-core and Fe-S-mantle structure (Leroux et al. 2008). Droplets of all sizes are typically spherical, but some exhibit deformation consistent with disruption and fission (similar to that reported from microgravity experiments by Wang et al. 1990, 1994) during rotational and translational motion in the molten state prior to quenching (Velbel and Harvey 2009). The composition of the larger droplets, including the presence of trace metals, has been found to vary substantially between and also within individual samples (Leroux et al. 2008). The similarity between Fe-Ni-S inclusions in Stardust melted grains and those in chondritic aggregate IDPs has drawn attention, although the IDP inclusions are typically larger and do not exhibit core-mantle structure, suggesting different formation environments and conditions (Zolensky et al. 2006; Rietmeijer et al. 2008).
A variety of formation mechanisms have been proposed for the larger droplets with core-mantle structure. Leroux et al. (2008) suggest crystallization sequences accounting for the range of droplet compositions, attributing the segregation of sulfide from metal to differing interfacial energies. Rietmeijer et al. (2008) hypothesize that transient extreme temperatures (>1500°C) may have facilitated the production of deep metastable eutectic compositions. Ishii et al. (2008) reproduce similar phases consisting of sulfide rims on reduced metal cores in laboratory experiments. Leroux et al. (2009) propose that, following the melting of the incident cometary material, S vaporizes and dissolves in the melted aerogel mixture and then condenses around droplets of metal that coalesced in a chemically reducing environment. There is some evidence that the cores of some larger droplets preserve cometary minerals that were not completely melted (Velbel and Harvey 2009). Abreu et al. (2011) have produced analogs of the small (<30 nm) Fe-Ni-S droplets in the laboratory by direct gas-phase condensation from a flash-cooled Fe-S-SiO-O2 vapor.
Velbel and Harvey (2009) documented a variety of compositional parameters (of large volumes analyzed at relatively low magnifications) that appear to vary systematically with penetration distance in melted-grain allocations from Stardust Track 35. They suggested that sorting by density—an attribute indigenous to individual volumes within the incident pre-capture particle—was responsible for the observed along-track compositional variation. Several downtrack compositional trends in melted grains (e.g., increases in Fe/Mg and Fe/S with increased penetration distance; Velbel and Harvey 2009; their Figures 7, 9 and 10) suggest that the farthest-traveled melted grains may have penetrated farthest because they contained larger amounts of beads of dense, Fe-bearing minerals (e.g., Fe-Ni metals, Fe-Ni sulfides) solidified from formerly molten pre-capture metal and sulfide mineral grains, as compared with melted grains that did not penetrate as far.
The size distribution of the nanoscale Stardust Fe-Ni-S droplets may preserve information relevant to interpretation of both their composition and morphology. In general, particle size distributions reflect the equilibrium reached between aggregation and fragmentation processes (for a recent review see Villermaux 2007). The kinetic properties of aerogel capture, which result in the bulb and stylus morphology of the debris track, correspond to along-track variations in temperature, pressure, and the chemical environment of the materials that could affect the equilibrium characteristics of the population of Fe-Ni-S droplets (Zolensky et al. 2006; Rietmeijer et al. 2008; Tomeoka et al. 2008). In particular, Leroux et al. (2000) find that small (<20 nm) droplets in the Tenham (L6) chondrite are composed almost entirely of FeS, while the largest (>20 nm) droplets contain additional Fe,Ni phases. Moreover, their average compositional analyses (their Table 3) indicate that the Fe/S ratio is higher within large droplets than in regions dominated by small droplets, while regions devoid of metal droplets contain essentially no Ni, S, or Fe outside of FeO. These results suggest that a high Fe/S ratio determined from bulk compositional analysis should be a signature of the presence of larger metal-sulfide droplets. If this relationship extends to metal-sulfide droplets in Stardust melted allocations, then the near-tripling of the Fe/S ratio in melted grains along Track 35 (Velbel and Harvey 2009; their Figure 10) might be expected to coincide with an increase in droplet size along Track 35.
The purpose of this study was to test the hypothesis that deeper penetrating melted grains in Stardust Track 35, with their higher Fe/S ratios (Velbel and Harvey 2009), contain larger metal-sulfide beads as suggested by the work of Leroux et al. (2000). The test we apply here seeks along-track variations in bead size, which would be consistent with the expectation deduced from our hypothesis. In doing so, we will produce a measurement of the droplet size distribution in each of several grains from Track 35 suitable for comparison with sulfide mineral grains in unmelted carbonaceous chondritic material. Such a detailed size distribution measurement may provide a basis for future comparisons with modeling and experimentation to isolate the effects of the capture heating and modification process and the textural properties of the pre-capture cometary material.