Giles A. Graham

Mineralogy and Petrology of Comet 81P/Wild 2 Nucleus Samples

The bulk of the comet 81P/Wild 2 (hereafter Wild 2) samples returned to Earth by the Stardust spacecraft appear to be weakly constructed mixtures of nanometer-scale grains, with occasional much larger (over 1 micrometer) ferromagnesian silicates, Fe-Ni sulfides, Fe-Ni metal, and accessory phases. The very wide range of olivine and low-Ca pyroxene compositions in comet Wild 2 requires a wide range of formation conditions, probably reflecting very different formation locations in the protoplanetary disk. The restricted compositional ranges of Fe-Ni sulfides, the wide range for silicates, and the absence of hydrous phases indicate that comet Wild 2 experienced little or no aqueous alteration. Less abundant Wild 2 materials include a refractory particle, whose presence appears to require radial transport in the early protoplanetary disk.

Isotopic Compositions of Cometary Matter Returned by Stardust

Hydrogen, carbon, nitrogen, and oxygen isotopic compositions are heterogeneous among comet 81P/Wild 2 particle fragments; however, extreme isotopic anomalies are rare, indicating that the comet is not a pristine aggregate of presolar materials. Nonterrestrial nitrogen and neon isotope ratios suggest that indigenous organic matter and highly volatile materials were successfully collected. Except for a single 17O-enriched circumstellar stardust grain, silicate and oxide minerals have oxygen isotopic compositions consistent with solar system origin. One refractory grain is 16O-enriched, like refractory inclusions in meteorites, suggesting that Wild 2 contains material formed at high temperature in the inner solar system and transported to the Kuiper belt before comet accretion.

Impact Features on Stardust: Implications for Comet 81P/Wild 2 Dust

Particles emanating from comet 81P/Wild 2 collided with the Stardust spacecraft at 6.1 kilometers per second, producing hypervelocity impact features on the collector surfaces that were returned to Earth. The morphologies of these surprisingly diverse features were created by particles varying from dense mineral grains to loosely bound, polymineralic aggregates ranging from tens of nanometers to hundreds of micrometers in size. The cumulative size distribution of Wild 2 dust is shallower than that of comet Halley, yet steeper than that of comet Grigg-Skjellerup.

Elemental compositions of comet 81P/Wild 2 samples collected by Stardust

We measured the elemental compositions of material from 23 particles in aerogel and from residue in seven craters in aluminum foil that was collected during passage of the Stardust spacecraft through the coma of comet 81P/Wild 2. These particles are chemically heterogeneous at the largest size scale analyzed (∼180 ng). The mean elemental composition of this Wild 2 material is consistent with the CI meteorite composition, which is thought to represent the bulk composition of the solar system, for the elements Mg, Si, Mn, Fe, and Ni to 35%, and for Ca and Ti to 60%. The elements Cu, Zn, and Ga appear enriched in this Wild 2 material, which suggests that the CI meteorites may not represent the solar system composition for these moderately volatile minor elements.

Infrared Spectroscopy of Comet 81P/Wild 2 Samples Returned by Stardust

Infrared spectra of material captured from comet 81P/Wild 2 by the Stardust spacecraft reveal indigenous aliphatic hydrocarbons similar to those in interplanetary dust particles thought to be derived from comets, but with longer chain lengths than those observed in the diffuse interstellar medium. Similarly, the Stardust samples contain abundant amorphous silicates in addition to crystalline silicates such as olivine and pyroxene. The presence of crystalline silicates in Wild 2 is consistent with mixing of solar system and interstellar matter. No hydrous silicates or carbonate minerals were detected, which suggests a lack of aqueous processing of Wild 2 dust.

Analytical scanning and transmission electron microscopy of laboratory impacts on Stardust aluminum foils: Interpreting impact crater morphology and the composition of impact residues

The known encounter velocity (6.1 kms(-1)) and particle incidence angle (perpendicular) between the Starchist spacecraft and the dust emanating from the nucleus of comet Wild-2 fall within a range that allows simulation in laboratory light-gas gun (LGG) experiments designed to validate analytical methods for the interpretation of dust impacts on the aluminum foil components of the Stardust collector. Buckshot of a wide size, shape, and density range of mineral, glass, polymer, and metal grains, have been fired to impact perpendicularly on samples of Stardust Al 1100 foil, tightly wrapped onto aluminum alloy plate as an analogue of foil on the spacecraft collector. We have not yet been able to produce laboratory impacts by projectiles with weak and porous aggregate structure, as may occur in some cometary dust grains. In this report we present information on crater gross morphology and its dependence on particle size and density, the pre-existing major- and trace-element composition of the foil, geometrical issues for energy dispersive X-ray analysis of the impact residues in scanning electron microscopes, and the modification of dust chemical composition during creation of impact craters as revealed by analytical transmission electron microscopy. Together, these observations help to underpin the interpretation of size, density, and composition for particles impacted on the Stardust aluminum foils.

The chemistry of micrometeoroid and space debris remnants captured on Hubble Space Telescope solar cells

Abstract Prior to the retrieval in 1993 from low Earth orbit (LEO), the “—V2” Solar Array wing of the Hubble Space Telescope was exposed to hypervelocity impacts (micrometre to millimetre scale) from both micrometeoroids and space debris. The initial survey of the damage (100–3500μm diameter sized craters) identified that micrometeoroid remnants dominated the flux in the 100–1000μm size regime, with debris dominating >1000μm. These residues were composed of remnants of silicate minerals, calcite, metal sulfides and metals that often appeared as complex poly-mineralic melts within melt pits. A further survey of 10–100μm diameter craters identified that the most common chemistry was space debris with the crossover from meteoroids to debris being at around 30μm DCO. Residues include remnants of specialised steels and paint fragments but the dominant type is aluminium and aluminium oxide, which are almost certainly remnants of solid rocket motor operations. It is found that the relative contribution of debris as a function of size, agrees remarkably with a prediction derived using flux data from Long Duration Exposure Facility and a meteoroid model.

Microparticle impacts upon HST solar cells

Analysis of individual solar cells removed from the returned array from the Hubble Space Telescope have yielded results on the residue chemistry of material preserved in impact features within the 1-3500μm size range. The initial survey of impact craters in the 100–3500μm size range identified that micrometeoroids were the dominant precursor chemistry. The second survey of impact features between 1–100μm identified space debris as the prevailing precursor chemistry. The categories of space debris remnants observed in this size range were dominated by aluminium and aluminium oxide chemistries indicative of solid rocket motor fuel debris. The results from the residue chemistry surveys have now been compared with a prediction derived from LDEF data and meteoroid modelling, and there is general agreement between the modelling and actual observations.

The collection of micrometeoroid remnants from low earth orbit

Abstract The solar array panel returned from the Hubble Space Telescope after 3.62 years of space exposure offered the opportunity to study individual solar cells for hypervelocity impact damage and residue. A detailed electron microscope investigation of impact craters (100–1000μm diameters) has identified that most are residue-rich and by digitised x-ray elemental mapping and semi-quantitative micro-spot analysis the original precursor composition of the impactor can almost unambiguously be identified. The residues contain diverse elemental compositions that can be associated with known meteorite mineralogies and directly compared with interplanetary dust particles and micrometeorites, possibly the most likely source object. The observation of a magnesium-rich residue with (Mg+Fe)/Si ratio similar to that of forsterite (end-member Mg-olivine identified in meteorites), indicates that it is possible in favourable conditions to define clearly the compsitional nature of the impactor. The identification of near-intact calcium-rich fragments, that are neither artefacts nor contamination, indicates that volatile chemistries can survive hypervelocity impacts in brittle glass substrates. The abundance of micrometeoroid residues in the individual solar cells has highlighted that valuable information can be retained from impact craters in returned space hardware which are essentially not designed as a dust collectors.

Hypervelocity impacts in low Earth orbit: Cosmic dust versus space debris

Abstract The understanding of the micron-sized populations of natural micrometeoroids and artificial space debris in low Earth orbit has benefited considerably from the post-flight investigations of retrieved surfaces from spacecraft, such as the Long Duration Exposure Facility. The returned solar array from the Hubble Space Telescope has added to this repository and has offered a further opportunity to document these particles. 25 individual solar cells were specially selected on the basis that they contained impact craters (diameter 100–1000 μm) which had the most potential to retain impactor residue chemistry. The solar cells were subject to a detailed investigation using analytical scanning electron microscopy which identified 29 impact craters, the analysis of which identified 3 residues as artificial in origin, 6 unclassified and 20 as natural in origin. The limited number of unclassified residues identified indicates that the methods of analysis employed in this investigation are a significant step forward for such studies and, if employed on a greater number of samples, will improve the calculations of the time-integrated flux rates for micrometeoroids and space debris in the low Earth orbit environment. Notwithstanding the small sample set examined, the observed chemical classification of the impact residues in terms of micrometeoroid to space debris (in the particle size range 8–80 μm) corresponds well to the flux model that predicts the dominance of natural particles.