Lindsay P. Keller

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.

Organics captured from comet 81P/Wild 2 by the Stardust spacecraft

Organics found in comet 81P/Wild 2 samples show a heterogeneous and unequilibrated distribution in abundance and composition. Some organics are similar, but not identical, to those in interplanetary dust particles and carbonaceous meteorites. A class of aromatic-poor organic material is also present. The organics are rich in oxygen and nitrogen compared with meteoritic organics. Aromatic compounds are present, but the samples tend to be relatively poorer in aromatics than are meteorites and interplanetary dust particles. The presence of deuterium and nitrogen-15 excesses suggest that some organics have an interstellar/protostellar heritage. Although the variable extent of modification of these materials by impact capture is not yet fully constrained, a diverse suite of organic compounds is present and identifiable within the returned samples.

The nature and origin of rims on lunar soil grains

Abstract Space weathering processes that operate in the lunar regolith modify the surfaces of lunar soil grains. Transmission electron microscope analysis of the lunar soil grains from the fine size fraction of several lunar soils show that most grains are surrounded by thin (60–200 nm thick) rims. The microstructure and chemical compositions of the rims can be used to classify rims into four broad categories: amorphous, inclusion-rich, multiple, and vesicular. Amorphous rims are noncrystalline, generally lack crystalline inclusions, show evidence for preferential sputtering of cations, and are produced largely by solar-wind irradiation damage. Inclusion-rich rims contain abundant nanometer-sized grains of Fe metal as randomly dispersed inclusions or as distinct layers embedded in an amorphous silica-rich matrix. Inclusion-rich rims are compositionally distinct from their host grains and typically contain accumulations of elements that are not indigenous to the host. Inclusion-rich rims are formed largely by the deposition of impact-generated vapors with a contribution from the deposition of sputtered ions. A continuum in the chemical and microstructural properties exists between typical amorphous rims and typical inclusion-rich rims. Multiple-rims consist of a distinct radiation-damaged layer up to 50 nm thick, that is overlain by vapor-deposited material of comparable thickness. Vesicular rims are compositionally similar to their hosts and are characterized by an abundance of small ( The formation of rims on lunar soils is complex and involves several processes whose effects may be superimposed. From this study, it is shown that one process does not dominate and that the relative importance of vapor-deposition is comparable to radiation-damage in the formation of rims on lunar silicate grains. The presence of rims on lunar soil grains, particularly those with nanometer-sized Fe metal inclusions, may have a major influence on the optical and magnetic properties of lunar soils.

Lunar Mare Soils: Space weathering and the major effects of surface‐correlated nanophase Fe

Lunar soils form the “ground truth” for calibration and modeling of reflectance spectra for quantitative remote sensing. The Lunar Soil Characterization Consortium, a group of lunar sample and remote sensing scientists, has undertaken the extensive task of characterization of lunar soils, with respect to their mineralogical and chemical makeup. This endeavor is aimed at deciphering the effects of space weathering of soils from the Moon, and these results should apply to other airless bodies. Modal abundances and chemistries of minerals and glasses in the <45 μm size fractions of nine selected mare soils have been determined, along with the bulk chemistry of each size fraction, and their IS/FeO values. These data can be addressed at http:/web.utk.edu/∼pgi/data.html. As grain size decreases, the bulk composition of each size fraction continuously changes and approaches the composition of the agglutinitic glasses. Past dogma had it that the majority of the nanophase Fe0 resides in the agglutinitic glasses. However, as grain size of a soil decreases, the percentage of the total iron present as nanophase-sized Fe0 increases dramatically, while the agglutinitic glass content rises only slightly. This is evidence for a large contribution to the IS/FeO values from surface-correlated nanophase Fe0, particularly in the <10 μm size fraction. This surficial nanophase Fe0 is present largely as vapor-deposited patinas on the surfaces of almost every particle of the mature soils. It is proposed that these vapor-deposited, nanophase Fe0-bearing patinas may have far greater effects upon reflectance spectra of mare soils than the agglutinitic Fe0.

Discovery of Vapor Deposits in the Lunar Regolith

Lunar soils contain micrometer-sized mineral grains surrounded by thin amorphous rims. Similar features have been produced by exposure of pristine grains to a simulated solar wind, leading to the widespread belief that the amorphous rims result from radiation damage. Electron microscopy studies show, however, that the amorphous rims are compositionally distinct from their hosts and consist largely of vapor-deposited material generated by micrometeorite impacts into the lunar regolith. Vapor deposits slow the lunar erosion rate by solar wind sputtering, influence the optical properties of the lunar regolith, and may account for the presence of sodium and potassium in the lunar atmosphere.

Organic globules in the Tagish Lake meteorite: remnants of the protosolar disk.

Coordinated transmission electron microscopy and isotopic measurements of organic globules in the Tagish Lake meteorite shows that they have elevated ratios of nitrogen-15 to nitrogen-14 (1.2 to 2 times terrestrial) and of deuterium to hydrogen (2.5 to 9 times terrestrial). These isotopic anomalies are indicative of mass fractionation during chemical reactions at extremely low temperatures (10 to 20 kelvin), characteristic of cold molecular clouds and the outer protosolar disk. The globules probably originated as organic ice coatings on preexisting grains that were photochemically processed into refractory organic matter. The globules resemble cometary carbon, hydrogen, oxygen, and nitrogen (CHON) particles, suggesting that such grains were important constituents of the solar system starting materials.

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.

Carbon abundance and silicate mineralogy of anhydrous interplanetary dust particles.

We have studied nineteen anhydrous chondritic interplanetary dust particles (IDPs) using analytical electron microscopy. We have determined a method for quantitative light element EDX analysis of small particles and have applied these techniques to a group of IDPs. Our results show that some IDPs have significantly higher bulk carbon abundances than do carbonaceous chondrites. We have also identified a relationship between carbon abundance and silicate mineralogy in our set of anhydrous IDPs. In general, these particles are dominated by pyroxene, olivine, or a subequal mixture of olivine and pyroxene. The pyroxene-dominated IDPs have a higher carbon abundance than those dominated by olivines. Members of the mixed mineralogy IDPs can be grouped with either the pyroxene- or olivine-dominated particles based on their carbon abundance. The high carbon, pyroxene-dominated particles have primitive mineralogies and bulk compositions which show strong similarities to cometary dust particles. We believe that the lower carbon, olivine-dominated IDPs are probably derived from asteroids. Based on carbon abundances, the mixed-mineralogy group represents particles derived from either comets or asteroids. We believe that the high carbon, pyroxene-rich anhydrous IDPs are the best candidates for cometary dust.

Aqueous alteration of the Bali CV3 chondrite: evidence from mineralogy, mineral chemistry, and oxygen isotopic compositions.

A petrographic, geochemical, and oxygen isotopic study of the Bali CV3 carbonaceous chondrite revealed that the meteorite has undergone extensive deformation and aqueous alteration on its parent body. Deformation textures are common and include flattened chondrules, a well-developed foliation, and the presence of distinctive (100) planar defects in olivine. The occurrence of alteration products associated with the planar defects indicates that the deformation features formed prior to the episode of aqueous alteration. The secondary minerals produced during the alteration event include well-crystallized Mg-rich saponite, framboidal magnetite, and Ca-phosphates. The alteration products are not homogeneously distributed throughout the meteorite, but occur in regions adjacent to relatively unaltered material, such as veins of altered material following the foliation. The alteration assemblage formed under oxidizing conditions at relatively low temperatures (<100 degrees C). Altered regions in Bali have higher Na, Ca, and P contents than unaltered regions which suggests that the fluid phase carried significant dissolved solids. Oxygen isotopic compositions for unaltered regions in Bali fall within the field for other CV3 whole-rocks, however, the oxygen isotopic compositions of the heavily altered material lie in the region for the CM and CR chondrites. The heavy-isotope enrichment of the altered regions in Bali suggest alteration conditions similar to those for the petrographic type-2 carbonaceous chondrites.

Aqueous alteration in the Kaba CV3 carbonaceous chondrite

The Kaba CV3 carbonaceous chondrite has undergone pervasive aqueous alteration that produced Fe-bearing saponite in chondrules and matrix. Saponite in chondrules is coarse grained and shows a crystallographic orientation relationship such that c∗ of saponite parallels a∗ of enstatite. Matrix saponite exhibits a variety of textures including coarse- and fine-grained clusters, isolated packets, and sparse, lamellar replacements of Fe-rich olivine. The coarse-grained clusters were probably derived from altered chondrules that were fragmented during regolith gardening. The fine-grained clusters and isolated packets formed from material released during the partial dissolution of matrix silicates. Much saponite is associated with submicron Fe-Ni sulfides, suggesting that conditions were not oxidizing during aqueous alteration. We believe that the alteration occurred after accretion and on the Kaba parent body because saponite is present throughout the sample. By analogy to terrestrial occurrences, the saponite in Kaba probably formed at low temperatures (<100°C). The distribution of saponite in Kaba is determined by local bulk compositions; enstatite in chondrules has been altered more extensively than the fine-grained olivine in matrix. The alteration products in Kaba resemble those in other aqueously altered carbonaceous chondrites such as the Mokoia CV3 chondrite and in certain CI chondrites. There are, however, significant differences. Kaba lacks the abundant high-Al phyllosilicates in chondrules and in CAIs reported from Mokoia; also lacking in Kaba is the serpentine and ferrihydrite found in Orgueil. Alteration products in Kaba matrix are dissimilar from those in CO3 chondrites even though their prealteration mineralogies were similar, an observation that probably reflects different temperatures during alteration.