ALBERT

Description: Infrared spectral properties of silicate grains in interplanetary dust particles (IDPs) were compared with those of astronomical silicates. The approximately 10-micrometer silicon-oxygen stretch bands of IDPs containing enstatite (MgSiO3), forsterite (Mg2SiO4), and glass with embedded metal and sulfides (GEMS) exhibit fine structure and bandwidths similar to those of solar system comets and some pre-main sequence Herbig Ae/Be stars. Some GEMS exhibit a broad, featureless silicon-oxygen stretch band similar to those observed in interstellar molecular clouds and young stellar objects. These GEMS provide a spectral match to astronomical "amorphous" silicates, one of the fundamental building blocks from which the solar system is presumed to have formed.

Description: It is the fine fractions that dominate the observed spectral signatures of bulk lunar soil, and the next to the smallest size fractions are the most similar to the overall properties of the bulk soil. Thus, our Lunar Soil Characterization Consortium has concentrated on understanding the inter-relations of compositional, mineralogical, and optical properties of the 〈45-micron size fraction and its component sizes (20-44 micron, 10-20 micron, and 〈10 micron size fractions). To be able to generalize our results beyond the particular sample set studied, it is necessary to quantitatively identify the observed effects of space weathering and evaluate the processes involved. For this, it is necessary to know the chemistry of each size fraction, modal abundances of each phase, average compositions of the minerals and glasses, I(sub s)/FeO values, reflectance spectra, and the physical makeup of the individual particles and their patinas. This characterization includes the important dissection of the pyroxene minerals into four separate populations, with data on both modes and average chemical compositions. Armed with such data, it should be possible to effectively isolate spectral effects of space weathering from spectral properties related to mineral and glass chemistry. Four mare soils from the Apollo 17 site were selected for characterization based upon similarities in bulk composition and their contrasting maturities, ranging from immature to submature to mature. The methodology of our characterization has been discussed previously. Results of the Apollo 17 mare soils, outlined herein, are being prepared for publication in MAPS. As shown, with decreasing grain size, the agglutinitic (impact) glass content profoundly increases. This is the most impressive change for the mare soils. In several soils we have examined, there is an over two-fold increase in the agglutinitic glass contents between the 90-150- micron and the 10-20-micron size fractions. Accompanying this increase in agglutinitic glass is a definite decrease in pyroxenes and to lesser extents, the oxides (ilmenite), volcanic glass, and olivine. Unexpectedly, however, the absolute plagioclase abundances stay relatively constant throughout the different grain sizes, although the abundance of plagioclase relative to the mafic minerals increases with decreasing particle size. These soils were chosen for study based upon their similarities in FeO and Ti02 content, allowing for direct comparisons between evolutions of chemistry between size fractions and among different maturities of soils. The bulk chemistry of these fractions was determined by EMP analyses of fused glass beads. In contrast to the systematic variations in bulk chemistry discussed below, the relatively uniform composition of agglutinitic glass with grain size and soil maturity is illustrated. The composition of the bulk fraction of each size fraction becomes more feldspathic with increasing maturity, with the effect being most pronounced for the finest fractions. The composition of the agglutinitic glass, however, is relatively invariant and more feldspathic (i.e., rich in Al2O3) than even the 〈10-micron fraction. This relation not only strengthens the "fusion of the finest fraction" (F(sup 3)) hypothesis, but also highlights the important role of plagioclase in the formation of agglutinitic glass. With decreasing grain size, FeO, MgO, and TiO2 contents decrease, whereas CaO, Na2O, and Al2O3 (plag components) increase for all soils. These chemical variations would appear to be coupled with the significant increase in agglutinitic glass and decrease in oxide (ilmenite),pyroxene, and volcanic glass. These changes in chemistry do not appear to be due to distinct changes in the compositions of individual phases but to their abundances. Values of I(sub s)/FeO increase with decreasing grain size, even though the bulk FeO contents decrease. That is, the percentage of the total Fe that is present as nanophase Fe(sup O) has increased substantially in the smaller size fraction. Note that the increase in nanophase FeO in smaller size fractions is significantly greater than the increase in agglutinitic glass content, with its single-domain FeO component. This would seem to indicate that at least some of the FeO is surface correlated. To illustrate this effect, if it is assumed that the nanophase FeO is entirely surface correlated, then equal masses of 15-micron and 6-micron spheres should have about 3x as much FeO in the finer fraction. The recent findings of Kelleret al. of the major role of vapor-deposited, nanophase FeO-containing patinas on most soil particles is a major breakthrough in our understanding of the distribution of FeO within agglutinitic glass and upon grain surfaces. Bidirectional reflectance spectra for a representative Apollo 17 soil (70181) are shown. The size separates all have similar albedo in the blue and follow a regular sequence in which the continuum slope increases, ferrous bands weaken, and albedo, increases with decreasing particle size. The bulk 〈45-micron soil is typically close to the 10-20 micron spectrum. It is important to note that although the finest fraction (〈10 micron) is close in composition to the abundant agglutinitic glass in each size fraction, this size fraction is relatively featureless and does not dominate the spectrum of the bulk 〈45-micron soil. It has long been suspected that agglutinitic glass, to a large extent, is the product of melting of the finest fraction of the soils, with a dominance of plagioclase. Given the low abundance of pyroxene in the finest fractions of each soil the source of the FeO in these Apollo 17 agglutinitic glasses is not fully identified. We suspect the abundant volcanic glass in these samples may be a significant contributor and this hypothesis will be tested with the suite under study from other Apollo sites.

Description: The effects of space weathering on the optical properties of lunar materials have been well documented. These effects include a reddened continuum slope, lowered albedo, and attenuated absorption features in reflectance spectra of lunar soils as compared to finely comminuted rocks from the same Apollo sites. However, the regolith processes that cause these effects are not well known, nor is the petrographic setting of the products of these processes fully understood. A Lunar Soil Characterization Consortium has been formed with the purpose of systematically integrating chemical and mineralogical data with the optical properties of lunar soils. Understanding space-weathering effects is critical in order to fully integrate the lunar sample collection with remotely-sensed data from recent robotic missions (e.g., Lunar Prospector, Clementine, and Galileo) We have shown that depositional processes (condensation of impact-derived vapors, sputter deposits, accreted impact material, e.g., splash glass, spherules, etc.) are a major factor in the modification of the optical surfaces of lunar regolith materials. In mature soils, it is the size and distribution of the nanophase metal in the soil grains that has the major effect on optical properties. In this report, we compare and contrast the space-weathering effects in an immature and a mature soil with similar elemental compositions. For this study, we analyzed 〈10 micron sieve fractions of two Apollo 17 soils, 79221 (mature, Is/FeO = 81) and 71061 (immature, Is/FeO = 14). Details of the sieving procedures and allocation scheme are given else where. The results of other detailed chemical, mineralogical, and spectroscopic analyses of these soil samples are reported elsewhere. A representative sample of each soil was embedded in low-viscosity epoxy, and thin sections (about 70nm thick) were obtained through ultra microtomy. The thin sections used for these analyses typically contained cross sections of up to 500 individual grains. The thin sections were studied using a JEOL 2010 transmission electron microscope (TEM) equipped with a thin window energy-dispersive X-ray (EDX) spectrometer. An individual thin section was selected from each soil, and for each grain in the section we determined (1) the elemental composition by EDX; (2) whether the grain was crystalline or glassy using electron diffraction and darkfield imaging; (3) the presence or absence of rims and accreted material; and (4) the distribution of nanophase Fe where present. Most of the categories are self-evident; however, we divide the agglutinate derived material into agglutinitic glass (glass with approximately the same composition as the bulk soil that contains nanophase Fe with or without vesicles) and agglutinate fragments, which are composed of crystalline grains and agglutinitic glass. Lithic fragments are defined as polymineralic grains with no glass. Pyroxene grains have been divided into high- and low-Ca groups. As expected, there are a number of differences in the petrography of the 〈10-microns fractions of 79221 and 71061 given the great difference in their respective maturities, but we focus here on two major distinctions: agglutinate content and the number of grains with micropatina. Slightly over 50% of the particles in 79221 consist of agglutinitic glass and agglutinate fragments, while the remainder are predominantly crystalline mineral grains. The agglutinic glass particles contain abundant nanophase Fe and vesicles. Angular particles are rare, with most showing smooth, rounded exteriors, Of the mineral grains analyzed thus far, over 90% of the grains have amorphous rims that contain nanophase Fe (these rims are believed to have formed by vapor deposition and irradiation effects). The nanophase Fe in these rims probably accounts for a significant fraction of the increase in Is/FeO measured in these size fractions. In addition to the rims, the majority of particles also show abundant accreted material in the form of glass splashes and spherules that also contain nanophase Fe. In stark contrast, the surfaces of the mineral grains in the 71061 sample are relatively prisitine, as only about 14% of the mineral grains in the sample exhibited amorphous rims. Furthermore, the mineral particles are more angular and show greater surface roughness than in the mature sample. Accreted material on particle surfaces is rare. Agglutinitic material is a major component of the 71061 sample; however, nanophase Fe and vesicles are not as well developed as in the 79221 sample. It is now recognized that nanophase Fe is probably the main agent in modifying the optical properties of lunar soil grains. The most important result of this study is the observation that in the fine size fractions of mature soils, nearly every grain has nanophase Fe within 100 run of the particle surface. (Additional Information contained in original)