[Show abstract][Hide abstract] ABSTRACT: Infrared wavelengths offer a better opportunity to deconvolve the spectra of carbonaceous chondrites into their constituent minerals. We will compare reflectance and emittance measurements with known mineral endmembers of the carbonaceous chondrites.
[Show abstract][Hide abstract] ABSTRACT: Absorption features can exist in disparate minerals at identical wavelengths due to similarities in chemistry and bond lengths; it is accordingly sometimes necessary to use absorption features at several wavelengths in order to correctly identify a specific mineral or mineral group, and thereby make assumptions about chemical processes. It is suggested that future space missions attempt to use a spectrometric wavelength coverage that allows observation of multiple diagnostic absorption features.
[Show abstract][Hide abstract] ABSTRACT: A combined laboratory and theoretical investigation is being conducted to determine the most accurate means of representing the particle size distribution of a mineral mixture for use in theoretical reflectance calculations. Radiative transfer theory is being used to model the laboratory reflectance spectra of mineral separates and mixtures which mimic carbonaceous and ordinary chondrites (the theoretical mixtures will be generated using the known chemistry, mineralogy, optical constants and grain size distributions which have been previously determined). Comparison of theoretical spectra computed using a single grain size (for example 52 micrometers) and the computed spectra using a cumulative power-law distribution to represent the same average grain size in mineral separates and mixtures show differences in the strength of absorption features which could affect estimates of minerals abundance.
[Show abstract][Hide abstract] ABSTRACT: We are developing a new approach to quantitative spectral characterization of remotely sensed objects. The work is an iterative and integrated theoretical and laboratory investigation. We are relating the spectral signature of the two types of carbonaceous chondrites (CV and CM) with their chemistry, mineralogy, and grain size distribution. In the future, the laboratory and theoretical portions of the study will be related to remote observations of asteroid surfaces. Radiative theory is being used to model the laboratory reflectance spectra of members of each of the two classes of carbonaceous chondrites (the theoretical mixtures will be generated using the known chemistry, mineralogy, optical constants and grain size distributions which have been previously determined) and verify the goodness of fit of the theoretical models to the laboratory spectra of meteorites. In using theoretical modeling assumptions about the physical and chemical properties of materials must be made. For the wavelength range we intend to consider (0.2-5 micrometers), we have previously obtained reasonable results using optical constants derived from reflectance spectra. That is, using a powdered sample in a narrow grain size range we invert a Hapke-based theoretical reflectance calculation to obtain an absorption coefficient. The primary disadvantage to this method is that it requires an assumption regarding the behavior of the index of refraction. However, as the index typically does not vary strongly in this wavelength interval, this is a reasonable assumption. Assumptions about the grain size of the materials being modelled are also important considerations when using Hapke-based theoretical modeling techniques. Commonly a single grain size range is deemed appropriate for a specific calculation and from that range an average grain size is estimated for the material. However, as most materials, including planetary regoliths and meteorites, are not composed of single mineralogies or single particle grain sizes we have investigated the disadvantages of making assumptions about a single grain size to represent the entire grain size distribution. For this preliminary study we selected pyroxene separates that were ground into narrow size intervals and for which an average grain size for each interval had been determined by Scanning Electron Microscope. Using a Hapke-based theoretical model we calculated the absorption coefficient. Then using a cumulative power-law distribution we calculated an absorption coefficient of the mixture. The average grain size of the cumulative distribution was determined to be 51.8 micrometers. This compound absorption coefficient was then used to compute a reflectance spectra of the pyroxene with a grain size of 52 micrometers. This spectrum was compared with the theoretical spectrum of the pyroxene in which the size distribution was a single size interval with an average grain size of 52 micrometers. Differences in the strengths of the resulting 2 micrometers absorption feature were nearly 7%, although no change was observed in the 1 micrometer absorption feature. Thus, suggesting that using a single grain size value to determine the absorption coefficient of a poly-grain size material needs to be reevaluated. Deriving the absorption coefficient based on a cumulative power-law distribution rather than a single average grain size is necessary, especially in multi-component mixtures where each mineral component, whether it is in the matrix or appears as an inclusion or chondrule, will likely have a unique grain size distribution. When modelling surfaces or materials with several mineralogic materials present, the failure to consider a cumulative size distribution will effect the estimates of mineral abundances.
[Show abstract][Hide abstract] ABSTRACT: Absorption features at 2.28 and 5.4 micrometers identified in Mariner 6/7 infrared spectrometer and terrestrial telescopic spectra are consistent with the spectra of hydrous magnesium carbonates such as hydromagnesite and artinite. Spectral characteristics of these hydrous carbonates are different from those of the anhydrous carbonates, as the former do not have the strong spectral features typically associated with anhydrous carbonates such as calcite and siderite. Theoretical mixing indicates that, depending on the type of hydrous carbonate, 10-20 wt % can be incorporated into the regolith without contradicting the spectral observations or the Viking x ray fluorescence chemical analysis. Hydrous carbonates form as weathering products of mafic minerals in the presence of H2O and CO2, even in the Antarctic. Their formation as evaporite minerals from either original magmas or hydrothermally altered rocks is consistent with the Martian environment, provided liquid water is or has been at least transiently present. On Earth, formation of hydrous Mg carbonates is associated with the production of amorphous iron oxides, which is consistent with both the environment and the inferred surface mineralogy of Mars. These minerals are about 60 wt % H2O, CO3, and OH; if they are abundant everywhere at the 10% level, then about 6% of the surface weight could be volatiles bound in this type of mineral. Although the spectroscopic evidence for anhydrous carbonates is scant, the possible presence of hydrous carbonates provides an appealing mechanism for the existence of carbonates on Mars.
Journal of Geophysical Research 08/1994; · 3.17 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have developed a digital reflectance spectral library, with management and spectral analysis software. The library includes 500 spectra of 447 samples (some samples include a series of grain sizes) measured from approximately 0.2 to 3.0 microns. The spectral resolution (Full Width Half Maximum) of the reflectance data is less than or equal to 4 nm in the visible (0.2-0.8 microns) and less than or equal 10 nm in the NIR (0.8-2.35 microns). All spectra were corrected to absolute reflectance using an NBS Halon standard. Library management software lets users search on parameters (e.g. chemical formulae, chemical analyses, purity of samples, mineral groups, etc.) as well as spectral features. Minerals from sulfide, oxide, hydroxide, halide, carbonate, nitrate, borate, phosphate, and silicate groups are represented. X-ray and chemical analyses are tabulated for many of the entries, and all samples have been evaluated for spectral purity. The library also contains end and intermediate members for the olivine, garnet, scapolite, montmorillonite, muscovite, jarosite, and alunite solid-solution series. We have included representative spectra of H2O ice, kerogen, ammonium-bearing minerals, rare-earth oxides, desert varnish coatings, kaolinite crystallinity series, kaolinite-smectite series, zeolite series, and an extensive evaporite series. Because of the importance of vegetation to climate-change studies we have include 17 spectra of tree leaves, bushes, and grasses.
[Show abstract][Hide abstract] ABSTRACT: The composition of the carbonaceous chondrites is dominated by a fine-grained opaque mineral mixture called matrix. In the lowest petrologic type C-chondrites significant alteration of matrix minerals has occured, resulting in compositions dominated by aqueous alteration products such as phyllosilicates, sulfates, oxides, hydroxides, and carbonates. The phyllosilicates top the list of abundant phases, and in the CM chondrites in particular, Fe-rich serpentines are the most important phases [e.g., 1]. King and Clark  have characterized the Mg-serpentines and chlorites, noting certain spectral similarities between chlorites and CI1 and CM2 chondrites. However, they found no exact spectral matches. We present here the results of an examination of the reflectance spectra of Fe-serpentines and two varieties of chamosite (chlorite group). We find these minerals can provide a reasonable spectral match to features seen in certain CM chondrites and by extension, the dark asteroids. We have measured the reflectance spectra of several different high-iron phyllosilicates. Samples were primarily obtained from the National Museum of Natural History (NMNH) with one extremely high iron chamosite provided by the University of Munster. Samples were hand picked and ground and measured in bidirectional reflectance from 0.3 to 25 micrometers. Samples were also characterized by x-ray. In the serpentine group we have measured samples of greenalite (Fe^2+,Fe^3+)(sub)2- 3(Si)2O(sub)5(OH)(sub)4, berthierine (Fe^2+,Fe^3+,Mg)(sub)2- 3(Si,Al)2O(sub)5(OH)4, and cronstedtite Fe^2+2Fe^3+(Si,Fe^3+)O5(OH)4. In the chlorite group we have measured two different samples of chamosite (Fe^2+,Mg,Fe^3+)(sub)5Al(Si(sub)3,Al)O(sub)10(OH,O)(sub)s. We have measured an additional Mg-serpentine amesite Mg(sub)2Al(Si,Al)O(sub)5(OH)(su)4, not examined by . (Chemical formulas cited reflect the ideal given in .) For comparison to spectra of CM-type chondrites, samples of Murchison and Murray were available to us and these were also measured in reflectance from 0.3 to 25 micrometers. There are a number of spectral differences between the Fe- and Mg- serpentines, most notably that the Fe-bearing minerals lack the strong, narrow feature at 1.4 micrometers. They also lack the strong Mg-OH features between 2.2 and 2.4 micrometers. In addition several of the samples exhibit absorptions near 0.7 and 0.9 micrometers. The absence of the near-infrared features coupled with the presence of absorptions at the long end of the visible allows the Fe endmembers to provide a much better spectral match to near-infrared characterisitics of CM chondrites like Murchison and Nogoya. Additionally, the general slope characteristics below 0.58 micrometers in CM2 chondrites are also well matched by those observed in the Fe-serpentines, particularly the berthierine that we measured. Vilas and Gaffey  compared the absorptions near 0.7 and 0.9 micrometers in several CM chondrites with those observed in main- and outer-belt asteroids. They argued for a similar origin for the spectral features so Fe-phyllosilicates may contribute to the observed spectral characteristics of certain asteroids as well. In the infrared the spectra of CM chondrites Murray and Murchison are quite similar with broad absorptions at 3 micrometers, and from 8-12 micrometers, with a narrower feature centered on 6.2 micrometers. The Mg-serpentine, amesite, has abundant spectral features beyond 13 micrometers, which are not seen in the CM chondrite spectra. The Fe-serpentines have absorptions that can contribute to those seen in the CM chondrites, but lacks the large absorptions beyond 13 micrometers, again providing a better spectral match than the Mg-serpentines. In the future we hope to compare the spectra of these Fe- serpentines with a wider variety of CM chondrites. Additionally a theoretical modeling study is planned, which will attempt to match meteorite spectra using their mineralogy and grain size distribution as the initial input to the models. Acknowledgements: This work was begun while W. M. Calvin was a Humboldt Research Fellow at the Inst. fur Planetologie at the University of Muenster. References:  Zolensky M. and McSween H. Y. (1988) Meteorites and the Early Solar System (Kerridge and Matthews, eds.), 114- 143.  King T. V. V. and Clark R. N. (1989) JGR, 94, 13997- 14008.  Fleischer M. and Mandarino J. A. (1991) Glossary of Mineral Species.  Vilas F. and Gaffey M. J. (1989) Science, 246, 790-792.
[Show abstract][Hide abstract] ABSTRACT: Spectra obtained from recent telescopic observation of 1-Ceres and laboratory measurements and theoretical calculations of three component mixtures of Ceres analog material suggest that an ammoniated phyllosilicate is present on the surface of the asteroid, rather than H(2)O frost as had been previously reported. The presence of an ammoniated phyllosilicate, most likely ammoniated saponite, on the surface of Ceres implies that secondary temperatures could not have exceeded 400 kelvin.
[Show abstract][Hide abstract] ABSTRACT: Although many of the spectral features of the Martian samples studied
are not unique mineralogical indicators, much of the current spectral
data is consistent with (possibly abundant) hydrous carbonates on the
surface of Mars. The absorption features in the measured samples were
quite weak compared with those of anhydrous carbonates. The weak
features imply that significantly more hydrous carbonates can be
incorporated onto the surface before becoming spectrally evident;
however, exact limits have yet to be determined. The stability of these
materials in the Martian environment is not known, but their formation
and occurrence in low temperature terrestrial environments makes them
appealing candidates for weathering products on Mars.
[Show abstract][Hide abstract] ABSTRACT: In December 1989 and January 1990, new observations of the leading and trailing edges of Callisto were made from the NASA Infrared Telescope Facility on Mauna Kea in Hawaii. Using the Cool Grating Array Spectrometer, spectral coverage was obtained from 1.89 to 2.46 microns and from 2.8 to 4.2 microns for both the leading and trailing hemispheres. In addition, spectral coverage of the leading hemisphere was obtained from 1.30 to 2.55 microns and from 4.2 to 4.8 microns. Interpretations of the data are given.
[Show abstract][Hide abstract] ABSTRACT: The reflectance spectra of minerals are studied as a function of spectral resolution in the range from 0.2 to 3.0 microns. Selected absorption bands were studied at resolving powers as high as 2240. At resolving powers of approximately 1000, many OH-bearing minerals show diagnostic sharp absorptions at the resolution limit. At low resolution, some minerals may not be distinguishable, but as the resolution is increased, most can be easily identified. As the resolution is increased, many minerals show fine structure, particularly in the OH-stretching overtone region near 1.4 micron. The fine structure can enhance the ability to discriminate between minerals, and in some cases the fine structure can be used to determine elemental composition.
[Show abstract][Hide abstract] ABSTRACT: High-resolution reflectance spectroscopy is used to determine the spectral characteristics of serpentines and chlorites. The structure and chemistry of hydrous layered silicates are reviewed. The possibility of distinguishing between isochemical end-members of the Mg-rich serpentine group and the capability of recognizing spectral variations in chlorites as a function of Fe/Mg ratio are demonstrated. Consideration is given to the application of the findings to terrestrial, asteroid, and meteorite studies.