Laboratory Reflectance Spectra Research Articles

Modeling of S-Type Asteroid Spectra Using Primitive Achondrites and Iron Meteorites

The stony iron model for the S asteroids has been tested using observed reflectance spectra of the S asteroids and modeled spectra of meteoritic components. Observed reflectance spectra of 40 S asteroids were fit with linear combinations of laboratory reflectance spectra of 6 representative primitive achondrites and an iron meteorite. Specular and diffuse reflection components were taken into account for metallic-iron reflectance. Even with this limited set of meteorite spectra, 16 of those 40 S asteroids were fit fairly well in terms of absorption-band features and spectral profiles. The number of well-fit S asteroids may be greater if the number and variety of metallic and stony domains are expanded. This initial result suggests that many of the S-asteroidal surfaces may be various kinds of regional mixtures of primitive achondrites and metallic iron. If true, the S asteroids could have been originally formed of chondrite-like source materials because mineral assemblages of primitive achondrites seem to have recrystallized from chondrites by segregation of metallic components and loss of partial melts.

Measured effects of desert varnish on the mid-infrared spectra of weathered rocks as an aid to TIMS imagery interpretation

The thermal infrared spectral properties (7-12 mu m) of natural rock surfaces from Silver Lake, CA, are discussed. Although the reflectance of weathered rocks is largely a function of the quartz content in rocks, the presence of desert varnish (clay coating) on rocks reduces the reflectance and spectral contrast with features unique to the rock spectra persisting if varnish is thin. Thick varnish has a spectrum with a reflectance peak near 9.6 mu m, due to clays, and resembles the spectra of clay-rich playa surfaces. Comparison of laboratory reflectance spectra for varnish and weathered rock samples with Thermal Infrared Multispectral Scanner (TIMS) emissivity spectra for Silver Lake suggests that TIMS signatures for felsic rocks are dominated by weathered rock and rock debris. In contrast, it is likely that varnish plays an important role in the TIMS signatures of mafic rocks. >

CCD reflectance spectra of selected asteroids: I. Presentation and data analysis considerations

Narrowband reflectance spectra of 35 asteroids have been obtained with a CCD spectrograph from 1984 to 1987 as part of a search for weak features in the reflectance spectra of asteroids that would expand our knowledge of their surface mineralogical composition. Spectra obtained of primitive asteroids were examined for evidence of aqueous alteration products. The spectrum of 1 Ceres shows weak absorption features at 0.60 and 0.67 μm not yet explained by laboratory reflectance spectra. Spectra of some main-belt and outer-belt C- and P-class asteroids show weak absorption features that have analogues among spectra of CM carbonaceous chondrites and terrestial phyllosilicates. Weak absorption features are seen in the spectra of some asteroids located in the Hilda and Cybele groups beyond the outer edge of the main asteroid belt that do not have analogues among laboratory spectra. The Trojan D-class asteroids observed show no sign of absorption features, suggesting that aqueous alteration did not operate at that heliocentric distance. Features present in the spectra of ordinary chondrites were sought in spectra of asteroids located in possible source regions for the ordinary chondrites. Spectra of asteroids located near the 3:1 Kirkwood Gap both in and outside of the proposed chaotic zone show no resemblance to laboratory spectra of ordinary chondrites.

Discrimination of active and inactive sand from remote sensing: Kelso dunes, Mojave desert, California

Landsat TM images, field data, and laboratory reflectance spectra were examined for the Kelso dunes, Mojave Desert, California to assess the use of visible and near-infrared (VNIR) remote sensing data to discriminate aeolian sand populations on the basis of spectral brightness. Results show that areas of inactive sand have a larger percentage of dark, fine-grained materials compared to those composed of active sand, ehich contain less dark fines and a higher percentage of quartz sand-size grains. Both areas are spectrally distinct in the VNIR, suggesting that VNIR spectral data can be used to discriminate active and inactive sand populations in the Mojave Desert. Analysis of laboratory spectra was complicated by the presence of magnetite in the active sands, which decreases their laboratory reflectance values to those of inactive sands. For this application, comparison of TM and laboratory spectra suggests that less than 35% vegetation cover does not influence the TM spectra.

Visible and near infrared reflectance characteristics of dry plant materials

Abstract Laboratory reflectance spectra of dry plant materials are distinctly different from green plant materials. The dry plant materials have diagnostic ligno-cellulose absorption features at 2·09 and in the 2·3μm region. The visible portion of spectrum is dominated by an absorption wing produced by intense blue and UV absorption. This absorption wing breaks down as plant decay progresses.

Photometric phase functions of common geologic minerals and applications to quantitative analysis of mineral mixture reflectance spectra

Hapke's model for bidirectional reflectance is used to calculate the mass fractional abundance of components in intimately mixed, particulate surfaces from laboratory reflectance spectra. Application of this model, simplified by the assumptions that all surfaces scatter light with the same constant phase function and the opposition surge is negligible, to binary mineral mixtures are summarized and compared with new results for ternary mixtures of olivine, enstatite, and anorthite. These experimental tests indicate that the simple model is accurate to within 7% for mixtures not containing low albedo components (<10% reflectance). However, several observed systematic deviations of the calculated mass fractions from the actual mass fractions suggest that the simple model may be improved by allowing for variable scattering between the minerals used. An empirical scattering function, based on the physically plausible scattering behavior that low albedo materials are backward scattering and high albedo, transparent materials are forward scattering, reduces the magnitude of the systematic deviations in mass fractions observed with the constant scattering model. To determine if the empirical scattering function is representative of the actual scattering behavior of particulate mineral surfaces, specific photometric parameters (single‐scattering albedo, single‐particle phase function) of five different mineral components (olivine, enstatite, anorthite, magnetite, hematite) and two mineral mixtures (90% olivine 10% magnetite, 25% olivine 75% magnetite) are derived from bidirectional reflectance spectra measured at 17 different viewing geometries between 0.5 and 1.6 μm. Mineral components were crushed and wet sieved with ethanol to a 45 to 75 μm particle size. All reflectance data are corrected for the non‐Lambertian scattering properties of halon. Analysis of the parameters of the single‐particle phase function indicate that each mineral defines a relatively unique suite of scattering parameters as a function of albedo. In particular, the silicates are forward scattering where the degree of forward scattering increases with albedo, magnetite is forward scattering but the degree of forward scattering decreases with albedo, and hematite is entirely backward scattering where the degree of backward scattering increases with albedo. It is apparent that the physical properties of the particles (roughness, transparency, shape) rather than albedo or chemical composition determine the gross scattering properties of the particulate surfaces studied here. The scattering parameters of the mixtures are similar to the parameters of the more abundant mineral in the mixture. The experimentally determined single‐scattering albedos are used to calculate the mass fractions of olivine and magnetite in these mixtures. These new fractions are much improved over the previous analyses and indicate that more detailed information regarding the geometric dependence of reflectance overcomes the systematic deficiencies of the simplified methods. In general, the simplified method is appropriate for mixtures not containing low albedo material using bidirectional reflectance measured at intermediate phase angles (20°–40°) and small emergence angles (near 0°). Abundance estimates using this approach are accurate to within 5–10% when information regarding the particle size of the surface components is available, and in a relative sense for surfaces of unknown particle size distributions. More precise abundance estimates can be derived if the scattering properties of the surface are well characterized and incorporated into the analysis.

On The spectral reflectance properties of materials exposed at the Viking landing sites

Bidirectional reflectances, calibrated to within 10% uncertainties, for blue (0.40–0.52 μm), green (0.50–0.59 μm), and red (0.60–0.74 μm) channels were determined for 31 block and soil exposures imaged by the Viking Lander Cameras. Reflectances were corrected for atmospheric attenuation by using optical depth measurements obtained by the Lander Cameras and for skylight illumination by subtracting the brightness values of adjacent shadowed areas. Reflectances for two facets on each of two large blocks were computed for a common geometry by determining facet orientations from high resolution Lander stereo images and by fitting the Hapke‐Irvine photometric function to reflectances estimated for several geometries. Results indicate that individual facets on the same block have different optical properties. The darkest, grayest block facets have properties similar to laboratory reflectance spectra of mafic rocks that are thinly coated with ferric iron‐rich palagonite. Most block surfaces have reflectances consistent with thin to optically thick covers of palagonitic material. Limited exposures of soil have spectra that are also similar to palagonitic material. The majority of the soils sampled and a few block surfaces have spectra with steeper slopes and greater spectral curvatures than the palagonite analog, consistent with greater degree of ferric iron crystallinity. The soils could be an intimate mixture of palagonitic material and other materials not seen in end member form, but, if so, these other materials would need an even greater slope and curvature than observed soils. The differences in reflectance properties for common block surfaces compared to common soils imply that local weathering of blocks has not contributed significantly to the soil exposed at the landing sites.

The Color of the Surface of Venus

Multispectral images of the basaltic surface of Venus obtained by Venera 13 were processed to remove the effects of orange-colored incident radiation resulting from interactions with the thick Venusian atmosphere. At visible wavelengths the surface of Venus appears dark and without significant color. High-temperature laboratory reflectance spectra of basaltic materials indicate that these results are consistent with mineral assemblages bearing either ferric or ferrous iron. A high reflectance in the near-infrared region observed at neighboring Venera 9 and 10 sites, however, suggests that the basaltic surface material contains ferric minerals and thus may be relatively oxidized.

The spectral reflectance of water‐mineral mixtures at low temperatures

Laboratory reflectance spectra in the 0.325–2.5μm region of bound water, water‐mineral mixtures, mineral grains on frost, and frost on minerals are presented. The materials used in this study are montmorillonite, kaolinite, beryl, Mauna Kea red cinder, and black charcoal. It is found that the wavelengths of bound water and bound OH absorptions do not shift appreciably with temperature and can be detected when large amounts of free water ice are present. The decrease in the visible reflectance seen in many planetary reflectance spectra containing strong water ice absorptions can be explained by water‐mineral mixtures, mineral grains on frost, or frost on mineral grains. Mineral grains on frost are detectable in very small quantities (fractional areal coverage ≲0.005) depending on the mineral reflectance features, while it takes a thick layer of frost (≳1 mm) to mask a mineral below 1.4 μm, again depending on the mineral reflectance. Frost on a very dark surface (albedo about 6%) is easily seen; however, a dark mineral mixed with water could completely mask the water absorptions (shortward of 2.5 μm).

Alteration mapping with airborne multispectral scanners

An airborne multispectral scanner was flown over the Gold Acres mining district, Nevada, as part of a joint NASA-Jet Propulsion Laboratory (JPL) program to determine the proper spectral bands for detecting hydrothermal alteration zones associated with mineral deposits. The Superior Oil Company acquired the computer tapes of this mission to analyze the flight data independently. A spectral ratio technique was used to enhance the information available in different spectral bands. Selection of the ratios was based on published laboratory reflectance spectra of several appropriate alteration minerals and vegetation. Several areas were chosen for field checking based on previous geologic mapping in the area. Laboratory spectrometry of field samples indicated the multispectral scanner was not calibrated on this flight and so the bands were normalized prior to determining the ratios. A color ratio composite image was created and, upon interpretation of the ratio image in conjunction with analyses of field samples, it is concluded that this technique is useful for discriminating a suite of alteration minerals which include clay minerals, iron oxides, and combinations of these. Areas of hydrothermal and supergene alteration, such as those that occur at the Gold Acres and Tenabo districts, are characterized by a combination of clays and iron oxides. Both of these districts, and altered extensions of the districts, are recognizable on the ratio image because of the enhanced discrimination of clays and iron oxides.

Spectra of SO2 frost for application to emission observations of Io

We have obtained, and describe here, laboratory reflection spectra of SO2 frost from 6 to 14 µm. For rapid deposition of the frost layer at a deposition temperature of 80 K, the frost exhibits a strong absorption band at 7.5 µm, consistent with the possible Voyager I identification of such a band due to suspended solid SO2 particles within Io's atmosphere. This compatibility does not, however, exist if the frost layer is formed at 140 K or if the layer is formed slowly. Thus it may be possible to obtain information concerning the particle formation process using more detailed laboratory SO2 frost spectra.

Near infrared reflection spectra of ammonia frost: Interpretation of the upper clouds of Saturn

Models of the uppermost cloud layer on Saturn indicate that the cloud is composed of solid ammonia, with the cloud particles being initially formed at about 150 K, and subsequently transported upward by convection to colder regions of the atmosphere. For the purpose of comparison with Saturn's near-i.r. reflection spectrum, we have obtained laboratory reflection spectra of ammonia frost corresponding to a variety of deposition rates and deposition temperatures. We find that the spectral location of the strong 3 μm absorption band of solid ammonia is highly dependent upon the manner in which the frost layer is formed. For rapid deposition on a 150 K cryosurface, which closely simulates what we believe to be the cloud formation process on Saturn, the band is centered at 3.29 μm. This band location, together with the apparent absence of gaseous ammonia overlying the solid ammonia cloud, is consistent with Saturn's observed reflection spectrum in the vicinity of 3 μm. Comparison of Saturn's reflection spectrum with that of Jupiter, in the vicinity of 1.3 and 1.6 μm, also supports the hypothesis of a Saturnian ammonia ice cloud. Differences in the two observed reflection spectra are interpreted as due to gaseous ammonia absorption bands for Jupiter being replaced by solid ammonia absorption features, at displaced wavelengths, for Saturn. An important conclusion of this study is that, for the purposes of either cloud interpretation or cloud radiation modeling, the polycrystalline phase of solid ammonia, and in turn its spectrum, is highly dependent upon the manner in which the soled is formed.

Laboratory reflection spectra for the determination of chemical composition of icy bodies

The visible (0.3 to 1.03 μm) spectral reflectances of H 2O, NH 3, H 2S, and NH 4HS frosts and mixtures of these frosts and their uv irradiated products have been measured. These spectra are compared with the spectra of the Galilean satellites of Jupiter, Saturn's rings and Saturn's satellites to place limits on their surface composition.

Infrared reflectivity of frost and the Venus clouds

Laboratory reflection spectra have been obtained for very small frost particles (less than 2 μ in diameter) in the spectral range from 0.9 to 3.4 μ. These spectra differ from the spectra of large ice particles, particularly in the near-absence of reflectivity minima near 1.5 and 2.0 μ. Clouds of these particles would exhibit a reflection spectrum in close agreement with that of the Venus clouds, despite any question of gaseous CO2 absorption in the Venus atmosphere. Ice remains the best candidate for the principal constituent of Venus clouds.