Edward A. Cloutis

Pyroxene spectroscopy revisited: Spectral‐compositional correlations and relationship to geothermometry

The authors wish to thank the various agencies which have provided the necessary support for this project including grants-in-aid of research from Sigma Xi, The Scientific Research Society (to E.A.C.), the Geological Society of America grant 3741-87 (to E.A.C.), and NASA Planetary Geology and Geophysics grant NAGW 642 (to M.J.G.).

Spectral Reflectance Properties of Hydrocarbons: Remote-Sensing Implications

The spectral reflectance properties of bituminous tar sands were examined in the wavelength range from 0.35 to 2.6 micrometers. Unique absorption features due to all the major phases, except quartz, appear in the spectra. The intensities of the absorption features correlate with the abundances of the various phases. The results of this study have applications to the remote sensing of many terrestrial and extraterrestrial targets because of potential similarities between tar sand and other hydrocarbon occurrences. For example, it is found that highly polymerized hydrocarbons are a plausible constituent of the dark material on one of Saturns satellites, Iapetus.

Delivery of dark material to Vesta via carbonaceous chondritic impacts

NASA’s Dawn spacecraft observations of Asteroid (4) Vesta reveal a surface with the highest albedo and color variation of any asteroid we have observed so far. Terrains rich in low albedo dark material (DM) have been identified using Dawn Framing Camera (FC) 0.75 lm filter images in several geologic settings: associated with impact craters (in the ejecta blanket material and/or on the crater walls and rims); as flow-like deposits or rays commonly associated with topographic highs; and as dark spots (likely secondary impacts) nearby impact craters. This DM could be a relic of ancient volcanic activity or exogenic in origin. We report that the majority of the spectra of DM are similar to carbonaceous chondrite meteorites mixed with materials indigenous to Vesta. Using high-resolution seven color images we compared DM color properties (albedo, band depth) with laboratory measurements of possible analog materials. Band depth and albedo of DM are identical to those of carbonaceous chondrite xenolith-rich howardite Mt. Pratt (PRA) 04401. Laboratory mixtures of Murchison CM2 carbonaceous chondrite and basaltic eucrite Millbillillie also show band depth and albedo affinity to DM. Modeling of carbonaceous chondrite abundance in DM (1–6 vol.%) is consistent with howardite meteorites. We find no evidence for large-scale volcanism (exposed dikes/pyroclastic falls) as the source of DM. Our modeling efforts using impact crater scaling laws and numerical models of ejecta reaccretion suggest the delivery and emplacement of this DM on Vesta during the formation of the � 400 km Veneneia basin by a low-velocity (<2 km/s) carbonaceous impactor. This discovery is important because it strengthens the long-held idea that primitive bodies are the source of carbon and probably volatiles in the early Solar System.

Sublimation in bright spots on (1) Ceres.

The dwarf planet (1) Ceres, the largest object in the main asteroid belt with a mean diameter of about 950 kilometres, is located at a mean distance from the Sun of about 2.8 astronomical units (one astronomical unit is the Earth–Sun distance). Thermal evolution models suggest that it is a differentiated body with potential geological activity. Unlike on the icy satellites of Jupiter and Saturn, where tidal forces are responsible for spewing briny water into space, no tidal forces are acting on Ceres. In the absence of such forces, most objects in the main asteroid belt are expected to be geologically inert. The recent discovery of water vapour absorption near Ceres and previous detection of bound water and OH near and on Ceres (refs 5, 6, 7) have raised interest in the possible presence of surface ice. Here we report the presence of localized bright areas on Ceres from an orbiting imager. These unusual areas are consistent with hydrated magnesium sulfates mixed with dark background material, although other compositions are possible. Of particular interest is a bright pit on the floor of crater Occator that exhibits probable sublimation of water ice, producing haze clouds inside the crater that appear and disappear with a diurnal rhythm. Slow-moving condensed-ice or dust particles may explain this haze. We conclude that Ceres must have accreted material from beyond the ‘snow line’, which is the distance from the Sun at which water molecules condense.

Near-infrared spectroscopic imaging in art conservation: investigation of drawing constituents

AbstractThe remote-sensing technique of spectroscopic imaging has been adapted to the non-destructive examination of works of art.The principleof near-infrared reflectance spectroscopic imaging is explained, and our instrumentation for art examination described. The technique allowsthe art materials to be distinguished by their composition, and under-drawings revealed. The initial results indicate that even over limitedwavelength ranges (650–1040 nm) and with relatively coarse spectral resolution (10 nm) a number of pigments can be distinguished on thebasis of variations in spectral properties such as spectral slope and the presence or absence of absorption bands. Software adapted from theremote-sensing image-processing field has been used to successfully map areas of different brown and black pigments across a drawing.Non-destructive identification of pigments can be used to address issues of attribution, age dating, and conservation.An additional advantageof this technique is that it can be performed off-site using portable instrumentation, and under relatively benign lighting conditions. Thetechnique has been applied to the examination of a 15th-century drawing,Untitled (The Holy Trinity), in the collection of the Winnipeg ArtGallery. Multivariate image analysis produced a set of principal component (PC) images highlighting different materials’ aspects of thedrawing. A color composite image produced from the PC images provided a direct visualization of the compositional characteristics of thework. Features of the under-drawing have been exposed, and its material tentatively identified as charcoal, by comparison with reference data.© 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

Near infrared spectroscopic reflectance imaging: a new tool in art conservation

The application of infrared spectroscopic imaging to non-destructive examination of works of art is described. Its advantages over infrared photography and reflectography are discussed, in particular its ability to provide spectroscopic information, which potentially allows identification of pigments, binders, and other materials. Near-infrared spectra of a selection of brown and black pigments are presented. Results are given of the application of infrared spectroscopic imaging to two works of art in different media: an ink drawing and an oil painting.

Reflectance Spectra of Mafic Silicate-Opaque Assemblages with Applications to Meteorite Spectra

Abstract The reflectance spectra of wustite and mixtures of mafic silicates plus carbon or magnetite can be used to interpret meteorite and asteroid spectra. Mafic silicate + magnetite spectra show many features characteristics of magnetite-bearing meteorites— an overall decline or constant reflectance and lown overall reflectance. Mafic silicate + amorphous carbon spectra show low overall reflectance and a red slope unlike that seen in CV and CO carbonaceous chondrite spectra, probably because the meteoritic carbon is in a more ordered form. The reflectance spectra of ureilites are largely consistent with an assemblage of mafic silicates and abundant carbon. Ordinary chondrite reflectance spectra cannot be reproduced by any of the laboratory mixture spectra. The reflectance spectrum of wustite is a reasonable match to the spectrum of ordinary chondrite metal, suggesting that most ordinary chondrite metal grains are probably coated with an optically thick layer of an oxide. Ordinary chondrite and mafic silicate reflectance spectra are consistently less red-sloped than S-class asteroid spectra. The various spectral criteria use to deco0nvolve mafic silicate spectra are also applicable to CV and CO carbonaceous chondrites, ureilites, and ordinary chondrites, because the opaque phases present in these meteorites are spectrally neutral.

Spectral-compositional variations in the constituent minerals of mafic and ultramafic assemblages and remote sensing implications

The 0.3–2.6 Μm reflectance spectra of most mafic and ultramafic assemblages can best be interpreted by considering the spectra as being composed of mafic silicate spectra modified by the presence of opaques, such as ilmenite or magnetite, and plagioclase feldspar. The systematic spectral-compositional relationships for olivine, orthopyroxene, and clinopyroxene have been examined and it has been determined that absorption band wavelength positions are correlated with ferrous iron content. Binary mafic silicate mixtures are generally less well understood, but certain spectral features such as reflectance maxima and minima wavelength positions and absorption band areas can be used to quantify or at least constrain end member abundances and compositions. The addition of opaques to a mafic silicate assemblage lowers overall reflectance and band depths. This differs from the effects of increasing grain size which are to lower overall reflectance but increase band depths. Plagioclase is relatively transparent compared to mafic silicates and must be present in appreciable amounts (tens of percent) to be spectrally detectable. The reflectance spectra of most mafic and ultramafic assemblages are dominated by mafic silicate absorption features and analysis of their spectra on this basis allows constraints to be placed on properties such as end member abundances and compositions.

Olivine or impact melt: Nature of the ``Orange'' material on Vesta from Dawn

Abstract NASA’s Dawn mission observed a great variety of colored terrains on asteroid (4) Vesta during its survey with the Framing Camera (FC). Here we present a detailed study of the orange material on Vesta, which was first observed in color ratio images obtained by the FC and presents a red spectral slope. The orange material deposits can be classified into three types: (a) diffuse ejecta deposited by recent medium-size impact craters (such as Oppia), (b) lobate patches with well-defined edges (nicknamed “pumpkin patches”), and (c) ejecta rays from fresh-looking impact craters. The location of the orange diffuse ejecta from Oppia corresponds to the olivine spot nicknamed “Leslie feature” first identified by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130–157) from ground-based spectral observations. The distribution of the orange material in the FC mosaic is concentrated on the equatorial region and almost exclusively outside the Rheasilvia basin. Our in-depth analysis of the composition of this material uses complementary observations from FC, the visible and infrared spectrometer (VIR), and the Gamma Ray and Neutron Detector (GRaND). Several possible options for the composition of the orange material are investigated including, cumulate eucrite layer exposed during impact, metal delivered by impactor, olivine–orthopyroxene mixture and impact melt. Based on our analysis, the orange material on Vesta is unlikely to be metal or olivine (originally proposed by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130–157)). Analysis of the elemental composition of Oppia ejecta blanket with GRaND suggests that its orange material has ∼25% cumulate eucrite component in a howarditic mixture, whereas two other craters with orange material in their ejecta, Octavia and Arruntia, show no sign of cumulate eucrites. Morphology and topography of the orange material in Oppia and Octavia ejecta and orange patches suggests an impact melt origin. A majority of the orange patches appear to be related to the formation of the Rheasilvia basin. Combining the interpretations from the topography, geomorphology, color and spectral parameters, and elemental abundances, the most probable analog for the orange material on Vesta is impact melt.

Visible/near-infrared spectral diversity from in situ observations of the Bagnold Dune Field sands in Gale Crater, Mars

As part of the Bagnold Dune campaign conducted by Mars Science Laboratory rover Curiosity, visible/near-infrared reflectance spectra of dune sands were acquired using Mast Camera (Mastcam) multispectral imaging (445–1013 nm) and Chemistry and Camera (ChemCam) passive point spectroscopy (400–840 nm). By comparing spectra from pristine and rover-disturbed ripple crests and troughs within the dune field, and through analysis of sieved grain size fractions, constraints on mineral segregation from grain sorting could be determined. In general, the dune areas exhibited low relative reflectance, a weak ~530 nm absorption band, an absorption band near 620 nm, and a spectral downturn after ~685 nm consistent with olivine-bearing sands. The finest grain size fractions occurred within ripple troughs and in the subsurface and typically exhibited the strongest ~530 nm bands, highest relative reflectances, and weakest red/near-infrared ratios, consistent with a combination of crystalline and amorphous ferric materials. Coarser-grained samples were the darkest and bluest and exhibited weaker ~530 nm bands, lower relative reflectances, and stronger downturns in the near-infrared, consistent with greater proportions of mafic minerals such as olivine and pyroxene. These grains were typically segregated along ripple crests and among the upper surfaces of grain flows in disturbed sands. Sieved dune sands exhibited progressive decreases in reflectance with increasing grain size, as observed in laboratory spectra of olivine size separates. The continuum of spectral features observed between the coarse- and fine-grained dune sands suggests that mafic grains, ferric materials, and air fall dust mix in variable proportions depending on aeolian activity and grain sorting.