Asteroid (354) Eleonora: Plucking an odd duck

During a survey of the S-type asteroids, Gaffey et al. (Gaffey, M.J., Bell, J.F., Brown, R.H., Burbine, T.H., Piatek, J., Reed, K.L., Chaky, D.A. [1993]. Icarus 106, 573–602) identified Asteroid (354) Eleonora as anomalous with a 1 μm absorption feature ∼2.5 times stronger than any S-asteroid of comparable size. Subsequent investigation revealed significant differences in the 1 μm absorption feature between the visible & very near-infrared CCD spectra (λ \u3c ∼1.0 μm) and other spectral data sets for this asteroid. There were also significant spectral differences among the several CCD survey spectra (SMASS-I, SMASS-II & S3OS2) of Eleonora. These differences could potentially arise from spectral variations across the asteroid surface, from observational phase angle differences, from surface temperature differences, from viewing geometry for a nonspherical body, or from the use of standard stars with deviated to different degrees from a true solar standard. In June 2011 Asteroid (354) Eleonora was observed over two nights using the NASA Infrared Telescope Facility (IRTF) at Mauna Kea Observatory in order to test these possible scenarios and to better understand the nature and history of Eleonora and its relationships to other asteroids and to the meteorites. Analysis of this data set has eliminated the following options as the cause of the differences in the 1 μm absorption feature within the CCD data sets and between the CCD data sets and the other spectral data: (1) rotational spectral variations; (2) variation in surface composition with latitude; (3) observation phase; (4) surface temperature variations with differing heliocentric distance in the asteroid’s elliptical orbit; (5) spectral effects of viewing geometry for a nonspherical body; and (6) differences in spectral standard stars. We conclude that none of the CCD spectra of (354) Eleonora are reliable, and that within the limits of their spectral coverage, analyses of the three CCD spectra would produce significantly different – and generally unreliable – indications of surface mineralogy. An effort needs to be made to determine whether “bad” CCD spectra are rare with the case of (354) Eleonora being an uncommon occurrence or whether there is a broader problem with the CCD asteroid survey data sets, and if so, how to identify the “bad” spectra. While CCD survey spectra show apparently irreconcilable differences, the near-infrared spectra of (354) Eleonora from various observers show only minor differences, primarily in the overall spectral slope, most of which can be attributed to slight differences in the standard stars used to calibrate the data. In June 2011, 226 near-infrared (∼0.76–2.5 μm) spectra of (354) Eleonora were obtained using the SpeX instrument on the NASA Infrared Telescope Facility at Mauna Kea Observatory. These spectra were consistent with the six sets of NIR spectra obtained for Eleonora by previous observers. The primary variation observed in this new data set was an approximately 10% variation in spectral slope between ∼0.8 μm and ∼1.6 μm during the rotation period of the asteroid. Mineralogically diagnostic spectral parameters extracted from this new data are most consistent with a surface assemblage of fine-grained intimately mixed olivine (∼60–70%, ∼Fo61–71) and low nickel (\u3c∼7–8% Ni) NiFe metal. The Fo estimate is consistent with previous estimates (Fo66±5) by Sanchez et al. (Sanchez, J.A., Reddy, V., Kelley, M.S., Cloutis, E.A., Bottke, W.F., Nesvorný, D., Lucas, M.P., Hardersen, P.S., Gaffey, M.J., Abell, P.A., Le Corre, L. [2014]. Icarus 228, 288–300), but not with the estimate (∼Fo90) of Sunshine et al. (Sunshine, J.M., Bus, S.J., Corrigan, C.M., McCoy, T.J., Burbine, T.H. [2007]. Meteorit. Planet. Sci. 42, 155–170). The surface assemblage appears to contain a small component (∼8–10%) of igneous pyroxene (weakly constrained at ∼Fs50Wo10). The parent lithology of the surface regolith may be similar to a pallasite assemblage, although none of the three known types of pallasites are good mineralogical matches

Exploring Exogenic Sources for the Olivine on Asteroid (4) Vesta

The detection of olivine on Vesta is interesting because it may provide critical insights into planetary differentiation early in our Solar System's history. Ground-based and Hubble Space Telescope (HST) observations of asteroid (4) Vesta have suggested the presence of olivine on the surface. These observations were reinforced by the discovery of olivine-rich HED meteorites from Vesta in recent years. However, analysis of data from NASA's Dawn spacecraft has shown that this olivine-bearing unit is actually impact melt in the ejecta of Oppia crater. The lack of widespread mantle olivine, exposed during the formation of the 19 km deep Rheasilvia basin on Vesta's South Pole, further complicated this picture. Ammannito et al., (2013a) reported the discovery of local scale olivine-rich units in the form of excavated material from the mantle using the Visible and InfraRed spectrometer (VIR) on Dawn. Here we explore alternative sources for the olivine in the northern hemisphere of Vesta by reanalyzing the data from the VIR instrument using laboratory spectral measurements of meteorites. We suggest that these olivine exposures could be explained by the delivery of olivine-rich exogenic material. Based on our spectral band parameters analysis, the lack of correlation between the location of these olivine-rich terrains and possible mantle-excavating events, and supported by observations of HED meteorites, we propose that a probable source for olivine seen in the northern hemisphere are remnants of impactors made of olivine-rich meteorites. Best match suggests these units are HED material mixed with either ordinary chondrites, or with some olivine-dominated meteorites such as R-chondrites.Comment: 62 pages, 12 figures, 4 tables; Icarus, Available online 30 January 2015, ISSN 0019-1035, http://dx.doi.org/10.1016/j.icarus.2015.01.01

Calibrations of phase abundance, composition, and particle size distribution for olivine-orthopyroxene mixtures from reflectance spectra

Spectral reflectance measurements of characterized (phase abundance, particle size) mixtures of olivine and orthopyroxene were utilized to define the correlations between spectral and albedo parameters of such assemblages and their mineralogical or textural properties. Thirty-three different spectral parameters falling into three general classes (relative or ratioed, absolute or albedo, and wavelength) were investigated for empirical sensitivity to one or more of the mixture properties. Theoretical considerations and previous experimental observations were utilized to understand their functional relationships. The ratio of areas for the 1- and 2-3µm absorption bands is shown to be a sensitive indicator of the olivine-orthopyroxene abundance and is very nearly independent of particle size and mineral composition. In conjunction with an abundance determination, the wavelength position of the 1-3tm absorption feature can be utilized to determine the molar iron contents of the olivine and orthopyroxene phases. This calibration is insensitive to particle size but will produce systematic deviations if the phases have significantly different iron contents or if more than a few percent of a clinopyroxene component is present. The spectral albedo in the 0.6- to 0.7-µm region is relatively insensitive to phase abundance and can be used to constrain particle size if phase composition has been determinedVarious portions of this work were supported at the University of Hawaii by NASA grant NSG-7312 and by a Sigma Xi Foundation grant to E.A.C. and at Rensselaer Polytechnic Institute by NASA grant NAGW-642. Planetary Geosciences Division, Hawaii Institute of Geophysics, University of Hawaii, publication4 70

Detection of Rotational Spectral Variation on the M-type asteroid (16) Psyche

The asteroid (16) Psyche is of scientific interest because it contains ~ 1% of the total mass of the asteroid belt and is thought to be the remnant metallic core of a protoplanet. Radar observations have indicated the significant presence of metal on the surface with a small percentage of silicates. Prior ground-based observations showed rotational variations in the near-infrared (NIR) spectra and radar albedo of this asteroid. However, no comprehensive study that combines multi-wavelength data has been conducted so far. Here we present rotationally resolved NIR spectra (0.7-2.5 microns) of (16) Psyche obtained with the NASA Infrared Telescope Facility. These data have been combined with shape models of the asteroid for each rotation phase. Spectral band parameters extracted from the NIR spectra show that the pyroxene band center varies from ~ 0.92 to 0.94 microns. Band center values were used to calculate the pyroxene chemistry of the asteroid, whose average value was found to be Fs30En65Wo5. Variations in the band depth were also observed, with values ranging from 1.0 to 1.5%. Using a new laboratory spectral calibration we estimated an average orthopyroxene content of 6+/-1%. The mass-deficit region of Psyche, which exhibits the highest radar albedo, also shows the highest value for spectral slope and the minimum band depth. The spectral characteristics of Psyche suggest that its parent body did not have the typical structure expected for a differentiated body or that the sequence of events that led to its current state was more complex than previously thought.Comment: 21 pages, 8 figures, 2 tables, published in The Astronomical Journa

Pyroxene spectroscopy revisited: Spectral-compositional correlations and relationship to geothermometry.

Pyroxene reflectance and transmittance spectra have been examined in a search for systematic relationships between spectral features and compositional variations and to assess the applicability of reflectance spectroscopy to pyroxene geothermometry. Orthopyroxenes containing up to ~11% Wollastonite show a positive correlation between Fe2+ content and wavelength positions of the major absorption bands. Aluminum-rich orthopyroxenes display absorption bands at lower than expected wavelengths. Spectral-compositional relationships are more complex for clinopyroxenes, showing both positive and negative correlations between band positions and major cation abundances. These relationships are further complicated by the presence of significant amounts of other transition series elements such as Ti and Cr and by the presence of exsolved phases and compositional zonations. Even small-scale exsolutions ( < 1 μm wide) seem to be sufficient to significantly alter spectral properties. Contours of the wavelength positions of band minima projected onto the pyroxene tetralateral generally exhibit concave downward shapes. The orientations of the contours relative to pyroxene geotherms are such as to effectively preclude the use of spectroscopy to significantly constrain the temperatures of formation of pyroxenes.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.).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.).https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/91JE0251

Diaspores and related hydroxides' Spectral-compositional properties and implications for Mars

Differences in spectral reflectance properties (0.3-26 μm) of a suite of metal hydroxides (gibbsite, böhmite, diaspore, goethite, and manganite) have been found to be a function of both structural and compositional differences between these minerals. The properties of the O-H stretching fundamental bands in particular can be used to identify the presence of these minerals and to discriminate isostructural and heteromorphous species. This is due to the fact that the O-H stretching fundamental bands are very intense in these minerals and occur at longer wavelengths than those of other minerals, and also due to the fact that these metal hydroxides lack strong absorption bands at shorter wavelengths. In the case of dimorphous minerals such as böhmite (ϒ-A1O(OH)) and diaspore (α-A1O(OH)), differences in the wavelength positions of O-H stretching fundamental bands can be of the order of 0.1-0.3 μm. Reflectance spectra of metal hydroxide-bearing "ores" (bauxites) and impure samples indicate that accessory phases, in particular iron-bearing minerals, have a more pronounced eftect on spectral reflectance toward shorter wavelengths. Intimate mixtures of diaspore + orthopyroxene indicate that diaspore abundances as low as 5 wt% (or less) can be detected by the appearance of characteristic absorption bands at 3.33 and 3.41μm . This finding has particular relevance to the study of near-infrared spectra of Mars, in that the conditions which favor diaspore formation on the Earth may have also been prevalent early in Mars' history. If diaspore did form on Mars in the past, it is likely to have persisted to the present.This study was supported by the University of Winnipeg through a start-up grant and research grant through the Discretionary Grants Program (to E.A.C.) and a NASA Planetary Geology and Geophysics Program grant (to J.F.B.; NAG5-4333This study was supported by the University of Winnipeg through a start-up grant and research grant through the Discretionary 'Grants Program (to E.A.C.) and a NASA Planetary Geology and Geophysics Program grant (to J.F.B.; NAG5-433

Ground-based Characterization of Hayabusa2 Mission Target Asteroid 162173 Ryugu: Constraining Mineralogical Composition in Preparation for Spacecraft Operations

Asteroids that are targets of spacecraft missions are interesting because they present us with an opportunity to validate ground-based spectral observations. One such object is near-Earth asteroid (NEA) (162173) Ryugu, which is the target of the Japanese Space Agency's (JAXA) Hayabusa2 sample return mission. We observed Ryugu using the 3-m NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii, on July 13, 2016 to constrain the object's surface composition, meteorite analogs, and link to other asteroids in the main belt and NEA populations. We also modeled its photometric properties using archival data. Using the Lommel-Seeliger model we computed the predicted flux for Ryugu at a wide range of viewing geometries as well as albedo quantities such as geometric albedo, phase integral, and spherical Bond albedo. Our computed albedo quantities are consistent with results from Ishiguro et al. (2014). Our spectral analysis has found a near-perfect match between our spectrum of Ryugu and those of NEA (85275) 1994 LY and Mars-crossing asteroid (316720) 1998 BE7, suggesting that their surface regoliths have similar composition. We compared Ryugu's spectrum with that of main belt asteroid (302) Clarissa, the largest asteroid in the Clarissa asteroid family, suggested as a possible source of Ryugu by Campins et al. (2013). We found that the spectrum of Clarissa shows significant differences with our spectrum of Ryugu, but it is similar to the spectrum obtained by Moskovitz et al. (2013). The best possible meteorite analogs for our spectrum of Ryugu are two CM2 carbonaceous chondrites, Mighei and ALH83100.Comment: 23 pages, 7 figures, 4 tables, accepted in Monthly Notices of the Royal Astronomical Society Main Journa

Statement on Spectroscopic Imaging in Art Conservation: A New Tool for Materials Investigations

http://leoalmanac.org/journal/Vol_10/lea_v10_n09.tx