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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.07.015 Byline: Vishnu Reddy (a), Michael J. Gaffey (b), Michael S. Kelley (c), Andreas Nathues (d), Jian-Yang Li (e), Robert Yarbrough (c) Keywords: Asteroid Vesta; Spectroscopy; Asteroids, Composition; Asteroids; Planetary formation Abstract: High signal-to-noise, rotationally-resolved spectra of Asteroid 4 Vesta's southern hemisphere from the 2007 opposition were used to constrain its compositional and mineralogical variations. The spectra were rotationally-phased using closely timed HST observations of Vesta by Li et al. (Li, J.-Y., McFadden, L.A., Thomas, P.C., Mutchler, M.J., Parker, J.Wm., Young, E.F., Russell, C.T., Sykes, M.V., Schmidt, B.E. [2010]. Icarus 208, 238-251). The average surface of Vesta's southern hemisphere is analogous to a howardite or polymict eucrite assemblage similar to the northern hemisphere, although the band parameters are distinctly shifted towards the diogenite zone on the Band-Band plot. A few distinct compositional units were detected and they might be related to albedo features detected by Hubble Space Telescope (Li et al., 2010). We have identified two compositionally distinct regions overlaying the background surface. The first unit is a polymict eucrite and/or low-Ca eucrite compositional unit at 143[degrees] longitude that border the eucrite zone on the Band-Band plot and the second is a diogenite unit at 159[degrees]. While we did not detect any distinct olivine units as suggested by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130-157), we cannot rule out the possibility of smaller olivine-rich units that are below the detection limit of the instrumentation we used. Based on the analysis and the limitations of the data, we do not suggest that Vesta's surface is olivine-free. Mean pyroxene chemistry estimates for both hemispheres broadly agree with one another (to within one-sigma) with the northern hemisphere ferrosilite (Fs) and wollastonite (Wo) values being slightly higher than southern hemisphere. Author Affiliation: (a) Department of Space Studies, Room 520, Box 9008, University of North Dakota, Grand Forks, ND 58202, USA (b) Department of Space Studies, Room 518, Box 9008, University of North Dakota, Grand Forks, ND 58202, USA (c) Department of Geology and Geography, Box 8149, Georgia Southern University, Statesboro, GA 30460, USA (d) Max-Planck Institute for Solar System Research, 37191 Katlenburg-Lindau, Germany (e) Department of Astronomy, University of Maryland, College Park, MD 20742, USA Article History: Received 12 March 2010; Revised 10 July 2010; Accepted 14 July 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.05.006 Byline: Michael J. Gaffey Keywords: Asteroids, Composition; Asteroids, Surfaces; Spectroscopy; Meteorites; Mineralogy Abstract: Lunar-style space weathering is well understood, but cannot be extended to asteroids in general. The two best studied Asteroids (433 Eros and 243 Ida) exhibit quite different space weathering styles, and neither exhibits lunar-style space weathering. It must be concluded that at this time the diversity and mechanisms of asteroid space weathering are poorly understood. This introduces a significant unconstrained variable into the problem of analyzing asteroid spectral data. The sensitivity of asteroid surface material characterizations to space weathering effects - whatever their nature - is strongly dependent upon the choice of remote sensing methodology. The effects of space weathering on some methodologies such as curve matching are potentially devastating and at the present time essentially unmitigated. On other methodologies such as parametric analysis (e.g., analyses based on band centers and band area ratios) the effects are minimal. By choosing the appropriate methodology(ies) applied to high quality spectral data, robust characterizations of asteroid surface mineralogy can be obtained almost irrespective of space weathering. This permits sophisticated assessments of the geologic history of the asteroid parent bodies and of their relationships to the meteorites. Investigations of the diversity of space weathering processes on asteroid surfaces should be a fruitful area for future efforts. Author Affiliation: Space Studies Department, John D. Odegard School of Aerospace Sciences, Box 9008, University of North Dakota, Grand Forks, ND 58202-9008, United States Article History: Received 24 November 2009; Revised 11 May 2010; Accepted 12 May 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.10.003 Byline: Paul S. Hardersen (a), Michael J. Gaffey (a), Edward A. Cloutis (b), Paul A. Abell (c), Vishnu Reddy (a) Keywords: Asteroids; composition; Mineralogy; Spectroscopy; Thermal histories Abstract: Near-infrared ([approximately equal to]0.7 to [approximately equal to]2.5 [mu]m) spectra of S-asteroids 138 Tolosa, 306 Unitas, 346 Hermentaria, and 480 Hansa suggest the presence of variable amounts of orthopyroxene [+ or -] clinopyroxene [+ or -] olivine [+ or -] plagioclase feldspar on the surfaces of these asteroids. The spectra of these asteroids were compared to laboratory mineral mixtures of orthopyroxene, clinopyroxene, and olivine [Singer, R.B., 1981. J. Geophys. Res. 86 (B9), 7967-7982; Cloutis, E.A., 1985. Master's thesis]. The band parameters (band centers, band areas) were quantified and temperature-corrected [Moroz et al., 2000. Icarus 147, 79-93; Gaffey et al., 2002. In: Bottke Jr., W.F., Cellino, A., Paolicchi, P., Binzel, R.P. (Eds.), Asteroids III. The University of Arizona Press, Tucson, pp. 183-204]. Each S-asteroid in this paper exhibits an overall spectral shape with band parameters that are inconsistent with ordinary chondrite near-infrared spectra and their inferred mineral abundances and/or pyroxene chemistries. 138 Tolosa displays a complex spectrum with a broad [approximately equal to]1 [mu]m absorption feature that displays a double Band I minimum, a well-defined absorption at [approximately equal to]1.3 [mu]m, and a broad, but weak absorption in the [approximately equal to]2 [mu]m region. Although different interpretations exist, the Tolosa spectrum is most consistent with a [approximately equal to]60/40 mixture of Type B clinopyroxene and orthopyroxene. Spectra of 306 Unitas suggest a surface with variable amounts of low-Ca pyroxene and olivine. Unitas is located in the S-(IV) and S-(VI) subtype regions in Gaffey et al. [1993. Icarus 106, 573-602]. 346 Hermentaria exhibits a complex, broad Band I absorption feature and a weak Band II feature, which suggests a [approximately equal to]50/50 mixture of clinopyroxene and orthopyroxene. Hermentaria is classified as an S-(III). Spectra of 480 Hansa suggest a dominant low-Ca pyroxene component with lesser amounts of olivine. Based on these characterizations, these four S-asteroids should not be considered as potential ordinary chondrite parent bodies. Furthermore, these results suggest that these S-asteroids experienced at least partial melting temperatures [T[greater than or equal to][approximately equal to]950a[degrees]C: Gaffey et al., 1993. Icarus 106, 573-602; Keil, K., 2000. Planet. Space Sci. 48, 887-903] during the formation epoch in the early Solar System. Continuing spectroscopic investigations will discern the relative abundance of chondritic and thermally-evolved objects among the S-type asteroids that have survived since the formation epoch [approximately equal to]4.56 billion years ago. Author Affiliation: (a) Department of Space Studies, Box 9008, University of North Dakota, Grand Forks, ND 58202, USA (b) Department of Geography, Room 5L13, University of Winnipeg, Manitoba, Canada (c) Planetary Astronomy Group, Astromaterials Research and Exploration Science, NASA Johnson Space Center, Mail Code SR, Houston, TX 77058, USA Article History: Received 27 April 2005; Revised 31 August 2005

A detailed description of the Halley-type Comet C/2001 O[G.sub.108] (LONEOS) has been derived from visible, near-infrared, and mid-infrared observations obtained in October and November 2001. These data represent the first high-quality ground-based observations of a bare Halley-type comet nucleus and provide the best characterization of a Halley-type comet other than 1P/Halley itself. Analysis of time series photometry suggests that the nucleus has a rotation period of 57.2 [+ or -] 0.5 h with a minimum nuclear axial ratio of 1.3, a phase-darkening slope parameter G of -0.01 [+ or -] 0.10, and an estimated H = 13.05 [+ or -] 0.10. The rotation period of C/2001 O[G.sub.108] is one of the longest observed among comet nuclei. The V-R color index for this object is measured to be 0.46 [+ or -] 0.02, which is virtually identical to that of other cometary nuclei and other possible extinct comet candidates. Measurements of the comet's thermal emission constrain the projected elliptical nuclear radii to be 9.6 [+ or -] 1.0 km and 7.4 [+ or -] 1.0 km, which makes C/2001 O[G.sub.108] one of the larger cometary nuclei known. The derived geometric albedo in V-band of 0.040 [+ or -] 0.010 is typical for comet nuclei. Visible-wavelength spectrophotometry and near-infrared spectroscopy were combined to derive the nucleus's reflectance spectrum over a 0.4 to 2.5 [micro]m wavelength range. These measurements represent one of the few nuclear spectra ever observed and the only known spectrum of a Halley-type comet. The spectrum of this comet nucleus is very nearly linear and shows no discernable absorption features at a 5% detection limit. The lack of any features, especially in the 0.8 to 1.0/[micro]m range such as are seen in the spectra of carbonaceous chondrite meteorites and many low-albedo asteroids, is consistent with the presence of anhydrous rather than hydrous silicates on the surface of this comet. None of the currently recognized meteorites in the terrestrial collections have reflectance spectra that match C/2001 O[G.sub.108]. The near-infrared spectrum, the geometric albedo, and the visible spectrophotometry all indicate that C/2001 O[G.sub.108] has spectral properties analogous to the D-type, and possibly P-type asteroids. Comparison of the measured albedo and diameter of C/2001 O[G.sub.108] with those of Damocloid asteroids reveals similarities between these asteroids and this comet nucleus, a finding which supports previous dynamical arguments that Damocloid asteroids could be composed of cometary-like materials. These observations are also consistent with findings that two Jupiter-family comets may have spectral signatures indicative of D-type asteroids. C/2001 O[G.sub.108] probably represents the transition from a typical active comet to an extinct cometary nucleus, and, as a Halley-type comet, suggests that some comets originating m the Oort cloud can become extinct without disintegrating. As a near-Earth object, C/2001 O[G.sub.108] supports the suggestion that some fraction of the near-Earth asteroid population consists of extinct cometary nuclei. Keywords: Comets, composition: Infrared observations: Visible observations: Comets, origin: Near-Earth asteroids, origin

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2004.10.017 Byline: Paul S. Hardersen (a), Michael J. Gaffey (b), Paul A. Abell (c) Abstract: The first verifiable near-infrared absorption features in the [approximately equal to]0.9-[mu]m spectral region are reported for Asteroids 16 Psyche, 69 Hesperia, 110 Lydia, 125 Liberatrix, 201 Penelope, and 216 Kleopatra. These weak features ([approximately equal to]1-3%) are attributed to orthopyroxenes present on the surfaces of these asteroids. 16 Psyche and 125 Liberatrix have full rotational coverage while 69 Hesperia, 110 Lydia, 201 Penelope, and 216 Kleopatra have [approximately equal to]75% rotational coverage. Qualitative [approximately equal to]2-[mu]m absorption features are present, but are very weak ( Author Affiliation: (a) Department of Space Studies, Room 526, Box 9008, University of North Dakota, Grand Forks, ND 58202, USA (b) Department of Space Studies, Room 518, Box 9008, University of North Dakota, Grand Forks, ND 58202, USA (c) NASA Johnson Space Center, Mail Code SR, Houston, TX 77058, USA

Detailed near-infrared spectral observations of Asteroid 1459 Magnya reveal an asteroid that is primarily composed of pyroxene and plagioclase feldspar, confirming earlier suggestions that Magnya has a basaltic composition. The average Magnya spectrum for March 23, 2002 has a Band I center of 0.926 [micro]m and a Band II center of 1.938 [micro]m. Observations over 4 1/2 hours show little variation in band center positions. The feldspar-to-pyroxene ratio is ~ 0.6 on Magnya's surface. Comparing Magnya with the spectral parameters from 4 Vesta shows discordant pyroxene chemistries; Magnya's pyroxenes contain ~ 10 mol% less Fs than Vesta's pyroxenes. This suggests that Magnya originated from a parent body other than 4 Vesta and that its progenitor formed in a more chemically reduced region of the solar nebula within the asteroid belt. Keywords: Asteroids, composition; Mineralogy; Spectroscopy: Infrared observations

In this paper, we present the first correlation of derived mineral abundances of V-class Asteroid 1929 Kollaa, 4 Vesta, and the HED meteorites. We demonstrate that 1929 Kollaa has a basaltic composition consistent with an origin within the crustal layer of 4 Vesta, and show a plausible genetic connection between Kollaa and the cumulate eucrite meteorites. These data support the proposed delivery mechanism of HED meteorites to the Earth from Vesta, and provide the first mineralogical constraint derived from the observation of a small V-class, Vesta family asteroid on the crustal thickness of 4 Vesta. Keywords: Asteroids; Vesta; Surfaces, asteroids; Asteroids, composition; Spectroscopy

High-resolution multispectral images of main-belt asteroid 3 Juno were taken at visible and near-IR wavelengths with the 100-inch telescope at Mount Wilson Observatory equipped with an adaptive optics system. The images show spectral features that may represent a large relatively recent impact that deeply excavated the coarse-grained olivine-pyroxene-rich crust of the asteroid.

We utilize the 56 images of Asteroid 4 Vesta obtained through four mineralogically diagnostic filters by the Hubble Space Telescope (Zellner et al. 1997, Icarus) to construct a geologic map. Vesta's surface is found to be geologically diverse and to be dichotomous at a hemispheric scale. The eastern hemisphere (longitude definition from Thomas et al. 1997, Icarus) is dominated by units interpreted to be impact excavated plutonic material composed of magnesium-rich and calcium-poor pyroxene. The mineralogy of this region is most analogous to diogenite meteorites. Several eastern hemisphere units which have the deepest and broadest 1 [[micro]meter] absorption bands may contain a substantial olivine component. The locations of these units are consistent with a previous map based on rotationally resolved groundbased spectroscopy (Gaffey 1997, Icarus). The western hemisphere is dominated by units interpreted to consist of a single component of iron-rich and relatively calcium-rich pyroxene, analogous to surface basalts such as eucrite meteorites. We investigate the spectral properties of 20 representative surface regions and find a correlation between the 1 [[micro]meter] absorption band depth and albedo: the units with lower albedos have shallower band depths. Such a relation could arise from differences in lithologies, differences in surface particle sizes, or from a weathering effect which decreases albedo and band depth over time. Because of Vesta's geologic diversity and hemispheric dichotomy, it is apparent that any major impact events, such as those possibly related to the formation of the Vesta family (Binzel and Xu 1993, Science 260, 186-191) did not globally resurface the planet. The spectral evidence for remnants of a basaltic crust implies that the oldest units on the surface date back to the time of their emplacement, most likely [approximately]4.5 byr ago. If a weathering process exists which alters the albedo, then the lowest albedo units, such as the feature with the proposed name 'Olbers,' most likely represent remnants of Vesta's ancient basaltic crust.

HST images obtained over an interval of 71 hr at a scale of 52 km/pixel have been used to determine the spin pole, size, and shape of Vesta. Several bright and dark surface features allow control point stereogrammetric measurement of spin pole orientation. An independent solution of spin pole and shape can be made from the limb and terminator coordinates. A combined control point and limb solution for Vesta's rotation pole is at [[Alpha].sub.0] = 308 [degrees] [+ or -] 10 [degrees], [[Delta].sub.0] = 48 [degrees] [+ or -] 10 [degrees], J2000). Its shape can be fit by an ellipsoid of radii 280, 272, 227 ([+ or -]12) km. The mean density of Vesta from the mass reported by Schubart and Matson (1979) is 3.8 [+ or -] 0.6 gm/[cm.sup.3]. For this density Vesta's shape is close to that of a Maclaurin spheroid with superposed variations of [approximately] 15 km.

High precision spectrophotometry of 4 Vesta, the third largest asteroid, was used to establish the surface composition of this body and to investigate mineralogical variations across its surface. The average surface of Vesta is analogous to howardite and/or polymict eucrite assemblages, regolith-derived members of the HED meteorite suite which consist of a eucrite matrix containing differing amounts of a diogenite component. Color and/or spectral changes, which exhibit a consistent relationship to rotational phasing, are evident in 16 of 18 studies of Vesta dating back to 1929, including the present work. After elimination of possible sources of spurious spectral or color changes, it is concluded that these variations arise from hemispheric variations in surface materials, and hence provide a means of spatially resolving subhemispheric compositional units on the surface of Vesta. The background surface of Vesta is a relatively dark howardite or polymict eucrite (pyroxene-plagioclase) assemblage with several compositionally distinct bright regions clustered in one hemisphere viewed around the maximum in Vesta's lightcurve. These include what appears to be an olivine-bearing unit (suggested name 'Leslie Formation') located near Vesta's equator which probably represents an impact basin (and/or its associated ejecta) that penetrated through the basaltic crust. Other high-albedo compositional units including an apparently low-calcium eucrite region and several diogenite (pyroxenite) regions, at least one located near the southern pole, may be smaller, shallower impact basins. By analogy to the eucrite meteorites, which represent surface flows or shallow intrusions and which constitute the major component of the regolith-derived howardites and polymict eucrites, we conclude that the howardite/polymict eucrite units represent a regolith-gardened original surface of Vesta. It is probable that the low albedo of the background surface on Vesta is due to an age-related darkening effect similar to that inferred from the Galileo images of Gaspra and Ida. This mechanism is consistent with the correlation of absorption feature intensity with the lightcurve. Vesta appears to have an old eucritic surface, darkened with age and represented among the meteorites by the regolith-derived howardites and polymict eucrites, on which several impacts on one hemisphere have exposed fresher brighter diogenite and olivine-bearing material. Based on qualitative analysis of the mineralogical variations as a function of rotation, a generalized lithologic map of Vesta was produced. There is reasonable control on the longitude of lithologic features but little control on their latitudes, except where specifically noted. In producing this map, discrete circular features were used on the plausible assumption that impacts have been the most important geologic process on this surface for most of the age of the solar system. The shape of the features has thus been assumed rather than derived and is subject to future revision as data improve.

Near-infrared reflected and thermal emitted lightcurves were measured concurrently for Asteroid 15 Eunomia and were used to calculate relative projected area and relative albedo variations over its surface. The technique for calculating relative variations in projected area and albedo from simultaneously measured reflected and emitted lightcurves can be used to detect changes in the shape and surface brightness of asteroids for which such data exist. Rotational spectral reflectance data obtained in 1981 were used to correlate variations in shape and relative surface albedo with changes in surface composition. Using these data combined with this relative projected area/relative albedo technique, it is confirmed that 15 Eunomia is an elongate asteroid with a generally ovoid ('egg') shape. Its surface composition varies from metalliferous with major olivine and lesser pyroxene in subequal metal/silicate ratios (much like stony-iron meteorites) for the more 'pointed' end to a higher concentration of high-Fe pyroxene and a higher general concentration of silicate (a basaltic composition) for the 'blunt' end (Gaffey and Ostro, 1987, Lunar Planet Sci. XVIII, 310-311). Asteroid 15 Eunomia may be a cross section through a completely or partially differentiated S-type asteroid parent body. The basaltic blunt end of 15 Eunomia may show a sample of the surface of the parent body with the metalliferous end showing a portion of the core/mantle boundary. If true, this would indicate a mantle thickness on the parent body of approximately 300 km.

A linear mixing model has been used to separate the high-resolution visible/near-infrared reflectance spectrum of the Iapetus dark material from the spectra of the leading and trailing sides of Iapetus. A distinct absorption feature centered at 0.67 [[micro]meter] is apparent in the spectrum. A slight inflection covering 0.4 to 0.6 [[micro]meter] and a change in slope near 0.73 [[micro]meter] suggesting the lower wavelength edge of an absorption are also present. These absorption features are attributed to the 6[A.sub.1] [right arrow] 4[T.sub.2](G) and 6[A.sub.1] [right arrow] 4[T.sub.1](G) charge transfer transitions in minerals such as goethite and hematite that are products of the aqueous alteration of anhydrous silicates. We confirm the strong, positive spectral slope attributed to organic material.

Anhydrous silicate alterations yield iron minerals that exhibit charge transfer transitions to generate iron absorption features in the spectra of five outer-belt and main-belt low-albedo asteroids. Weak absorption characteristics at 0.6 micrometer indicates that phyllosilicates and iron alteration minerals coexist on the surface of asteroid materials and resemble terrestrial aqueous alterations. The low-albedo asteroid number with iron alteration characteristics is less than those with phyllosilicate spectral properties.

The variation in the 0.3 to 2.6 micrometer spectral reflectance properties of carbides, polymorphs and macromolecular organic-bearing materials indicates the existence of different structure and composition. The visible-near infrared spectral shapes and slopes enable the distinction of the different carbon polymorphs such as diamond, carbon black and graphite. The characteristic features observed in the spectra of interstellar molecules can be used to identify the various compounds in the meteorites.

Spectral analysis of the S-type asteroid population shows that absorption bands are less intense than those of the pure mafic silicate assemblages. This shows that an additional phase consisting of ferrous nickel metal phase is present. This is caused by the presence of fully differentiated, partially differentiated and undifferentiated objects inside the S-asteroid population.

Calcium-aluminum inclusions (CAI) reflectance spectra of asteroids is investigated to determine the influence of the constituent minerals over the CAI reflectance spectra. Spinel is the most influential constituent, while other elements such as melilite, fassaite and plagioclase felspar have less effect on the CAI spectra. Minerals such as perovskite and grossulars have no influence rover the spectra. The CAI absorption features are exhibited by the CO carbonaceous chondrite perfectly. Hence, the asteroids with CO carbonaceous chondrites can be easily detected.

An investigation of the reflectance spectra variations caused by glass, end member abundance and compositional variations was conducted to determine their effect on lunar regoliths. Results indicate that laboratory produced glass are sensitive to fusion techniques and initial composition. The variations exceed those in lunar samples. Introduction of glass to a mafic silicate bearing mixture results in a reduction of the variances.