Article

The origin of Vesta’s crust: Insights from spectroscopy of the Vestoids

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Abstract

High quality VNIR spectra of 15 Vestoids, small asteroids that are believed to originate from Vesta, were collected and compared to laboratory spectra and compositional data for selected HED meteorites. A combination of spectral parameters such as band centers, and factors derived from Modified Gaussian Model fits (band centers, band strengths, calculation of the low to high-Ca pyroxene ratio) were used to establish if each Vestoid appeared most like eucrite or diogenite material, or a mixture of the two (howardite). This resulted in the identification of the first asteroid with a ferroan diogenite composition, 2511 Patterson. This asteroid can be used to constrain the size of diogenite magma chambers within the crust of Vesta. The Vestoids indicate that both large-scale homogeneous units (>5km) and smaller-scale heterogeneity (

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... Because these resonances act as escape hatches from the main belt and provide trajectories into the inner solar system, the Vestoids were likely the immediate parent bodies for most howardite-eucrite-diogenite (HED) meteorites [Migliorini et al., 1997]. Indeed, the reflectance spectra of Vesta [McCord et al., 1970;Gaffey, 1997] and of Vestoids [Binzel and Xu, 1993;Vilas et al., 2000;Burbine et al., 2001;Kelley et al., 2003;Shestopalov et al., 2008;Moskovitz et al., 2010;De Sanctis et al., 2011a;Mayne et al., 2011;Reddy et al., 2011] are very similar to those of howardites, eucrites, and diogenites. Thus, there is likely to be a corresponding linkage between the HED meteorites and Rheasilvia, a connection to be explored in this paper. ...
... Miyamoto and Takeda [1994] estimated a minimum crustal thickness of~15 km, from cooling rates based on exsolution lamellae in eucrite pyroxenes. Spectral studies of Vestoids have identified eucrite chunks as large as 6-10 km [Binzel and Xu, 1993;Mayne et al., 2011]. Taking all these constraints into account, we suggest that a plausible thickness of the Vestan crust is 15-20 km. ...
... [16] Previous possible interpretations of olivine in Vesta's interior were based on a 1 mm band (henceforth called BI) that is broader and deeper than for the rest of Vesta in terrains interpreted to be Rheasilvia ejecta Gaffey, 1997;Thomas et al., 1997]. High-Ca pyroxene might make the 1 mm band appear broader, but significant amounts of high-Ca pyroxene have not been found in harzburgites, and none of the spectra of 15 Vestoids examined by Mayne et al. [2011] show 1 mm broadening. Harzburgite and dunite in the HED collection confirm the occurrence of olivine, usually in proportions varying from <30 to >90 vol.%. ...
Article
Combined analyses of the surface elemental composition and mineralogy of Vesta and Ceres provide insights into their interior evolution, crustal formation, and regolith processes. Compositional data acquired by Dawn's Visible to Infrared Mapping Spectrometer (VIR) and Gamma Ray and Neutron Detector (GRaND) are sensitive to different depths and spatial scales. To compare these data sets, high-resolution maps of absorption band strengths from VIR are degraded to the broad spatial scales sampled by GRaND using a physics-based smoothing algorithm that accounts for the shape and topography of Vesta and Ceres. On Vesta, the distributions of elemental hydrogen and hydroxyl are similar, which implies that hydrogen is primarily in the form of hydroxyl, likely as phyllosilicates delivered by the infall of carbonaceous chondrite impactors. Small differences in the spatial patterns of hydroxyl and hydrogen imply that hydrogen is layered in some locations. In Vesta's dark hemisphere, hydrogen deposits are more extensive than hydroxyl, which indicates higher concentrations of hydrated minerals at depth. In contrast, the distributions of elemental hydrogen and hydrogen-bearing species (OH and NH_4^+) on Ceres are dissimilar. High concentrations of hydrogen in the Ceres’ polar regions (approaching 30 wt.% equivalent H_2O) indicate the presence of subsurface ice as predicted by ice stability theory. The concentration of iron follows a water-dilution trend when plotted as a function of regolith hydrogen content, consistent with the presence of subsurface water ice. The VIR and GRaND data jointly constrain aspects of Ceres’ surface chemistry and evolution. GRaND iron measurements place a firm upper bound on magnetite content, which supports graphitized carbon as an alternative to magnetite as a darkening agent. Lower-bounds on the concentration of carbon in carbonates implied by VIR, together with the ratio of carbonates to organics in carbonaceous chondrite meteorite analogs suggest high concentrations of carbon within Ceres’ regolith. GRaND neutron measurements permit elevated carbon concentrations, equal to or in excess of that found in CI chondrites (greater than a few wt.%). Organic matter, detected by VIR at Ernutet crater, might be widespread and may have been converted to graphite, e.g. via UV exposure, elsewhere on the surface. Furthermore, elevated concentrations of carbonaceous material can explain the difference between iron and hydrogen concentrations measured by GRaND and the CI carbonaceous chondrites, which are representative of the materials from which Ceres accreted. The elemental measurements indicate that ice and rock fractionated during Ceres’ evolution producing a crust that differs in composition from the whole body.
... Because these resonances act as escape hatches from the main belt and provide trajectories into the inner solar system, the Vestoids were likely the immediate parent bodies for most howardite-eucrite-diogenite (HED) meteorites [Migliorini et al., 1997]. Indeed, the reflectance spectra of Vesta [McCord et al., 1970;Gaffey, 1997] and of Vestoids [Binzel and Xu, 1993;Vilas et al., 2000;Burbine et al., 2001;Kelley et al., 2003;Shestopalov et al., 2008;Moskovitz et al., 2010;De Sanctis et al., 2011a;Mayne et al., 2011;Reddy et al., 2011] are very similar to those of howardites, eucrites, and diogenites. Thus, there is likely to be a corresponding linkage between the HED meteorites and Rheasilvia, a connection to be explored in this paper. ...
... Miyamoto and Takeda [1994] estimated a minimum crustal thickness of~15 km, from cooling rates based on exsolution lamellae in eucrite pyroxenes. Spectral studies of Vestoids have identified eucrite chunks as large as 6-10 km [Binzel and Xu, 1993;Mayne et al., 2011]. Taking all these constraints into account, we suggest that a plausible thickness of the Vestan crust is 15-20 km. ...
... [16] Previous possible interpretations of olivine in Vesta's interior were based on a 1 mm band (henceforth called BI) that is broader and deeper than for the rest of Vesta in terrains interpreted to be Rheasilvia ejecta Gaffey, 1997;Thomas et al., 1997]. High-Ca pyroxene might make the 1 mm band appear broader, but significant amounts of high-Ca pyroxene have not been found in harzburgites, and none of the spectra of 15 Vestoids examined by Mayne et al. [2011] show 1 mm broadening. Harzburgite and dunite in the HED collection confirm the occurrence of olivine, usually in proportions varying from <30 to >90 vol.%. ...
Article
The estimated excavation depth of the huge Rheasilvia impact basin is nearly twice the likely thickness of the Vestan basaltic crust, so the mantle should be exposed. Spectral mapping by the Dawn spacecraft reveals orthopyroxene-rich materials, similar to diogenite meteorites, in the deepest parts of the basin and within its walls. Significant amounts of olivine are predicted for the mantles of bulk-chondritic bodies like Vesta, and its occurrence is demonstrated by some diogenites that are harzburgite and dunite. However, olivine has so far escaped detection by Dawn's instruments. Spectral detection of olivine in the presence of orthopyroxene is difficult in samples with <25% olivine, and olivine in Rheasilvia might have been diluted during impact mixing or covered by the collapse of basin walls. The distribution of diogenite inferred from its exposures in and around Rheasilvia provides a geologic context for the formation of these meteorites, but does not clearly distinguish between a magmatic cumulate versus partial melting restite origin for diogenites. The former is favored by geochemical arguments, and crystallization in either a magma ocean or multiple plutons emplaced near the crust-mantle boundary is permitted by Dawn observations.
... Because these resonances act as escape hatches from the main belt and provide trajectories into the inner solar system, the Vestoids were likely the immediate parent bodies for most howardite-eucrite-diogenite (HED) meteorites [Migliorini et al., 1997]. Indeed, the reflectance spectra of Vesta [McCord et al., 1970;Gaffey, 1997] and of Vestoids [Binzel and Xu, 1993;Vilas et al., 2000;Burbine et al., 2001;Kelley et al., 2003;Shestopalov et al., 2008;Moskovitz et al., 2010;De Sanctis et al., 2011a;Mayne et al., 2011;Reddy et al., 2011] are very similar to those of howardites, eucrites, and diogenites. Thus, there is likely to be a corresponding linkage between the HED meteorites and Rheasilvia, a connection to be explored in this paper. ...
... Miyamoto and Takeda [1994] estimated a minimum crustal thickness of~15 km, from cooling rates based on exsolution lamellae in eucrite pyroxenes. Spectral studies of Vestoids have identified eucrite chunks as large as 6-10 km [Binzel and Xu, 1993;Mayne et al., 2011]. Taking all these constraints into account, we suggest that a plausible thickness of the Vestan crust is 15-20 km. ...
... [16] Previous possible interpretations of olivine in Vesta's interior were based on a 1 mm band (henceforth called BI) that is broader and deeper than for the rest of Vesta in terrains interpreted to be Rheasilvia ejecta Gaffey, 1997;Thomas et al., 1997]. High-Ca pyroxene might make the 1 mm band appear broader, but significant amounts of high-Ca pyroxene have not been found in harzburgites, and none of the spectra of 15 Vestoids examined by Mayne et al. [2011] show 1 mm broadening. Harzburgite and dunite in the HED collection confirm the occurrence of olivine, usually in proportions varying from <30 to >90 vol.%. ...
Article
We present and describe the maps of spectral parameters such as pyroxene band centers and depths, reflectance at 1.4 μm and 2.8μm band depth in the Rheasilvia quadrangle. We found a broad anti-correlation between pyroxene band centers and depths while the reflectance is not correlated with the pyroxene spectral parameters. In addition, we found that the Rheasilvia quadrangle is free of OH absorption signatures. We also derived lithological maps with improvements in the spatial resolution with respect to previous lithological maps of the same region. We confirm that the central mound is dominated by eucritic/howarditic pyroxene while diogenitic lithology has been found mainly in a region delineated by Tarpeia, Severina and Mariamne craters. We found small scale variations in the composition of pyroxene. These variations identify lithological units that extend for tens of km, although small units of less than 1 km have also been found. We consider this fact as an indication of a high level of compositional heterogeneity within the Vestan crust.
... The presence of crustal intrusions would imply a more heterogeneous distribution of HED lithologies in the interior of Vesta and has implications for the magma ocean model. Looking at the mineralogy of Vestoids, Mayne et al. (2011) also favored the partial melting option to account for both large-scale homogeneity and small-scale heterogeneity observed on their surfaces. By modeling magmatic intrusions migrating to the surface of Vesta, Wilson and Keil (1996) suggested the possible formation of two types of intrusions: intrusions at shallow depth forming dikes no larger than 10 km vertically and $1 m in width, and dikes or sills at the base of the crust extending up to 30 km with a thickness up to 3 m. ...
... According to the same authors, olivine could be detectable by the Dawn VIR instrument using the BAR parameter for an exposure of dunitic diogenite (>90 vol.%) over a large area. It is important to note, as explained in Duffard et al. (2004) and Mayne et al. (2011), that the BAR calculation cannot be used to retrieve a precise estimation of the olivine abundance if olivine is mixed with high-calcium pyroxene (HCP), which is a component in eucrites À5.4% to 29% HCP according to Mayne et al. (2010)À and howardites. Mayne et al. (2011) discussed the mineralogy of Vestoids presented in Duffard et al. (2004) and inferred the absence of olivine by using MGM analysis and observation of the Band I center. ...
... It is important to note, as explained in Duffard et al. (2004) and Mayne et al. (2011), that the BAR calculation cannot be used to retrieve a precise estimation of the olivine abundance if olivine is mixed with high-calcium pyroxene (HCP), which is a component in eucrites À5.4% to 29% HCP according to Mayne et al. (2010)À and howardites. Mayne et al. (2011) discussed the mineralogy of Vestoids presented in Duffard et al. (2004) and inferred the absence of olivine by using MGM analysis and observation of the Band I center. This is in contradiction with the conclusions drawn by Duffard et al. (2004) using the BAR method from Gaffey et al. (2002). ...
... Hiroi et al., 1996) or starting with a priori assumptions about composition to generate a mineralogical model (e.g. Mayne et al., 2011). For example, the relative strengths of pyroxene and olivine bands in the vicinity of 1 µm led Binzel et al. (2001) to correctly predict an LL chondrite-like composition for (25143) Itokawa (Fig. 4). ...
... Lim et al., 2011). MGM analyses of V-type asteroids suggest that very roughly 25-40% of their total pyroxene is in the high-Ca phase (Duffard et al., 2006;Mayne et al., 2011), consistent with the pyroxene composition of the HED meteorites (Mittlefehldt et al., 1998) and therefore supporting the generally accepted link between these asteroids and meteorites. ...
Article
Full-text available
Methods to constrain the surface mineralogy of asteroids have seen considerable development during the last decade with advancement in laboratory spectral calibrations and validation of our interpretive methodologies by spacecraft rendezvous missions. This has enabled the accurate identification of several meteorite parent bodies in the main asteroid belt and helped constrain the mineral chemistries and abundances in ordinary chondrites and basaltic achondrites. With better quantification of spectral effects due to temperature, phase angle, and grain size, systematic discrepancies due to non-compositional factors can now be virtually eliminated for mafic silicate-bearing asteroids. Interpretation of spectrally featureless asteroids remains a challenge. This paper presents a review of all mineralogical interpretive tools currently in use and outlines procedures for their application.
... We ultimately found that a set of parameters modeling four bands and a sloped continuum yielded the best fit while minimizing free parameters. However, in the case of three Eucrite meteorites-ALHA81001, BTN00300, and MAC02522-the output band centers varied too significantly from those measured in Mayne et al. (2011), and so these objects were excluded from our sample. ...
... MGM is thus able to accurately fit spectra and determine individual band parameters even in the presence of overlapping bands, which is particularly important when analyzing pyroxene spectra featuring absorption bands centered near 1 µm and 1.2 µm (Mayne et al., 2010).Previous research using MGM to analyze basaltic asteroid and meteorite spectra has employed a physically-motivated fit in order to yield specific mineralogy(Mayne et al., 2010; Ieva et al., 2015). However, our approach was a purely mathematically-motivated one, similar to that employed byThomas & Binzel (2010) andMayne et al. (2011). We aimed to determine a single set of input parameters which would yield the best possible fit when used with all of our HED meteorite and asteroid spectra while simultaneously minimizing free parameters by using the fewest possible bands. ...
Article
Full-text available
We present near-infrared (0.78-2.45 μm) reflectance spectra for nine middle and outer main belt (a > 2.5 AU) basaltic asteroids. Three of these objects are spectrally distinct from all classifications in the Bus-DeMeo system and could represent spectral end members in the existing taxonomy or be representatives of a new spectral type. The remainder of the sample are classified as V- or R-type. All of these asteroids are dynamically detached from the Vesta collisional family, but are too small to be intact differentiated parent bodies, implying that they originated from differentiated planetesimals which have since been destroyed or ejected from the solar system. The 1- and 2-μm band centers of all objects, determined using the Modified Gaussian Model (MGM), were compared to those of 47 Vestoids and fifteen HED meteorites of known composition. The HEDs enabled us to determine formulas relating Band 1 and Band 2 centers to pyroxene ferrosilite (Fs) compositions. Using these formulas we present the most comprehensive compositional analysis to date of middle and outer belt basaltic asteroids. We also conduct a careful error analysis of the MGM-derived band centers for implementation in future analyses. The six outer belt V- and R-type asteroids show more dispersion in parameter space than the Vestoids, reflecting greater compositional diversity than Vesta and its associated bodies. The objects analyzed have Fs numbers which are, on average, between five and ten molar percent lower than those of the Vestoids; however, identification and compositional analysis of additional outer belt basaltic asteroids would help to confirm or refute this result. Given the gradient in oxidation state which existed within the solar nebula, these results tentatively suggest that these objects formed at either a different time or location than 4 Vesta.
... Besides Vesta, most studies of V-type asteroids have been associated with the first Vestoids discovered by Binzel and Xu (1993), but subsequent efforts have contributed to the spectral and mineralogical characterizations of a somewhat broader range of potential Vestoids in the visible and/or near-infrared (VNIR) spectral regions (Vilas et al., 2000;Burbine et al., 2001;Kelley et al., 2003;Cochran et al., 2004;Alvarez-Candal et al., 2006;Duffard et al., 2004Duffard et al., , 2006Roig et al., 2008;Duffard and Roig, 2009;Moskovitz et al., 2008Moskovitz et al., , 2010De Sanctis et al., 2011aMayne et al., 2011;Reddy et al., 2011a;Solontoi et al., 2012;Jasmim et al., 2013;Hicks et al., 2014). ...
... These previous efforts have largely confirmed that the original Vestoids likely originate from (4) Vesta, but some interesting discrepancies have appeared among the results of various workers. This is the case for (1929) Kollaa, which has been studied by Kelley et al. (2003), Moskovitz et al. (2010), and Mayne et al. (2011). While these works reported consistent Band I absorption centers, the Band II absorption centers are quite divergent and range from 1.914-to 1.980-lm (Kelley et al., 2003;Moskovitz et al., 2010). ...
Article
Full-text available
This work reports high quality NIR spectra, and their respective interpretations, for eight Vp type asteroids, as defined by Carvano et al. (2010), that were observed at the NASA Infrared Telescope Facility on January 14, 2013 UT. They include (3867) Shiretoko, (5235) Jean-Loup, (5560) Amytis, (6331) 1992 FZ1, (6976) Kanatsu, (17469) 1991 BT, (29796) 1999 CW77, and (30872) 1992 EM17. All eight asteroids exhibit the broad 0.9 and 1.9 micron mineral absorption features indicative of pyroxene on each asteroid's surface. Data reduction and analysis via multiple techniques produced consistent results for the derived spectral absorption band centers and average pyroxene surface chemistries for all eight asteroids (Reddy et al., 2012; Lindsay et al., 2013,2014; Gaffey et al., 2002; Burbine et al., 2009). (3867) Shiretoko is most consistent with the eucrite meteorites while the remaining seven asteroids are most consistent with the howardite meteorites. The existing evidence suggests that all eight of these Vp type asteroids are genetic Vestoids that probably originated from Vesta's surface.
... We analysed visible and NIR spectra of V-type asteroids belonging to different dynamical families (vestoids, fugitives, V-type NEOs and V-type non vestoids) to highlight possible differences among dynamical classes and compare them with HED meteorites. We made use of several spectral parameters (reflectivity gradients, band centres, band separation) to infer mineralogical properties [6,7], as well as Modified Gaussian Model to state different pyroxene composition [8]. Finally we compared band parameters from V-types with data available from the Visible and InfraRed spectrometer (VIR) on board of the Dawn mission [9], who mapped almost the whole surface of Vesta in 2011-2012. ...
Conference Paper
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The majority of V-type objects in the main belt are dynamically connected to the largest basaltic asteroid known, (4) Vesta, although others, due to their current orbital parameters, cannot be easily linked to Vesta and could be fragments of another differentiated asteroid. We present a statistical analysis of previously published visible and infrared V-type spectra, with the intent to compare dynamical families and study the possibility that some of these V-type asteroids have actually a different mineralogy from Vesta, suggesting a possible different origin.
... Additionally, all our studies sites fall outside the diogenite-rich areas making temperature correction a lesser concern. It is also interesting to note that Mayne et al. (2011) argues that temperature corrections are not needed. Therefore, given that we observed only small shifts in band centers due to temperature, we will not apply any temperature correction to the spectral data. ...
Conference Paper
NASA’s Dawn spacecraft entered orbit around asteroid (4) Vesta in July 2011 for a year-long mapping orbit, which allowed for complete imaging of the surface with the Dawn Framing Camera (FC). During the approach phase, enigmatic “orange” material was discovered on Vesta (Reddy et al. 2012). This “orange” material exhibits a red spectral slope and therefore is seen in FC ratio images (with red=0.75/0.45 microns, green=0.75/0.92 microns, and blue=0.45/0.75 microns) as orange. Here the green channel helps quantify the pyroxene band depth at 0.90 microns (greener areas have higher ratio, i.e. deeper band). The two other channels depend on the visible spectral slope (redder areas have positive visible slope; bluer areas have negative visible slope). FC images revealed diffuse “orange” ejecta around three impact craters: 34-km diameter Oppia, 30-km diameter Octavia, and 11-km Arruntia as well as numerous lobate “orange” deposits distributed around Rheasilvia’s basin rim. Additionally, some fresh-looking craters (like Cornelia or Rubria) display impact ejecta rays made of “orange” material. Even though some of the “orange” deposits are not directly linked with nearby impact craters, these observations suggest an impact related origin. The distribution of all of the lobate “orange” material is constrained to outside Rheasilvia, which suggests a link with the basin’s formation. Interestingly, as noted in Reddy et al. (2013) and Le Corre et al. (2013), the “orange” ejecta around Oppia corresponds to the olivine-rich unit that was postulated by Gaffey (1997). We will present our study of the geomorphology and composition of this material and explore meteoritical analogs (Le Corre et al. 2013). Several possible options for the composition are investigated in-depth including a cumulate eucrite layer exposed during impact, metal delivered by impactors, an olivine-orthopyroxene mixture, and impact melt. Combining all our analyses, the most probable analog for the orange material on Vesta appears to be impact melt. This work is supported by Dawn UCLA subcontract# 2090-S-MB170, NASA Dawn Participating Scientist Program Grant NNH09ZDA001N-DAVPS, and NASA Planetary Geology and Geophysics Grant NNX07AP73G.
... Additionally, all our studies sites fall outside the diogenite-rich areas making temperature correction a lesser concern. It is also interesting to note that Mayne et al. (2011) argues that temperature corrections are not needed. Therefore, given that we observed only small shifts in band centers due to temperature, we will not apply any temperature correction to the spectral data. ...
Article
The Dawn mission revealed distinct units on Vesta with red spectral slope. Oppia ejecta is identified as “Leslie unit" postulated by Gaffey (1997). Orange material composition is unlikely to be metal or olivine. Its nature is most likely impact melt.
... Fig. 5). If we take $20% as the typical maximum reflectance at 0.555 lm, and assume that the surface of Vesta is fine grained, as seems likely (Hiroi et al., 1995), this suggests that the regions we are examining are not optically equivalent to powdered HEDs measured in the laboratory, which are much brighter (Hiroi et al., 1995(Hiroi et al., , 2001Sunshine et al., 2004;Duffard et al., 2005;Burbine et al., 2007;Donaldson Hanna and Sprague, 2009;Moskovitz et al., 2010;Mayne et al., 2010Mayne et al., , 2011Beck et al., 2011;Reddy et al., 2012a). If we use the full suite of Millbillillie + Murchison intimate mixtures as a guide to the DM terrains, the brightest regions ($20% reflectance at 0.653 lm) are most similar to the 80/20 Fig. 13. ...
... The Vesta asteroid family (PDS ID 401) has historically been one of the most well-studied families, due to its clear association with one of the largest known asteroids, the high albedos and locations in the inner Main Belt making members favorable for ground-based observations, and association with the HED meteorites leading to the interpretation of (4) Vesta as a differentiated parent body (McCord et al., 1970;Zappala et al., 1990;Binzel & Xu 1993;Consolmagno & Drake, 1977;Moskovitz et al., 2010;Mayne et al., 2011). With the recent visit of the Dawn spacecraft to (4) Vesta (see the chapter by Russell et al. in this volume) ground-truth data can be compared to remote-sensing observations of family members. ...
Article
Full-text available
An asteroid family is typically formed when a larger parent body undergoes a catastrophic collisional disruption, and as such family members are expected to show physical properties that closely trace the composition and mineralogical evolution of the parent. Recently a number of new datasets have been released that probe the physical properties of a large number of asteroids, many of which are members of identified families. We review these data sets and the composite properties of asteroid families derived from this plethora of new data. We also discuss the limitations of the current data, and the open questions in the field.
... Additionally, all our studies sites fall outside the diogenite-rich areas making temperature correction a lesser concern. It is also interesting to note that Mayne et al. (2011) argues that temperature corrections are not needed. Therefore, given that we observed only small shifts in band centers due to temperature, we will not apply any temperature correction to the spectral data. ...
Article
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, 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 (1997) 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). 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.
... Fig. 3B displays the spectral parameter space BT vs. MR in which we defined several regions (polygons) that allow us to largely discriminate olivine-orthopyroxenites, harzburgites, dunites and specific high-Ca pyroxenes (HCP)s that we termed HCP/HED . The latter are those HCPs that are consistent with pyroxene compositions among eucrites in HEDs Mayne et al., 2009Mayne et al., , 2011, in which all other HCPs have been omitted . According to our lab results FC color data is capable of identifying olivine if the olivine content reaches 40 wt.% in an intimate mixture. ...
Article
In this paper we present the results of a global survey of olivine-rich lithologies on (4) Vesta. We investigated Dawn Framing Camera (FC) High Altitude Mapping Orbit (HAMO) color cubes (∼60 m/pixel resolution) by using a method described in Thangjam et al. (Thangjam, G., Nathues, A., Mengel, K., Hoffmann, M., Schäfer, M., Reddy, V., Cloutis, E.A., Christensen, U., Sierks, H., Le Corre, L., Vincent, J.-B, Russell, C.T. [2014b]. Meteorit. Planet. Sci. arXiv:1408.4687 [astro-ph.EP]). In total we identified 15 impact craters exhibiting olivine-rich (>40 wt.% ol) outcrops on their inner walls, some showing olivine-rich material also in their ejecta and floors. Olivine-rich sites are concentrated in the Bellicia, Arruntia and Pomponia region on Vesta’s northern hemisphere. From our multi-color and stratigraphic analysis, we conclude that most, if not all, of the olivine-rich material identified is of exogenic origin, i.e. remnants of A- or/and S-type projectiles. The olivine-rich lithologies in the north are possibly ejecta of the ∼90 km diameter Albana crater. We cannot draw a final conclusion on their relative stratigraphic succession, but it seems that the dark material (Nathues, A., Hoffmann, M., Cloutis, E.A., Schäfer, M., Reddy, V., Christensen, U., Sierks, H., Thangjam, G.S., Le Corre, L., Mengel, K., Vincent, J.-B., Russell, C.T., Prettyman, T., Schmedemann, N., Kneissl, T., Raymond, C., Gutierrez-Marques, P., Hall, I. Büttner, I. [2014b]. Icarus (239, 222--237)) and the olivine-rich lithologies are of a similar age. The origin of some potential olivine-rich sites in the Rheasilvia basin and at crater Portia are ambiguous, i.e. these are either of endogenic or exogenic origin. However, the small number and size of these sites led us to conclude that olivine-rich mantle material, containing more than 40 wt.% of olivine, is basically absent on the present surface of Vesta. In combination with recent impact models of Veneneia and Rheasilvia (Clenet, H., Jutzi, M., Barrat, J.-A., Gillet, Ph. [2014]. Lunar Planet Sci. 45, #1349; Jutzi, M., Asphaug, E., Gillet, P., Barrat, J.-A., Benz, W. [2013]. Nature 494, 207–210), which predict an excavation depth of up to 80 km, we are confident that the crust–mantle depth is significantly deeper than predicted by most evolution models (30 km; Mittlefehldt, D.W. [2014]. Asteroid 4 Vesta: A Fully Differentiated Dwarf Planet. NASA Technical Reports Server (20140004857.pdf)) or, alternatively, the olivine-content of the (upper) mantle is lower than our detection limit, which would lead to the conclusion that Vesta’s parent material was already depleted in olivine compared to CI meteorites.
... The spectroscopic properties of the HED clan have been the subject of a number of studies (e.g., Hiroi et al., 1995Hiroi et al., , 2001Sunshine et al., 2004;Duffard et al., 2005;Burbine et al., 2007;Donaldson Hanna and Sprague, 2009;Moskovitz et al., 2010;Mayne et al., 2010Mayne et al., , 2011Beck et al., 2011;Reddy et al., 2012b). In the present study we examine the spectral reflectance properties of intimate and areal mixtures of the Millbillillie eucrite and the Murchison CM2 carbonaceous chondrite, as well as multiple grain size powders and slabs of the CM2-bearing howardite PRA 04401, and a mineralogically diverse suite of HEDs, most of which have not been previously characterized. ...
Article
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We have studied the spectral reflectance properties of the CM2 xenolith-bearing howardite PRA 04401, intimate and areal mixtures of a eucrite (Millbillillie) and a CM2 chondrite (Murchison), and a mineralogically diverse suite of 12 HED meteorites. The main goal is to ascertain whether the presence of CM2-type material can be discriminated from grain size variations in HEDs. This project was motivated by the presence of CM2 material in a number of howardites. This study is of high relevance to Vesta because of the detection of low-albedo features on its surface by the Framing Camera (FC) aboard NASA’s Dawn spacecraft. The addition of CM2 material and increasing HED grain size both lead to decreasing overall albedo. However these two processes can be recognized by how they affect pyroxene band depths and shapes and, to a lesser extent, band widths. Pyroxene band depths and widths decrease with increasing CM2 abundance and increase with increasing HED grain size, as do various FC reflectance ratios. HED pyroxene absorption bands appear to reach saturation when grain size is in the region of 60–170 μm, with band I saturating at smaller grain sizes than band II. Band I centers are largely insensitive to CM2 abundances or HED grain size variations, while Band II center positions increase by up to 40 nm with increasing CM2 abundance, and vary non-systematically by up to 22 nm as a function of grain size. The variation with CM2 abundance is probably due to the red-sloped nature of the CM2 spectrum and the broad and shallow nature of the pyroxene band II absorption. Band area ratios show less consistent behavior, likely due to CM2-induced slope changes in the mixtures and band saturation effects in the HEDs. Because HED pyroxene is so strongly featured, CM2 abundances must be well in excess of 80 wt.% to allow for the appearance of their much weaker CM2 phyllosilicate absorption bands. CM2 material may also cause a reddening of spectral slope and a shifting of pyroxene band minima to shorter wavelengths, although CM2 chondrites can exhibit a range of spectral slopes. The presence of submicron opaques (specifically chromite) in HED pyroxenes can lead to large variations in all spectral parameters. Discriminating the spectrum-altering effects of CM2 material from HED grain size variations is possible, with the confidence in the interpretation increasing as a larger range of spectral parameters are applied to the analysis.
... The Vesta family is one of the best known asteroid families, closely related to one of the largest asteroids within the solar sys- tem, (4) Vesta. This family presents characteristic high albedos (< p NIR >= 0.52 ± 0.17), and is associated with the howarditeeucrite-diogenite (HED) meteorites (McCord et al. 1970;Binzel & Xu 1993;Moskovitz et al. 2010;Mayne et al. 2011). ...
Article
The aim of this work is to study the compositional diversity of asteroid families based on their near-infrared colors, using the data within the MOVIS catalog. As of 2017, this catalog presents data for 53 436 asteroids observed in at least two near-infrared filters (Y, J, H, or Ks). Among these asteroids, we find information for 6299 belonging to collisional families with both Y -J and J-Ks colors defined. The work presented here complements the data from SDSS and NEOWISE, and allows a detailed description of the overall composition of asteroid families. We derived a near-infrared parameter, the ML∗, that allows us to distinguish between four generic compositions: two different primitive groups (P1 and P2), a rocky population, and basaltic asteroids. We conducted statistical tests comparing the families in the MOVIS catalog with the theoretical distributions derived from our ML∗ in order to classify them according to the above-mentioned groups. We also studied the background populations in order to check how similar they are to their associated families. Finally, we used this parameter in combination with NEOWISE and SDSS to check for possible bimodalities in the data. We found 43 families with ML∗err < 0.071 and with at least 8 asteroids observed: 5 classified as P1, 10 classified as P2, 19 families associated with the rocky population, and 9 families that were not linked to any of the previous populations. In these cases, we compared our samples with different combinations of these theoretical distributions to find the one that best fits the family data. We also show, using the data from MOVIS and NEOWISE, that the Bapistina family presents a two-cluster distribution in the near-infrared albedo vs. ML∗ parameter space that might be related to a common differentiated parent body. Finally, we show that the backgrounds we defined seem to be linked to their associated families.
... Beyond this evidence, though, our knowledge of main-belt basaltic asteroids gets decidedly murkier. Recent work has been attempting to better define the basalt that resides throughout the main belt (Vilas et al. 2000;Burbine et al. 2001;Kelley et al. 2003;Cochran et al. 2004;Duffard et al. 2006;Moskovitz et al. 2008a;Nesvorny et al. 2008;Duffard & Roig 2009;Lim et al. 2011;Mayne et al. 2011;Reddy et al. 2011;Hardersen et al. 2014Hardersen et al. , 2015Leith et al. 2017). A larger abundance of V-type asteroids has been identified and classified, which includes a tantalizing collection of possible non-Vesta-related basaltic asteroids (Hasselmann et al. 2012;Mainzer et al. 2012). ...
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Investigations of the main asteroid belt and efforts to constrain that population's physical characteristics involve the daunting task of studying hundreds of thousands of small bodies. Taxonomic systems are routinely employed to study the large-scale nature of the asteroid belt because they utilize common observational parameters, but asteroid taxonomies only define broadly observable properties and are not compositionally diagnostic. This work builds upon the results of work by Hardersen et al., which has the goal of constraining the abundance and distribution of basaltic asteroids throughout the main asteroid belt. We report on the near-infrared (NIR: 0.7 to 2.5 μm) reflectance spectra, surface mineralogical characterizations, analysis of spectral band parameters, and meteorite analogs for 33 Vp asteroids. NIR reflectance spectroscopy is an effective remote sensing technique to detect most pyroxene group minerals, which are spectrally distinct with two very broad spectral absorptions at ∼0.9 and ∼1.9 μm. Combined with the results from Hardersen et al., we identify basaltic asteroids for ∼95% (39/41) of our inner-belt Vp sample, but only ∼25% (2/8) of the outer-belt Vp sample. Inner-belt basaltic asteroids are most likely associated with (4) Vesta and represent impact fragments ejected from previous collisions. Outer-belt Vp asteroids exhibit disparate spectral, mineralogical, and meteorite analog characteristics and likely originate from diverse parent bodies. The discovery of two additional likely basaltic asteroids provides additional evidence for an outer-belt basaltic asteroid population. © 2018. The American Astronomical Society. All rights reserved.
... Oszkiewicz et al. (2015) and Oszkiewicz et al. (2017) show that some V-type prograde rotators in the inner main belt have Yarkovsky drift direction that is inconsistent with origin in the Vesta family and may represent parent bodies of those anomalous HEDs. Spectroscopic surveys of this region (Burbine et al. 2001;Moskovitz et al. 2010;Mayne et al. 2011) have yet to unambiguously identify V-type asteroids that are not related to Vesta. ...
Article
Context. Asteroid (2579) Spartacus is a small V-type object located in the inner main belt. This object shows spectral characteristics unusual for typical Vestoids, which may indicate an origin deeper than average within Vesta or an origin from an altogether different parent body. Aims. Our main goal is to study the origin of Spartacus. We derive the spin of Spartacus and a convex shape model of Spartacus in order to increase the knowledge of the body’s physical properties. The rotational parameters are then used to investigate dynamical evolution of the object as well as to distinguish regions sampled by spectral observations to determine whether its surface displays heterogeneity. Methods. We collected lightcurves available from the literature (oppositions of 2009, 2012) and obtained additional photometric observations at various telescopes in 2016, 2017, and 2018. We used the lightcurve inversion method to derive a spin and convex shape model. We have collected spectral observations over two rotational periods of Spartacus and determined its spectral parameters using the modified Gaussian model (MGM). We then dynamically integrated the orbital elements of Spartacus, taking into account existing information, including its thermal properties, size and the derived spin axis orientation. Results. We find two models for (2579) Spartacus: (a) λ = 312° ± 5°, β = −57° ± 5° and (b) λ = 113° ± 5°, β = −60° ± 5° both retrograde. We find that the drift direction for Spartacus is consistent with separation from Vesta, and after a backward integration of 1 Gyr the asteroid reaches the boundary of the family. We did not observe spectral variations with rotation, thus the body most likely has a homogeneous surface. Additionally, new spectral analysis indicates that the 1.0 and 2.0 μ m band centers are within ranges that are typical for Vestoids while the area ratio of these bands is about half that of typical Vestoids. Conclusions. The asteroid (2579) Spartacus is in retrograde rotation and has a drift direction consistent with an origin from Vesta. The revised spectral band centers are within ranges typical for Vestoids, while band area ratio (BAR) is unusually low compared to that of other V-types. The dynamical model shows that the asteroid could have migrated to its current location from the edges of the Vesta family within 1 Gyr, but an origin from an earlier impact on Vesta could also be plausible.
... Although the spectral link between Vesta and HEDs was initially persuasive, Vesta was recognized to be in an orbit that made transferring its impact debris into Jovian or secular resonances, and thence into Earth-crossing orbits, difficult (Wetherill et al. 1987). This problem was surmounted by the discovery of numerous Vesta family asteroids ("Vestoids"), generally <10 km in size, having HED-like spectra (Binzel and Xu 1993;Vilas et al. 2000;Burbine et al. 2001;De Sanctis et al. 2011a;Mayne et al. 2011;Reddy et al. 2011). The orbits of the Vestoids extend between that of Vesta and the Jovian 3:1 and m6 resonances, demonstrating a migration path from Vesta to nearby escape hatches (Wisdom 1985). ...
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The Dawn mission has provided new evidence strengthening the identification of asteroid Vesta as the parent body of the howardite, eucrite, and diogenite (HED) meteorites. The evidence includes Vesta's petrologic complexity, detailed spectroscopic characteristics, unique space weathering, diagnostic geochemical abundances and neutron absorption characteristics, chronology of surface units and impact history, occurrence of exogenous carbonaceous chondritic materials in the regolith, and dimensions of the core, all of which are consistent with HED observations and constraints. Global mapping of the distributions of HED lithologies by Dawn cameras and spectrometers provides the missing geologic context for these meteorites, thereby allowing tests of petrogenetic models and increasing their scientific value.
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The Dawn spacecraft mission has provided extensive new and detailed data on Vesta that confirm and strengthen the Vesta-howardite-eucrite-diogenite (HED) meteorite link and the concept that Vesta is differentiated, as derived from earlier telescopic observations. Here, we present results derived by newly calibrated spectra of Vesta. The comparison between data from the Dawn imaging spectrometer—VIR—and the different class of HED meteorites shows that average spectrum of Vesta resembles howardite spectra. Nevertheless, the Vesta spectra at high spatial resolution reveal variations in the distribution of HED-like mineralogies on the asteroid. The data have been used to derive HED distribution on Vesta, reported in Ammannito et al. (2013), and to compute the average Vestan spectra of the different HED lithologies, reported here. The spectra indicate that, not only are all the different HED lithologies present on Vesta, but also carbonaceous chondritic material, which constitutes the most abundant inclusion type found in howardites, is widespread. However, the hydration feature used to identify carbonaceous chondrite material varies significantly on Vesta, revealing different band shapes. The characteristic of these hydration features cannot be explained solely by infalling of carbonaceous chondrite meteorites and other possible origins must be considered. The relative proportion of HEDs on Vesta's surface is computed, and results show that most of the vestan surface is compatible with eucrite-rich howardites and/or cumulate or polymict eucrites. A very small percentage of surface is covered by diogenite, and basaltic eucrite terrains are relatively few compared with the abundance of basaltic eucrites in the HED suite. The largest abundance of diogenitic material is found in the Rheasilvia region, a deep basin, where it clearly occurs below a basaltic upper crust. However, diogenite is also found elsewhere; although the depth to diogenite is consistent with one magma ocean model, its lateral extent is not well constrained.
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Many early-forming asteroids differentiated, as a result of incorporating the heat-generating isotope 26Al, and experienced various kinds of volcanic activity. The Dawn spacecraft's investigation of the best-preserved of these asteroids, 4 Vesta, warrants a reappraisal of the factors controlling asteroid volcanism. We conclude that silicate melts were removed efficiently from the mantles of all asteroids by complex networks of small and large veins and dikes. This implies that only a few percent of the interior consisted of melt at any one time. Thus although large amounts of mantle melting may ultimately have occurred, “magma oceans”, in the sense of mantles containing many tens of percent of melt, can not have formed. The total rate of silicate melt production in an asteroid of a given size can be predicted as a function of time after formation. If magma were erupted directly to the surface it must have done so through a small number of major dikes, brittle fractures that penetrated the outer thermal boundary crustal layer ∼10 km deep within which heat was conducted to the surface fast enough that temperatures stayed below the silicate solidus. By modeling the link between magma flow rate and the stresses needed to keep fractures open and allow magma to rise through them without excessive cooling, we show that continuous eruptions direct from mantle to surface were impossible on asteroids with radii less than ∼190–250 km. Instead, magma must have accumulated, in sills at the base of the thermal boundary layer or in magma reservoirs in its lower part. Magma could then erupt intermittently to the surface from these steadily replenished reservoirs. The eruption rates from the reservoirs are not linked directly to the melt production rate in the mantle and could be very large, hundreds to thousands of m3 s−1, comparable to rates in historic basaltic eruptions on Earth. Many asteroid magmas are expected to have contained at least a few hundred ppm of volatiles. On asteroids with radii less than ∼100 km, the gases and small (sub-mm) pyroclastic melt droplets ejected in explosive eruptions as volatiles expanded into the surrounding vacuum will have had velocities exceeding escape speed. Only pyroclasts of at least cm size will have had small enough speeds to be retained on the surface. Asteroids significantly larger than ∼100 km in radius will have retained all pyroclasts, and most clasts will have reached the surface after passing through optically dense fire fountains. These clasts suffered negligible cooling and coalesced into lava ponds feeding lava flows. Only if eruption rates were low and volatile contents high will enough clasts have suffered sufficient cooling that spatter or cinder deposits formed. Thicknesses of lava flows are controlled by the acceleration due to gravity, surface slope, and the effective yield strength that lava develops due to cooling. Low gravity on asteroids caused flows to be thick and, coupled with high eruption rates, induced initially turbulent flow. Cooling caused a change to laminar flow and eventually brought flows to a halt, but comparison of expected cooling rates and flow thicknesses suggests that many flows attained lengths of tens of km and stopped as a result of cessation of magma supply from the reservoir rather than cooling. If more than ∼30% melting of the mantle of an asteroid occurred and the resulting erupted volcanic products were retained on the surface, as is expected for asteroids with radii >∼100 km, the volcanic deposits will have buried the original chondritic surface layers of the asteroid to such great depths that they were melted, or at least heavily thermally metamorphosed, leaving no meteoritical evidence of their existence today. Tidal stresses caused by close encounters between asteroids and proto-planets may have briefly and temporarily increased melting and melt migration speeds in asteroid interiors but will not greatly have changed volcanic histories unless gross structural disruption took place.
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The hypothesis that Vesta is the parent body of HED meteorites is consistent with, and strengthened by, the geologic context for HEDs provided by Dawn.
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Here we examine laboratory spectra of harzburgitic and orthopyroxenitic diogenites. In the VNIR, these two meteorites are indistinguishable; suggesting that vestan Ol-rich terranes will be difficult to identify using the VIR spectrometer on Dawn.
Chapter
Remote Compositional Analysis - edited by Janice L. Bishop November 2019
Article
We present global lithological maps of the Vestan surface based on Dawn mission's Visible InfraRed (VIR) Spectrometer acquisitions with a spatial sampling of 200 m. The maps confirm the results obtained with the data set acquired by VIR with a spatial sampling of 700 m, that the reflectance spectra of Vesta's surface are dominated by pyroxene absorptions that can be interpreted within the context of the distribution of howardites, eucrites, and diogenites (HEDs). The maps also partially agree with the ground and Hubble Space Telescope observations: they confirm the background surface being an assemblage of howardite or polymict eucrite, as well as the location of a diogenitic-rich spot; however, there is no evidence of extended olivine-rich regions in the equatorial latitudes. Diogenite is revealed on the Rheasilvia basin floor, indicating that material of the lower crust/mantle was exposed. VIR also detected diogenites along the scarp of Matronalia Rupes, and the rims of Severina and a nearby, unnamed crater, and as ejecta of Antonia crater. The diogenite distribution is fully consistent with petrological constraints; although the mapped distribution does not provide unambiguous constraints, it favors the hypothesis of a magma ocean.
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The asteroid 4 Vesta was recently found to have two large impact craters near its south pole, exposing subsurface material. Modelling suggested that surface material in the northern hemisphere of Vesta came from a depth of about 20 kilometres, whereas the exposed southern material comes from a depth of 60 to 100 kilometres. Large amounts of olivine from the mantle were not seen, suggesting that the outer 100 kilometres or so is mainly igneous crust. Here we analyse the data on Vesta and conclude that the crust–mantle boundary (or Moho) is deeper than 80 kilometres.
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Cambridge Core - History of Astronomy and Cosmology - Remote Compositional Analysis - edited by Janice L. Bishop
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The young Marcia crater on Vesta displays several interesting features, including pitted and smooth terrains, exposure of relatively bright and dark material, and enrichments of hydrated material in the ejecta. Several questions arise about the origin of Marcia and of the dark material (exogenic material vs volcanic or impact melts) and the smooth and pitted terrains. Here we describe the results of the spectral and thermal analysis of the Marcia crater, with a particular effort to assess the composition of the different units, identifying the presence of OH and its correlation with dark material. Detailed studies of the Marcia crater wall, smooth and floor units reveal a compositional rich terrain with small areas enriched in diogenites with respect to the general eucritic regolith dominating the equatorial region of Vesta. The signature of OH is particularly clear in the pitted floor, dark material, smooth unit, and ejecta. The pitted terrains, beside their appearance, also show thermal anomalies, being colder with respect to the surrounding terrains. The presence of OH, concentrated in darker layers, and the pitted crater floor indicate that the area where the Marcia impact event occurred was rich in volatiles. The results show how the relatively young impact events have modified the surface of Vesta, disrupting a layer of dark material once present on Vesta's equatorial terrain and exposing fresh, bright material rich in pyroxene.
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The equatorial Marcia quadrangle region is characterized by the large, relatively young impact craters Marcia and Calpurnia and their surrounding dark ejecta field, a hill with a dark-rayed crater named Aricia Tholus, and an unusual diffuse material surrounding the impact crater Octavia. The spectral analysis indicates that while this region is relatively uniform in the pyroxene band centers, it instead shows large differences in pyroxene band depths and reflectance. A large variation of reflectance is seen in the quadrangle: bright and dark materials are present as diffuse material, and as concentrated spots and outcrops. Moreover, OH signature is pervasive in the quadrangle, with a few exceptions. The region, especially the Marcia ejecta field, is characterized by spectra showing the 2 μm band shifted at long wavelengths. This is commonly associated with eucritic material, believed to have crystallized as lava on Vesta’s surface or within relatively shallow-level dikes and plutons, thus suggesting that this region is a remnant of the old Vestan basaltic crust. However, other characteristics of the spectra do not fully fit the eucritic composition, indicating an alternative explanation for the band center distribution, including the presence of carbonaceous chondritic material mixed with the native Vestan pyroxene. The detailed mineralogical analysis of the Marcia quadrangle indicates that this quadrangle is the result of the mixture of the Vestan “endogenic” minerals with the “exogenic” carbonaceous chondrites. The stratigraphic units around Marcia clearly show the bright, uncontaminated material interlaced and mixed with the dark material that contains a strong OH signature. Only few small areas can be considered as representative of the old Vestan original material.
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In the present communication, we show that Mössbauer spectra of cumulate eucrites are characteristically different from those of non-cumulate eucrites. In cumulate eucrites, iron occupies only one site, i.e. the M2 site in pyroxene crystal lattice, while it is distributed in both M1 and M2 sites in non-cumulate eucrites. We discuss the importance of asymmetric doublet observed in the Mössbauer spectrum of cumulate body, especially in Vissannapeta cumulate eucrite (fallen in Andhra Pradesh, India), where an appreciable asymmetry was observed. The Mössbauer parameters of diogenite almost exactly replicate those of cumulate eucrites, suggesting that they originate from the same magma ocean. Howardites which are mixed eucrites and diogenites, show iron mineralogy which is different from what one would expect from a physical mixture model. The possible reasons for this difference are also discussed.
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Improving constraints on the abundance of basaltic asteroids in the main asteroid belt is necessary for better understanding the thermal and collisional environment in the early solar system, for more rigorously identifying the genetic family for (4) Vesta, for determining the effectiveness of Yarkovsky/YORP in dispersing asteroid families, and for better quantifying the population of basaltic asteroids in the outer main belt (a greater than 2.5 AU) that are likely unrelated to (4) Vesta. NIR spectral observations in this work were obtained for the Vp-type asteroids (2011) Veteraniya, (5875) Kuga, (8149) Ruff, (9147) Kourakuen, (9553) Colas, (15237) 1988 RL6, (31414) Rotaryusa, and (32940) 1995 UW4 during August and September 2014 utilizing the SpeX spectrograph at the NASA Infrared Telescope Facility (IRTF), Mauna Kea, Hawaii. Spectral band parameter (band centers, Band Area Ratios) and mineralogical analysis (pyroxene chemistry) for each average asteroid NIR reflectance spectrum suggests a howardite-eucrite-diogenite (HED) meteorite analog for each asteroid. (5875) Kuga is most closely associated with the eucrite meteorites, (31414) Rotaryusa is most closely associated with the diogenites, and the remaining other six asteroids are most closely associated with the howardite meteorites. Along with orbital locations in the inner main belt and in the vicinity of (4) Vesta, the existing evidence suggests that these eight Vp-type asteroids are also likely Vestoids.
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We present the spectra of 60 asteroids, including 47 V-types observed during the first phase of the Adler V-Type Asteroid (AVAST) Survey. SDSS photometry was used to select candidate V-type asteroids for follow up by nature of their very blue $i-z$ color. 47 of the 61 observed candidates were positively classified as V-type asteroids, while an additional six show indications of a 0.9 $\mu$m feature consistent with V-type spectra, but not sufficient for formal classification. Four asteroids were found to be S-type, all of which had $i-z$ values very near the adopted AVAST selection criteria of $i-z \le -0.2$, including one candidate observed well outside the cut (at a mean $i-z$ of -0.11). Three A-type asteroids were also identified. Six V-type asteroids were observed beyond the 3:1 mean motion resonance with Jupiter, including the identification of two new V-type asteroids (63085 and 105041) at this distance. Six V-type asteroids were observed with low ($<5\deg$) orbital inclination, outside of the normal dynamical range of classic Vestoids, and are suggestive of a non-Vesta origin for at least some of the population.
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A systematic spectral analysis has been Carried out on a large subset (39 of 144) of the S-type asteroid population. The S-asteroid class includes a number of distinct compositional subtypes [designated S(I)-S(VII)] which exhibit surface silicate assemblages ranging from pure olivine (dunites) through olivine-pyroxene mixtures to pure pyroxene or pyroxene-feldspar mixtures (basalts). S-asteroid absorption bands are weaker than expected for pure mafic silicate assemblages, indicating the presence of an additional phase, most probably FeNi metal, although the abundance of metallic or feldspar components is not well constrained, The diversity within the S-class probably arises from several sources, including the coexistence of undifferentiated, partially differentiated, and fully differentiated bodies within the general S-asteroid population and the exposure of compositionally distinct units from within metamorphosed and partially and fully differentiated parent bodies. Partial differentiation within planetesimals appears to be an important source of this diversity. The surface assemblages of these subtypes include both analogues to known meteorite classes (e.g., pallasites, mesosiderites, ureilites, lodranites, brachinites, winonaites) and materials not sampled in our present meteorite collections. No specific ordinary chondrite parent bodies have been identified within the S-class, but silicate mineralogy provides a strong test for possible ordinary chondritic affinities. This test is failed by 75% of the S-asteroids. Only the S(IV)-subtype objects have silicates consistent with ordinary chondrites (OC). This subtype provides the only viable OC parent body candidates among the large main-belt S-asteroid population, although the individual objects remain to be evaluated. The S(IV) objects are concentrated near the 3:1 Kirkwood gap at 2.5 AU, and their ejecta can be readily injected into the associated chaotic region and rapidly converted into Earth-crossing orbits which may contribute to the high abundance of OC meteorites. S-asteroid absorption band depth correlates with asteroid diameter. It is relatively constant for objects larger than 100 km and increases steeply toward smaller sizes. This suggests that some equilibrium has been attained in the optical surfaces of S-asteroids larger than 100 km but not on smaller bodies.
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Differentiation of silicate bodies was widespread in the inner solar system, producing an array of igneous meteorites. While the precursor material was certainly chondritic, direct links between chondrites and achondrites are few and tenuous. Heating by short-lived radionuclides (26Al and 60Fe) initiated melting. Formation of a basaltic crust would occur soon after the onset of melting, as would migration of the cotectic Fe, Ni-FeS component, perhaps forming a S-rich core. While some asteroids appear to be arrested at this point, many continued to heat up, with efficient segregation of metal to form large cores. Extensive melting of the silicate mantle occurred, perhaps reaching 40-75%. In the largest asteroids, the original, basaltic crust may have been resorbed into the mantle, producing a magma ocean. The crystallization of that magma ocean, whether by equilibrium or fractional crystallization, could have produced the layered structure typical of large differentiated asteroids like 4 Vesta. Spacecraft exploration of 4 Vesta promises to further extend our knowledge of asteroid differentiation.
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RELAB is a multiuser facility that provides high quality spectroscopy data (0.3 to 50 µm) without charge to scientists and students in NASA planetary exploration programs.
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To explain why Vesta has an unweathered surface, we suggest two hypotheses: "rheological" and "dynamical."
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Hubble Space Telescope images of asteroid 4 Vesta obtained during the favorable 1996 apparition show an impact crater 460 kilometers in diameter near the south pole. Color measurements within the 13-kilometer-deep crater are consistent with excavation deep into a high-calcium pyroxene-rich crust or olivine upper mantle. About 1 percent of Vesta was excavated by the crater formation event, a volume sufficient to account for the family of small Vesta-like asteroids that extends to dynamical source regions for meteorites. This crater may be the site of origin for the howardite, eucrite, and diogenite classes of basaltic achondrite meteorites.
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We model the formation and subsequent collisional evolution of the Vesta asteroid family. The outcomes of the cratering event(s) which generated the family are predicted from current cratering physics, whereas the subsequent erosion of the family members due to collisions with background asteroids is simulated according to the model of Marzari et al. (1995Icar..113..168M). Comparing the size and orbital distribution of the model Vesta families with the observed family, we estimate the number and size of the projectile(s) which have impacted Vesta. The observed morphology of the family suggests two possible scenarios: (1) The family is the outcome of a major cratering event, resulting from the impact of an asteroid =~40km in diameter on the surface of Vesta about 1Gyr ago, and followed by another more recent lower-energy impact (by a projectile =~20km in diameter), producing the family's subgrouping close to the 3:1 mean motion Jovian resonance. (2) A single impact occurred =~1 Gyr ago and formed the whole family at the same time. In this case we have to assume that the fragments were ejected isotropically over a hemispheric region of Vesta, instead of being concentrated near the surface of a 90deg aperture cone, as suggested by laboratory impact experiments with planar targets. This different ejection geometry yields a more scattered distribution of the orbital elements, resulting into a better agreement with the observed family. In both scenarios the cratering event(s) which formed the family is/are likely to have injected a significant number of km-sized and smaller fragments into the 3:1 resonance, thus generating V-type near-Earth asteroids and HED meteorites. However, it appears likely that the current influx of HED meteorites cannot be directly traced back to the family-forming event(s), but results from more recent, smaller impacts on Vesta (or other family members).
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An asteroid family is a group of asteroids with similar orbits and spectra that was produced by a collisional breakup of a large parent body. To identify asteroid families, researchers look for clusters of asteroid positions in the space of proper orbital elements. These elements, being more constant over time than osculating orbital elements, provide a dynamical criterion of whether a group of bodies has a common ancestor. More than fifty asteroid families have been identified to date. Their analysis produced several important insights into the physics of large scale collisions, dynamical processes affecting small bodies in the Solar System, and surface and interior properties of asteroids.
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This paper describes the status of the 2004 edition of the HITRAN molecular spectroscopic database. The HITRAN compilation consists of several components that serve as input for radiative transfer calculation codes: individual line parameters for the microwave through visible spectra of molecules in the gas phase; absorption cross-sections for molecules having dense spectral features, i.e., spectra in which the individual lines are unresolvable; individual line parameters and absorption cross-sections for bands in the ultra-violet; refractive indices of aerosols; tables and files of general properties associated with the database; and database management software. The line-by-line portion of the database contains spectroscopic parameters for 39 molecules including many of their isotopologues.The format of the section of the database on individual line parameters of HITRAN has undergone the most extensive enhancement in almost two decades. It now lists the Einstein A-coefficients, statistical weights of the upper and lower levels of the transitions, a better system for the representation of quantum identifications, and enhanced referencing and uncertainty codes. In addition, there is a provision for making corrections to the broadening of line transitions due to line mixing.
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We present the design, construction, and performance of SpeX, a medium-resolution 0.8-5.5 mum cryogenic spectrograph and imager, now in operation at the 3.0 m NASA Infrared Telescope Facility (IRTF) on Mauna Kea. The design uses prism cross-dispersers and gratings to provide resolving powers up to R~2000 simultaneously across 0.8-2.4, 1.9-4.2, or 2.4-5.5 mum, with a 15" long slit. A high-throughput low-resolution R~200 prism mode is also provided for faint-object and occultation spectroscopy. Single-order 60" long-slit modes with resolving powers up to R~2000 are available for extended objects. The spectrograph employs an Aladdin 3 1024×1024 InSb array and uses narrow slits and a spatial scale of 0.15" pixel-1 for optimum sensitivity on point sources. An autonomous infrared slit viewer is used for object acquisition, infrared guiding, and scientific imaging in the wavelength range 0.8-5.5 mum. The imager employs an Aladdin 2 512×512 InSb array that covers a 60''×60'' field of view at 0.12" pixel-1. SpeX was successfully commissioned on IRTF during 2000 May, June, and July. Astronomical observations are presented to illustrate performance.
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Reflectance spectra of medium and high spectral resolution of 19 minerals from the tremolite-actinolite solid series are used to characterize systematic variations in absorption band parameters as a function of composition. The systematic changes in the energy, width, intensity, and number of OH overtone bands are easily quantified using the modified Gaussian model, and the results are highly correlated with the Fe/Mg ratios of the samples. These results indicate that the modified Gaussian model can be used to quantify objectively the bulk chemistry of the minerals of the tremolite-actinolite solid solution series using high-resolution reflectance spectra of the OH overtone bands. -from Author
Article
Ansrn-c.cr Reflectance spectra of medium (0.005 pm between 0.3 and 2.7 pm) and high (0.0002 pm between 1.38 and 1.42 pm and 0.002 pm between 2.2 and2.5 pm) spectral resolution of 19 minerals from the tremolite-actinolite solid solution series are used to characteiue systematic variations in absorption band parameters as a function of composition. The medium-resolution data are used to characterize broad absorptions associated with elec- tronic processes in Fe3+ and Fe2*, and the high-resolution data are used to characterize overtones ofOH vibrational bands. Quantitative analyses ofthese data were approached using a model that treats absorptions as modified Gaussian distributions (Sunshine et al., 1990). Each spectrum was deconvolved into a set of additive model absorption bands superimposed on a reflectance continuum. For absorptions associated with electronic pro- cesses, the model bands were shown to provide estimates of the Fe and Mg mineral chemistry with correlations of greater than 0.98. The systematic changes in the energy, width, intensity, and number of OH overtone bands are easily quantified using the mod- ified Gaussian model, and the results are highly correlated with the FelMg ratios of the samples. These results indicate that the modified Gaussian model can be used to quantify objectively the bulk chemistry of the minerals of the tremolite-actinolite solid solution series using high-resolution reflectance spectra ofthe OH overtone bands.
Article
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.
Article
Remote sensing analysis of common mafic silicates on bodies like asteroid 433 Eros may be in error unless temperature effects are considered. In this paper, spectral sensitivity to temperature as a function of wavelength from 0.4 to 2.5 microns is quantified using new measurements of reflectance spectra of olivine and two ordinary chondrites. The new data were obtained at higher temperature resolution and with greater accuracy than previous measurements. We use a simple thermal model to show that the temperature difference between terrestrial ambient conditions and those prevailing on main belt asteroids, as well as the temperature variations expected on the surfaces of individual asteroids during observations by spacecraft, are large enough to cause easily detectable spectral differences. Therefore, interpretations of asteroid spectra using spectra of minerals and meteorites obtained at terrestrial ambient conditions are suspect.
Article
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.
Article
Seismic refraction data obtained at the Apollo 14, 16, and 17 landing sites permit a compressional wave velocity profile of the lunar near surface to be derived. Although the regolith is locally variable in thickness, it possesses surprisingly similar seismic characteristics. Beneath the regolith at the Apollo 14 Fra Mauro site and the Apollo 16 Descartes site is material with a seismic velocity of ∼300 m/s, believed to be brecciated material or impact-derived debris. Considerable detail is known about the velocity structure at the Apollo 17 Taurus-Littrow site. Seismic velocities of 100, 327, 495, 960, and 4700 m/s are observed. The depth to the top of the 4700-m/s material is 1385 m, compatible with gravity estimates for the thickness of mare basaltic flows, which fill the Taurus-Littrow valley. The observed magnitude of the velocity change with depth and the implied steep velocity-depth gradient of >2 km/s/km are much larger than have been observed on compaction experiments on granular materials and preclude simple cold compaction of a fine-grained rock powder to thicknesses of the order of kilometers. The large velocity change from 960 to 4700 m/s is more indicative of a compositional change than a change of physical properties alone. This high velocity is believed to be representative of the material that forms the lunar highlands.
Article
Y-75032 type, A 881839, Dho 700 are among the most ferroan diogenites, slightly more magnesian than cumulate eucrites. Minor element compositions in pyroxenes and chromite compositions indicate that Y-75032 type and the ferroan diogenites formed from different parental melts.
Article
We use basic principles of fluid mechanics to explore the intrusive and eruptive consequences of early, large-scale silicate melting within the asteroid 4 Vesta and relate our findings to the properties of the howardite-eucrite-diogenitee meteorites. Surface lava flows on Vesta would commonly have had widths ranging from a few hundred meters to a few kilometers, and lengths lying between a few kilometers and at least several tens of kilometers. Flow thicknesses would have occupied a much narrower range, lying between about 5 and 20 m. These thicknesses imply cooling rates and plagioclase crystal growth rates which are extremely similar to those measured in the eucrite meteorites. Intrusions may have formed at very shallow depths, as cooling dikes failed to erupt their contents completely to the surface, or at the base of the crust, where rising magmas would have been neutrally buoyant as the density structure of Vesta's crust evolved. Deep intrusions would have had lateral extents up to 30 km and thicknesses up to 3 m; shallow intrusions would have had vertical extents less than 10 km and widths of ̃1 m. None of these intrusions and surface flows would have been thick enough or have formed frequently enough at a given location to have caused extensive thermal alteration of earlier eruptives, lending support to the idea that burial and subsequent long-term exposure to the rising geothermal gradient was the main mechanism of thermal metamorphism.
Article
A space-weathering-related trend of 20 Vestoids [1] was studied using HED meteorites [2], laser-irradiated diogenite [3], eucrite impact melt [4], and lunar soils based on the following parameters: 1-mu m band depth = lnRM - lnRC, Visible redness = lnRM - lnR55 where RM, RC, and R55 indicate reflectance at the maximum around 0.74 mu m, the 1-mu m band center, and 0.55 mu m, respectively. The relation between these two parameters is shown in Fig. 1. Vestoids are shown with circles with a size proportional to estimated ejection velocity from Vesta [1]. There is a trend formed by HEDs, majority of Vestoids, and lunar soils. About 8 Vestoids diverge from this trend. These Vestoids do not belong to Vesta family with one exception. Furthermore, the three Vestoids with the largest ejection velocity all fall in this unusual group. The data indicate that many of the Vestoids far from Vesta's orbit are dynamically and spectrally distinct from the others. Perhaps they are more associated with the projectile. Laboratory spectra were measured at RELAB, a multiuser facility operated under NAGW-748. References: [1] Binzel R. P. and Xu S. (1993) Science 260, 186. [2] Hiroi T. et al. (1995) Icarus 115, 374. [3] Wasson J. T. et al. (1997) Lunar Planet. Sci. 28, 1505. [4] Hiroi T. (1997) Antarct. Meteorites 22. \epsfysize=5.0cm \epsfbox{7022.eps}
Article
The mineralogical composition of asteroids can be constrained using visible and near-IR (VNIR) spectroscopy. The most prominent spectral features observed over this wavelength range are due to olivine and pyroxene, the two most abundant minerals in both chondritic and achondritic meteorites. The observed ratio of these two minerals is highly dependent on the amount of heating that an asteroid has undergone. The 1-micron band minimum and the band area ratio between the 2- and 1-micron bands (BAR) reveal relative abundances of olivine and/or pyroxene on an asteroid surface (Gaffey et al. 1993, Icarus 106:573). A large sample of S-, A-, V-, and R-type asteroid spectra was collected over the visible and near-IR wavelengths during the second phase of the Small Main-belt Asteroid Spectroscopic Survey (Bus and Binzel 2002, Icarus 158:106) and using the low-resolution SpeX spectrograph (Rayner et al. 2003, PASP 115:362) at NASA's Infrared Telescope Facility (IRTF). Here we present a methodology for calculating the location of the 1-micron band minimum and BAR with appropriate 1- sigma uncertainties. This method was used to characterize approximately 200 S-type asteroids throughout the main belt. We will also present the distribution of olivine / pyroxene throughout the main belt by measuring how the S-type mineralogy varies with heliocentric distance. This will provide a better understanding of both the thermal processing across the main belt and subsequent mixing of asteroids through collisional and dynamical processes. This work was conducted through a Research Experience for Undergraduates (REU) position at the University of Hawaii's Institute for Astronomy and funded by the NSF.
Article
Abstract– A few relatively unbrecciated olivine-rich diogenites consist of an equilibrium assemblage of olivine and magnesian orthopyroxene (harzburgite). More common diogenites with smaller amounts of olivine are breccias containing two distinct orthopyroxenes—one magnesian and one ferroan. These diogenites are mixtures of a harzburgite lithology that is more magnesian, with the “normal” orthopyroxenite lithology that is ferroan and may contain small amounts of plagioclase. Both lithologies likely formed by fractional crystallization in multiple plutons emplaced within the crust of asteroid 4 Vesta. Minor element trends in orthopyroxenes indicate that these plutons exhibited a range of compositions. We propose a revised taxonomy for the HED (howardites, eucrites, and diogenites) suite where all ultramafic samples are referred to as diogenites. Within this group, the prefixes dunitic, harzburgitic, and orthopyroxenitic are used to distinguish diogenites consisting of more than or equal to 90% olivine, olivine + orthopyroxene, and more than or equal to 90% orthopyroxene, respectively. The prefix polymict is used to describe brecciated mixtures of any of these rock types. The recognition that olivine is a significant phase in some diogenites is consistent with spectral interpretations of olivine in a deeply excavated crater on Vesta, and has important implications for the bulk composition and petrogenesis of that body.
Article
Abstract— Available evidence strongly suggests that the HED (howardite, eucrite, diogenite) meteorites are samples of asteroid 4 Vesta. Abundances of the moderately siderophile elements (Ni, Co, Mo, W and P) in the HED mantle indicate that the parent body may have been completely molten during its early history. During cooling of a chondritic composition magma ocean, equilibrium crystallization is fostered by the suspension of crystals in a convecting magma ocean until the crystal fraction reaches a critical value near 0.80, when the convective system freezes and melts segregate from crystals by gravitational forces. The extruded liquids are similar in composition to Main Group and Stannern trend eucrites, and the last pyroxenes to precipitate out of this ocean (before convective lockup) span the compositional range of the diogenites. Subsequent fractional crystallization of a Main Group eucrite liquid, which has been isolated as a body of magma, produces the Nuevo Laredo trend and the cumulate eucrites. The predicted cumulate mineral compositions are in close agreement with phase compositions analyzed in the cumulate eucrites. Thus, eucrites and diogenites are shown to have formed as part of a simple and continuous crystallization sequence starting with a magma ocean environment on an asteroidal size parent body that is consistent with Vesta.
Article
Abstract— We have calculated pyroxene mineralogies of seven near-Earth asteroids (NEAs) with reflectance spectra similar to HEDs (howardites, eucrites, and diogenites). Two different sets of formulas (Gaffey et al. 2002; Burbine et al. 2007) are used to calculate the pyroxene mineralogies of the NEAs from their Band I and II centers. The band centers have been adjusted to compensate for the low temperatures on the asteroid surfaces. All of the derived mineralogies from the Gaffey et al. (2002) formulas and the Burbine et al. (2007) formulas overlap. The derived wollastonite (Wo) contents are very similar with differences being only approximately 1 mol%. The derived ferrosilite (Fs) contents differ by only 3 to 8 mol%. The determined pyroxene mineralogies for all seven near-Earth vestoids are consistent with eucrites or howardites. None of the objects have pyroxene mineralogies consistent with diogenites. The absence of near-Earth vestoids with pyroxene mineralogies similar to diogenites may indicate that it is difficult to produce sizeable (km-sized or larger) bodies that are predominantly composed of diogenitic material, suggesting these objects are rubble piles of mixed ejecta.
Article
Abstract— Spectra of asteroid 4 Vesta and 21 small (estimated diameters less than 10 km) asteroids with Vesta-like spectral properties (Vestoids) were measured at visible and near-infrared wavelengths (∼0.44 to ∼1.65 μm). All of the measured small asteroids (except for 2579 Spartacus) have reflectance spectra consistent with surface compositions similar to eucrites and howardites and consistent with all being derived from Vesta. None of the observed asteroids have spectra similar to diogenites. We find no spectral distinction between the 15 objects tabulated as members of the Vesta dynamical family and 6 of the 7 sampled “non-family” members that reside just outside the semi-major axis (a), eccentricity (e), and inclination (i) region of the family. The spectral consistency and close orbital (a-e-i) match of these “non-family” objects to Vesta and the Vesta family imply that the true bounds of the family extend beyond the subjective cut-off for membership. Asteroid 2579 Spartacus has a spectrum consistent with a mixture of eucritic material and olivine. Spartacus could contain olivine-rich material from Vesta's mantle or may be unrelated to Vesta altogether. Laboratory measurements of the spectra of eucrites show that samples having nearly identical compositions can display a wide range of spectral slopes. Finer particle sizes lead to an increase in the slope, which is usually referred to as reddening. This range of spectral variation for the best-known meteoritic analogs to the Vestoids, regardless of whether they are actually related to each other, suggests that the extremely red spectral slopes for some Vestoids can be explained by very fine-grained eucritic material on their surfaces.
Article
The 1.2 µm band in near-infrared spectra of pyroxenes results from Fe 2+ in the M1 crystallographic site. The distribution of Fe and Mg between the M1 and M2 sites is in part a function of the cooling rate and thermal history of a pyroxene. Combining near-infrared and Mössbauer spectra for a series of compositionally controlled synthetic Mg, Fe, Ca pyroxenes, we quantify the strength of the 1.2 µm band as a function of Fe 2+ in the M1 site. Near-infrared spectra are deconvolved into component absorptions that can be assigned to the M1 and M2 sites using the modified Gaussian model. The relative strength of the 1.2 µm band is shown to be directly related to the amount of Fe 2+ in the M1 site measured by Mössbauer spectroscopy. The strength of the 1.2 µm band relative to the combined strengths of the 1.2 and 2 µm bands, or the M1 intensity ratio, is calculated for 51 howardite, eucrite, and diogenite (HED) meteorites. Diogenites and cumulate eucrites exhibit the lowest M1 intensity ratios, consistent with their formation as slowly cooled cumulates. Basaltic eucrites exhibit a large range of M1 intensity ratios, all of which are consistently higher than the diogenites and cumulate eucrites. This example illustrates how the M1 intensity ratio can be a used as a tool for characterizing the cooling history of remotely detected pyroxene-dominated rocks.
Article
Visible and near-infrared spectra of reflected sunlight from asteroid surfaces exhibit features that hold the promise for identifying surface mineralogy. However, the very surfaces that are observed by remote-sensing are also subject to im-pingement by micrometeoroids and solar wind particles, which are believed to play the dominant role in space weathering, which is the time-dependent modification of an as-teroid's reflectance spectrum. Such space weathering has confused the interpretations of telescopic spectra of asteroids, especially concerning the possible association of common ordinary chondritic meteorites with so-called S-type asteroids. Recent space-craft studies of asteroids (especially of Eros by NEAR-Shoemaker) have documented aspects of space weathering processes, but we still do not understand the physics of space weathering well enough to confidently assay mineralogy of diverse asteroids by remote-sensing. A review of the intellectual history of this topic reveals the complexity of interdisciplinary research on far-away astronomical bodies.
Article
Abstract— If Vesta is the parent body of the howardite, eucrite, and diogenite (HED) meteorites, then geo-chemical and petrologic constraints for the meteorites may be used in conjunction with astronomical constraints for the size and mass of Vesta to (1) determine the size of a possible metal core in Vesta and (2) model the igneous differentiation and internal structure of Vesta. The density of Vesta and petrologic models for HED meteorites together suggest that the amount of metal in the parent body is <25 mass%, with a best estimate of ∼5%, assuming no porosity. For a porosity of up to 5% in the silicate fraction of the asteroid, the permissible metal content is <30%. These results suggest that any metal core in the HED parent body and Vesta is not unusually large. A variety of geochemical and other data for HED meteorites are consistent with the idea that they originated in a magma ocean. It appears that diogenites formed by crystal accumulation in a magma ocean cumulate pile and that most noncumulate eucrites (excepting such eucrites as Bouvante and Statinem) formed by subsequent crystallization of the residual melts. Modelling results suggest that the HED parent body is enriched in rare earth elements by a factor of ∼2.5–3.5 relative to CI-chondrites and that it has approximately chondritic Mg/Si and Al/Sc ratios. Stokes settling calculations for a Vesta-wide, nonturbulent magma ocean suggest that early-crystallizing magnesian olivine, orthopyroxene, and pigeonite would have settled relatively quickly, permitting fractional crystallization to occur, but that later-crystallizing phases would have settled (or floated) an order of magnitude more slowly, allowing, instead, a closer approach to equilibrium crystallization for the more evolved (eucritic) melts. This would have inhibited the formation of a plagioclase-flotation crust on Vesta. Plausible models for the interior of Vesta, which are consistent with the data for HED meteorites and Vesta, include a metal core (<130 km radius), an olivine-rich mantle (∼65–220 km thick), a lower crustal unit (∼12–43 km thick) composed of pyroxenite, from which diogenites were derived, and an upper crustal unit (∼23–42 km thick), from which eucrites originated. The present shape of Vesta (with ∼60 km difference in the maximum and minimum radius) suggests that all of the crustal materials, and possibly some of the underlying olivine from the mantle, could have been locally excavated or exposed by impact cratering.
Article
Abstract— Zagami and Nakhla are achondrites and belong to the Shergotty-Nakhla-Chassigny (SNC) meteorite group. It is generally accepted that Mars is their parent body. Mineralogical and chemical analyses have revealed that the major mineral phases of these two meteorites are pyroxene, olivine, maskelynite, and plagioclase. In this work, near-infrared biconical reflectance measurements were performed on sawed surfaces of chips from Zagami and Nakhla. Spectra obtained with an analytical spot diameter on the order of the mineral grain size reflect the heterogeneous distribution of different mineral phases. The characteristic absorption bands of the pyroxenes are numerically evaluated in terms of the modified Gaussian model. Spectra with overlapping absorption features are resolved into the basic absorption bands. From these results, it can be estimated what kind of clinopyroxenes belong to the investigated mineral assemblages. As a result, the major clinopyroxene phase in Nakhla is Ca-rich augite, whereas in Zagami both Ca-rich and Ca-poor pyroxenes are present. By means of such a procedure, laboratory spectra of minerals become more informative and may help in discussing Martian remote sensing data in the near-infrared region.
Article
Abstract— Understanding the fundamental crystal chemical controls on visible and near-infrared reflectance spectra of pyroxenes is critical to quantitatively assessing the mineral chemistry of pyroxenes viewed by remote sensing. This study focuses on the analysis of spectroscopic measurements of a comprehensive set of synthetic Mg-Fe pyroxenes from the visible through the near-infrared (0.3–2.6 μm) to address the constraints of crystal structure and Fe2+ content on spin-forbidden and spin-allowed crystal field absorptions in Ca-free orthopyroxenes. The chemistry and oxidation state of the synthetic pyroxenes are characterized. Coordinated Mössbauer spectroscopy is used to determine site occupancy of Fe2+ in the M1 and M2 crystallographic sites. Properties of visible and near-infrared absorption bands of the synthetic pyroxenes are quantified using the modified Gaussian model. The 1 and 2 μm spin-allowed crystal field absorption bands move regularly with increasing iron content, defining a much tighter trend than observed previously. A spin-allowed crystal field absorption band at 1.2 μm is explicitly verified, even at low total iron contents, indicating that some portion of Fe2+ resides in the M1 site. The 1.2 μm band intensifies and shifts to longer wavelengths with increasing iron content. At visible wavelengths, spin-forbidden crystal field absorptions are observed in all iron-bearing samples. The most prominent absorption near 506 nm, attributed to iron in the M2 site, shifts to slightly longer wavelengths with iron content. The purity and extent of this pyroxene series allows visible wavelength absorption bands to be directly assigned to specific transitions of Fe2+ in the M1 and M2 sites.
Article
Abstract— Knowledge of regolith depth structure is important for a variety of studies of the Moon and other bodies such as Mercury and asteroids. Lunar regolith depths have been estimated using morphological techniques (i.e., Quaide and Oberbeck 1968; Shoemaker and Morris 1969), crater counting techniques (Shoemaker et al. 1969), and seismic studies (i.e., Watkins and Kovach 1973; Cooper et al. 1974). These diverse methods provide good first order estimates of regolith depths across large distances (tens to hundreds of kilometers), but may not clearly elucidate the variability of regolith depth locally (100 m to km scale). In order to better constrain the regional average depth and local variability of the regolith, we investigate several techniques. First, we find that the apparent equilibrium diameter of a crater population increases with an increasing solar incidence angle, and this affects the inferred regolith depth by increasing the range of predicted depths (from ∼7–15 m depth at 100 m equilibrium diameter to ∼8–40 m at 300 m equilibrium diameter). Second, we examine the frequency and distribution of blocky craters in selected lunar mare areas and find a range of regolith depths (8–31 m) that compares favorably with results from the equilibrium diameter method (8–33 m) for areas of similar age (∼2.5 billion years). Finally, we examine the utility of using Clementine optical maturity parameter images (Lucey et al. 2000) to determine regolith depth. The resolution of Clementine images (100 m/pixel) prohibits determination of absolute depths, but this method has the potential to give relative depths, and if higher resolution spectral data were available could yield absolute depths.
Article
Abstract– We investigate the relationship between the petrology and visible–near infrared spectra of the unbrecciated eucrites and synthetic pyroxene–plagioclase mixtures to determine how spectra obtained by the Dawn mission could distinguish between several models that have been suggested for the petrogenesis of Vesta’s crust (e.g., partial melting and magma ocean). Here, we study the spectra of petrologically characterized unbrecciated eucrites to establish spectral observables, which can be used to yield mineral abundances and compositions consistent with petrologic observations. No information about plagioclase could be extracted from the eucrite spectra. In contrast, pyroxene dominates the spectra of the eucrites and absorption band modeling provides a good estimate of the relative proportions of low- and high-Ca pyroxene present. Cr is a compatible element in eucrite pyroxene and is enriched in samples from primitive melts. An absorption at 0.6 μm resulting from Cr3+ in the pyroxene structure can be used to distinguish these primitive eucrites. The spectral differences present among the eucrites may allow Dawn to distinguish between the two main competing models proposed for the petrogenesis of Vesta (magma ocean and partial melting). These models predict different crustal structures and scales of heterogeneity, which can be observed spectrally. The formation of eucrite Allan Hills (ALH) A81001, which is primitive (Cr-rich) and relatively unmetamorphosed, is hard to explain in the magma ocean model. It could only have been formed as a quench crust. If the magma ocean model is correct, then ALHA81001-like material should be abundant on the surface of Vesta and the Vestoids.
Article
Abstract— Our analyses of high quality spectra of several S-type asteroids (17 Thetis, 847 Agnia, 808 Merxia, and members of the Agnia and Merxia families) reveal that they include both low- and high-calcium pyroxene with minor amounts of olivine (<20%). In addition, we find that these asteroids have ratios of high-calcium pyroxene to total pyroxene of >∼0.4. High-calcium pyroxene is a spectrally detectable and petrologically important indicator of igneous history and may prove critical in future studies aimed at understanding the history of asteroidal bodies. The silicate mineralogy inferred for Thetis and the Merxia and Agnia family members requires that these asteroids experienced igneous differentiation, producing broadly basaltic surface lithologies. Together with 4 Vesta (and its smaller “Vestoid” family members) and the main-belt asteroid 1489 Magnya, these new asteroids provide strong evidence for igneous differentiation of at least five asteroid parent bodies. Based on this analysis of a small subset of the near-infrared asteroid spectra taken to date with SpeX at the NASA IRTF, we expect that the number of known differentiated asteroids will increase, consistent with the large number of parent bodies inferred from studies of iron meteorites.
Article
Spectral reflectance measurements of powdered olivine–orthopyroxene mixtures and two ordinary chondrites have been performed in the temperature range between 293 and 80 K. The decrease in temperature produced a number of significant spectral effects. The composite absorption feature due to olivine and pyroxene in the 1-μm region (band I) resolves into distinct bands, provided the opx/(opx+ol) ratio is less than 0.5. Since such a splitting makes it possible to better constrain the mineral abundances, we suggest that spectral data of future space missions acquired at large insolation angles (low surface temperatures) may be useful for the compositional interpretation. The pyroxene absorption band near 2 μm (band II) moves to shorter wavelength, as expected, and splits into two distinct bands even if the calcic pyroxene abundance is very low. Therefore, a possible detection of a clinopyroxene band beyond 2 μm in asteroid spectra does not indicate the presence of an abundant calcic pyroxene component. The increase in reflectivity of the interband peak near 1.5 μm with decreasing temperature suggests that the puzzling turnover of the infrared continuum slope of S-type asteroids might be caused, at least in part, by their lower surface temperatures. The position of a composite 1-μm absorption band (band I) is subject to an offset with decreasing temperature. The direction of the wavelength shift depends on the opx/(opx+ol) ratio. The band II/band I area ratio increases moderately. For relatively olivine-rich assemblages these effects offset the data points on the plot of band I center wavelength position versus band area ratio. The upward displacement of S(II) and possibly S(III) asteroid subgroups with respect to the olivine–orthopyroxene mixing line on the plot does not necessarily indicate high contents of calcic clinopyroxene on their surfaces.
Article
We performed our investigations on mono-mineralogical phases of two terrestrial pyroxenes, a diopsidic augite and a hyperstene as well as on four meteoritic samples, Millbillillie, Ellemeet, Soko-Banja and Vaca Muerta, which represent extraterrestrial matter and could be considered to be analogs for surface material of some asteroid classes. Beside the known spectral variations characterizing ortho- and clinopyroxenes as well as basaltic assemblages, temperature related effects on the reflectance spectra are discussed.
Article
Low pressure melting experiments on eucritic meteorites demonstrate that the compositions of most eucrites can be generated by low pressure fractionation of pigeonite and plagioclase from liquids similar in composition to the Sioux County and Juvinas eucrites. It is unlikely that the liquids with compositions similar to Sioux County and Juvinas were themselves residual liquids produced by extensive fractionation of more magnesian parental liquids. The compositions of Stannern and Ibitira cannot be produced by fractionation of liquids with compositions similar to other known eucrites. Liquid compositions similar to Stannern, Ibitira, and Sioux County could have been generated by increasing degrees of low pressure partial melting of source regions composed of olivine (~Fo65), pigeonite (~Wo5En65), plagioclase (~An94), Cr-rich spinel, and metal. These source assemblages may have been primitive, undifferentiated material of the basaltic achondrite parent body and the eucrites may represent melts produced in early stages of its melting and differentiation. Further melting in these source regions, after exhaustion of plagioclase, may have produced magnesian liquids from which the magnesian pyroxenes and olivines in howardites, diogenites, and mesosiderites crystallized in closed-system plutonic environments. Most of the cumulate eucrites (e.g. Moama, Moore County, Serra de Magé) could not have equilibrated with liquids similar in composition to known eucrites. These cumulates may have accumulated from liquids produced by extensive fractionation of advanced partial melts of the source regions of eucritic liquids. A depletion in Na, K, and Rb in Ibitira is noted.
Article
V-type asteroids in the inner Main Belt (a < 2.5 AU) and the HED meteorites are thought to be genetically related to one another as collisional fragments from the surface of the large basaltic Asteroid 4 Vesta. We investigate this relationship by comparing the near-infrared (0.7–2.5 μm) spectra of 39 V-type asteroids to laboratory spectra of HED meteorites. The central wavelengths and areas spanned by the 1 and 2 μm pyroxene–olivine absorption bands that are characteristic of planetary basalts are measured for both the asteroidal and meteoritic data. The band centers are shown to be well correlated, however the ratio of areas spanned by the 1 and 2 μm absorption bands are much larger for the asteroids than for the meteorites. We argue that this offset in band area ratio is consistent with our currently limited understanding of the effects of space weathering, however we cannot rule out the possibility that this offset is due to compositional differences. Several other possible causes of this offset are discussed. Amongst these inner Main Belt asteroids we do not find evidence for non-Vestoid mineralogies. Instead, these asteroids seem to represent a continuum of compositions, consistent with an origin from a single differentiated parent body. In addition, our analysis shows that V-type asteroids with low inclinations (i < 6°) tend to have band centers slightly shifted towards long wavelengths. This may imply that more than one collision on Vesta’s surface was responsible for producing the observed population of inner belt V-type asteroids. Finally, we offer several predictions that can be tested when the Dawn spacecraft enters into orbit around Vesta in the summer of 2011.
Article
The visible and near-IR spectral reflectance of some minerals is a function of temperature. Hence, surface temperature variation must be considered for mineralogical interpretation of spectral reflectance data obtained from a spatially resolved surface which has a temperature distribution. An example is presented to illustrate the errors which can be encountered when interpreting the spectral reflectance of a silicate mineral assemblage, measured at various temperatures that might be expected across an asteroid surface. Independent knowledge of the surface temperature distribution would allow the optimal extraction of mineralogical information in such cases.
Article
The second phase of the Small Main-belt Asteroid Spectroscopic Survey (SMASSII) produced an internally consistent set of visible-wavelength charge-coupled device (CCD) spectra for 1447 asteroids (Bus and Binzel 2002, Icarus, ). These data provide a basis for developing a new asteroid taxonomy that utilizes more of the information contained in CCD spectra. Here we construct a classification system that builds on the robust framework provided by existing asteroid taxonomies. In particular, we define three major groupings (the S-, C-, and X-complexes) that adhere to the classical definitions of the S-, C-, and X-type asteroids. A total of 26 classes are defined, based on the presence or absence of specific spectral features. Definitions and boundary parameters are provided for each class, allowing new spectral observations to be placed in this system. Of these 26 classes, 12 bear familiar single-letter designations that follow previous conventions: A, B, C, D, K, O, Q, R, S, T, V, and X. A new L-class is introduced to describe 35 objects with spectra having a steep UV slope shortward of 0.75 μm, but which are relatively flat longward of 0.75 μm. Asteroids with intermediate spectral characteristics are assigned multiletter designations: Cb, Cg, Cgh, Ch, Ld, Sa, Sk, Sl, Sq, Sr, Xc, Xe, and Xk. Members of the Cgh- and Ch-classes have spectra containing a 0.7-μm feature that is generally attributed to hydration. Although previously considered featureless, CCD observations reveal distinct features of varying strengths in the spectra of asteroids in the X-complex, thus allowing the Xc-, Xe-, and Xk-classes to be established. Most notably, the spectra of Xe-type asteroids contain an absorption feature centered near 0.49 μm that may be associated with troilite. Several new members are identified for previously unique or sparsely populated classes: 12 A-types, 3 O-types, and 3 R-types. Q-types are common within the near-Earth asteroid population but remain unobserved in the main belt. More than 30 new V-types are found in the vicinity of Vesta. The heliocentric distribution of the SMASSII taxonomic classes is similar to that determined from previous studies, though additional structure is revealed as a result of the larger sample size.
Article
We present reflectance spectra of 19 V-type asteroids obtained at the 3.6 m Telescopio Nazionale Galileo covering 0.8 to 2.5 μm. For 8 of these asteroids we obtained also visible spectra in the same observational run. The range from 0.8 to 2.5 μm, encompassing the 1 and 2 μm pyroxene features, allows a precise mineralogical characterization of these asteroids. The obtained data suggests the possible coexistence of distinct mineralogical groups among the V-type asteroids, either probing different layers of (4) Vesta or coming from different bodies. No clear correlation was found between mineralogies and the objects being, or not, member of the Vesta dynamical family.
Article
The recent discovery of the first V-type asteroid in the middle belt, (21238) 1995WV7, located at ~2.54 AU, raises the question of whether it came from (4) Vesta or not. In this paper, we present spectroscopic observations indicating the existence of another V-type asteroid at ~2.53 AU, (40521) 1999RL95, and we investigate the possibility that these two asteroids evolved from the Vesta family to their present orbits by drifting in semi-major axis due to the Yarkovsky effect. The main problem with this scenario is that the asteroids need to cross the 3/1 mean motion resonance with Jupiter, which is highly unstable. Combining numerical simulations of the orbital evolution, that include the Yarkovsky effect, with Monte Carlo models, we compute the probability of an asteroid of given diameter D to evolve from the Vesta family and to cross over the 3/1 resonance, reaching a stable orbit in the middle belt. Our results indicate that an asteroid like (21238) 1995WV7 has a low probability of having evolved through this mechanism due to its large size (~5 km). However, the mechanism might explain the orbit of smaller bodies like (40521) 1999RL95 (~3 km), provided that we assume that the Vesta family formed > 3.5 Gy ago. We estimate that about 10% or more of the V-type bodies with D>1 km may come from the Vesta family by crossing over the 3/1 resonance. The remaining 90% must have a different origin.
Article
We utilize the 56 images of Asteroid 4 Vesta obtained through four mineralogically diagnostic filters by the Hubble Space Telescope (Zellneret 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 Thomaset 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 μm 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 μm 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,Science260,186–191) 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 ∼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.
Article
We investigate the ability to refine pyroxene composition and modal abundance from laboratory and remotely acquired spectra. Laboratory data including the martian meteorites, Shergotty, Zagami, MIL03346, and ALH84001 as well as additional pyroxene-rich spectra obtained from the OMEGA (Observatoire pour la Minéralogie, l'Eau, les Glaces, et l'Activité) spectrometer for Mars are characterized using the Modified Gaussian Model (MGM), a spectral deconvolution method developed by Sunshine et al. [Sunshine, J.M., Pieters, C.M., Pratt, S., 1990. J. Geophys. Res. 95, 6955–6966]. We develop two sensitivity tests to assess the extent to which the MGM can consistently predict (1) pyroxene composition and (2) modal abundance for a compositionally diverse suite of pyroxene spectra. Results of the sensitivity tests indicate that the MGM can be appropriately applied to remote spectroscopic measurements of extraterrestrial surfaces and can estimate pyroxene composition and relative abundance within a derived uncertainty. Deconvolved band positions for laboratory spectra of the meteorites Shergotty and Zagami are determined within ±17 nm while remotely acquired OMEGA spectra are defined within ±50 nm. These results suggest that absolute compositions can be uniquely derived from laboratory pyroxene-rich spectra and non-uniquely derived from the remote measurements of OMEGA at this time. While relative pyroxene chemistries are not assessed from OMEGA measurements at this time, relative pyroxene abundances are estimated using a normalized band strength ratio between the low-calcium (LCP) and high-calcium (HCP) endmember components and are constrained to ±10%. The fraction of LCP in a two-pyroxene mixture is the derived value from the normalized band strength ratio, LCP/(LCP + HCP). This calculation for relative abundance is robust in the presence of up to 10–15% olivine. Deconvolution results from the OMEGA spectra indicate that the ancient terrain in the Syrtis Major region is uniquely enriched in LCP (59±10% LCP) relative to HCP while the volcanics of Syrtis Major are uniquely enriched in HCP (39±10% LCP).
Article
Twenty-nine unbrecciated eucrites have been thoroughly characterized in terms of the petrologic factors that affect their spectra, such as mineral chemistry, modal adundances, grain sizes, and textures. We have conducted a combined petrologic and spectral study designed to provide insight into the petrogenesis of the basaltic crust of Vesta and the variety of rock-types that exist within it, as well as aid in the petrologic interpretation of spectra to be collected by the Dawn orbiting spacecraft. This paper details the petrology part of the study. Unbrecciated eucrite samples were selected to avoid the complications of lithologic mixing in the accompanying spectral study. A wide variety of textural types are seen within the basaltic eucrites, encompassing quenched, coarse-grained, and granoblastic samples. Zoned pyroxenes in eucrites and those that preserve a history of initial rapid cooling are rare. Nearly all eucrite samples have been thermally metamorphosed and would commonly be classified as equilibrated; however, this term reflects only the quadrilateral (Mg, Fe, and Ca) compositions of pyroxenes, and considerable variations are seen within the minor elements (Al, Ti, and Cr) in pyroxenes as well as plagioclase compositions. Determination of both pyroxene and plagioclase compositions together with pyroxene geothermometry provides a better estimate for the relative degree of thermal metamorphism a eucrite has experienced. The petrologic differences observed here might allow different eucrites to be distinguished spectrally. This is especially true for the varying pyroxene compositions as the spectra of eucrites are dominated by absorption features attributed to pyroxene.
Article
Predictions of future potential Earth impacts by near-Earth objects (NEOs) have become commonplace in recent years, and the rate of these detections is likely to accelerate as asteroid survey efforts continue to mature. In order to conveniently compare and categorize the numerous potential impact solutions being discovered we propose a new hazard scale that will describe the risk posed by a particular potential impact in both absolute and relative terms. To this end, we measure each event in two ways, first without any consideration of the event's time proximity or its significance relative to the so-called background threat, and then in the context of the expected risk from other objects over the intervening years until the impact. This approach is designed principally to facilitate communication among astronomers, and it is not intended for public communication of impact risks. The scale characterizes impacts across all impact energies, probabilities and dates, and it is useful, in particular, when dealing with those cases which fall below the threshold of public interest. The scale also reflects the urgency of the situation in a natural way and thus can guide specialists in assessing the computational and observational effort appropriate for a given situation. In this paper we describe the metrics introduced, and we give numerous examples of their application. This enables us to establish in rough terms the levels at which events become interesting to various parties.
Article
A sample of over 12,487 asteroid proper element triplets, computed by A. Milani and Z. Knežević (1994, Icarus 107, 219-254), has been searched for statistically significant and robust families by both the hierarchical clustering (HCM) and the wavelet analysis (WAM) automated techniques. The current sample includes almost 8000 unnumbered objects with relatively well-determined orbits, which allow us to extend the previous family classifications to much smaller asteroid sizes. With both clustering methods about one-third of the whole asteroid population is found to belong to groupings of varying significance and robustness. Nearly all the families identified in previous searches by Ph. Bendjoya (1993, Astron. Astrophys. Suppl. Ser. 102, 25-55) and V. Zappalà et al. (1994, Astron. J. 107, 772-801) are confirmed. Many more groupings are now recognized having a small number of members and/or a large percentage of unnumbered ones, particularly when the WAM technique is applied to asteroids located in the middle region of the main belt, between the 3:1 and the 5:2 Kirkwood gaps. Most of these new small families appear to derive from the (possibly recent) break-up of small parent asteroids, <50 km across. Some are very compact, suggesting ejection speeds not much in excess of the parent's escape velocity. Some fairly populous families can be readily interpreted as the outcome of giant cratering events, including those associated with (4) Vesta and (10) Hygiea. Others (e.g., Meliboea in the outer belt) are so depleted in small members that probably an old age is implied. Several populous families, and in particular those named "clans" (e.g., Eunomia, Vesta, Nysa), show a complex and size-dependent internal structure, suggesting a complex collisional history (i.e., multiple successive disruptive events). A problem with family searches applied to very numerous asteroid samples, such as the current one, is that neighboring, physically distinct groupings often partially overlap each other, and thus merge according to the clustering techniques.
Article
We performed instrumental neutron activation analysis on a large suite of antarctic and nonantarctic eucrites, including unbrecciated, brecciated, and polymict eucrites and cumulate and noncumulate eucrites. We evaluate the use of Hf and Ta, two highly incompatible elements, as sensitive indicators of partial melting or fractional crystallization processes. Comparison with rare earth element (REE) data from nonantarctic and antarctic eucrites shows that Hf and Ta are unaffected by the terrestrial alteration that has modified the REE contents and patterns of some antarctic eucrites. The major host phases for Hf and Ta—zircon, baddeleyite, ilmenite, and titanite—are much less susceptible to terrestrial alteration than the phosphate hosts of REEs. The host phases for Hf and Ta are minor or trace phases, so sample heterogeneity is a serious concern for obtaining representative compositions. The trace lithophile and siderophile element contents of noncumulate eucrites do not allow for a single, simple model for the petrogenesis of the howardite-eucrite-diogenite suite. Fractional crystallization models cannot reproduce the compositional relationship between eucrites of the main group–Nuevo Laredo trend and those of the Stannern trend. Equilibrium crystallization models cannot explain the trace element diversity observed among diogenites. Partial melting models cannot explain the W variations among eucrites, unless source regions had different metal contents. We suggest that slight variations in oxygen fugacity of eucrite source regions during partial melting can explain the W variations without requiring different metal contents. This hypothesis may fail to account for eucrite Co contents, however.
Article
A finite element code has been developed to model the thermal history of Asteroid 4 Vesta. This is the first attempt to model the thermal history of a differentiated asteroid through core and crust formation and subsequent cooling until geochemical closure is attained. The results of the simulation are consistent with chronological measurements and other constraints provided by cumulate and noncumulate eucrites believed to have been derived from Vesta. The work solves two major problems with the hypothesis of heating by decay of26Al, an extinct radionuclide, postulated to be a plausible heat source in the early Solar System. First, the model demonstrates that it is possible to keep the mantle of Vesta hot for ∼100 Ma, thereby explaining the observed difference in ages between cumulate and noncumulate eucrites. Second, the simulation offers a possible explanation of why detectable excesses of26Mg (the decay product of26Al) are not observed in noncumulate eucrites. The simulation draws a model chronology of Vesta and predicts times (relative to CAI formation) for accretion at 2.85 Myr, core formation at 4.58 Myr, crust formation at 6.58 Myr, and geochemical closure at ∼100 Myr for a H-chondrite asteroidal bulk composition. Decay of60Fe is found to cause no perceptible difference in the thermal history of Vesta, even when sequestered into a central core. Although chondritic xenoliths have not been described in HED igneous lithologies, the thermal model suggests the possibility that a veneer of unmelted near-surface material should remain.
Article
Bidirectional reflectance measurements are the only type of reflectance data available to the remote observer. For compositional interpretations, data are desired not only for identification of possible mineral components but also for modal abundance. The latter requires detailed information about the strength of absorption features. Using a new laboratory facility, the RELAB, laboratory data in the near infrared are presented that document effects of particle size, mineral mixtures, and viewing geometry for selected materials with well-developed absorption bands. The commonly observed increase in reflectance with decrease in particle size is also observed for absorption bands as well as a related decrease in absorption strength. For small particles in parts of the spectrum of maximum reflectance, however, a minor decrease in reflectance with a decrese in particle size is sometimes observed. Small particles dominate the observed characteristics of particulate surfaces, which contain a range of particle sizes. The mean optical path length (transmission through particles) of reflected radiation measured for a variety of particle sizes has an apparent upper limit of about 2 mm for particles of less than 250 microns. The typical number of particles involved in the optical path is less than 50.
Article
The present paper provides the results from a series of laboratory experiments which were conducted to investigate the variable spectral behavior of diagnostic Fe(++) crystal field absorptions in common rock-forming minerals as a function of temperature. The general goal of the study is to refine existing techniques of mineralogic and chemical characterization, based on specific optical behavior which has been measured and correlated in a systematic way. It is pointed out that the presented work, taken together with a number of recent studies, provides new information on the manner in which specific features of mafic minerals combine in rocks and soil spectra, taking into account also variations of identification and subsequent interpretation as a function of planetary surface temperature. The obtained results are directly applicable to interpretation of remotely sensed data for many solar system objects, particularly Mars, asteroids, the moon, earth, and Mercury.
Article
Although visible and near IR reflectance spectra contain absorption bands that are characteristic of the composition and structure of the absorbing species, deconvolving a complex spectrum is nontrivial. An improved approach to spectral deconvolution is presented that accurately represents absorption bands as discrete mathematical distributions and resolves composite absorption features into individual absorption bands. The frequently used Gaussian model of absorption bands is shown to be inappropriate for the Fe(2+) electronic transition absorptions in pyroxene spectra. A modified Gaussian model is derived using a power law relationship of energy to average bond length. The modified Gaussian model is shown to provide an objective and consistent tool for deconvolving individual absorption bands in the more complex orthopyroxene, clinopyroxene, pyroxene mixtures, and olivine spectra.