Exogenic olivine on Vesta from Dawn Framing Camera color data

Icarus 258 (2015) 467–482

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Exogenic olivine on Vesta from Dawn Framing Camera color data Andreas Nathues a,⇑, Martin Hoffmann a, Michael Schäfer a, Guneshwar Thangjam a, Lucille Le Corre b, Vishnu Reddy b, Ulrich Christensen a, Kurt Mengel c,a, Holger Sierks a, Jean-Baptist Vincent a, Edward A. Cloutis d, Christopher T. Russell e, Tanja Schäfer a, Pablo Gutierrez-Marques a, Ian Hall a, Joachim Ripken a, Irene Büttner a a

Max-Planck-Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719-2395, USA Clausthal University of Technology, Adolph-Roemer-Straße 2a, 38678 Clausthal-Zellerfeld, Germany d Department of Geography, University of Winnipeg, 515 Portage Avenue Winnipeg, Manitoba R3B 2E9, Canada e Institute of Geophysics and Planetary Physics, University of California, 3845 Slichter Hall, 603 Charles, CA 90095, USA b c

a r t i c l e

i n f o

Article history: Received 5 August 2014 Revised 23 September 2014 Accepted 24 September 2014 Available online 5 October 2014 Keywords: Asteroid Vesta Asteroids, composition Mineralogy Spectroscopy

a b s t r a c t 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 Stype 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. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction To our current knowledge, Vesta is geologically the most diverse differentiated planetesimal body that survived the collisional history of our Solar System (Russell et al., 2012, 2013; Keil, ⇑ Corresponding author. E-mail address: [email protected] (A. Nathues). http://dx.doi.org/10.1016/j.icarus.2014.09.045 0019-1035/Ó 2014 Elsevier Inc. All rights reserved.

2002). Thus Vesta is one of the most interesting proto-planets, whose study enables us to reveal the early phase of planet formation, since evolution models favor a magmatic differentiation of Vesta that ceased in the first few million years of accretion (Keil, 2002). Most of the Howardite–Eucrite–Diogenite (HEDs) meteorites originate from Vesta (McCord et al., 1970; Thomas et al., 1997; Migliorini et al., 1997; Russell et al., 2012, 2013). These meteorites or their parent bodies have been ejected by one or more

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large impacts from Vesta, reached escape velocity, and were transferred via the 3:1 mean-motion resonance and the m6 secular resonance to near Earth space (Migliorini et al., 1997). Evolution models, based on the petrogenesis of HEDs (Ruzicka et al., 1997; Righter and Drake, 1997; Warren, 1997), generally favor the evolution of Vesta through extensive melting, termed ‘magma ocean’. A further model, the ‘serial magmatism model’ (Yamaguchi et al., 1996, 1997), describes Vesta’s evolution through a sequential development of eruptions in multiple shallow magma chambers. This model is supported by the measured variations in incompatible trace element abundances in diogenites (Mittlefehldt, 1994; Fowler et al., 1995; Shearer et al., 1997), the wide range of Mg (mol.%) in pyroxene, and olivine among olivine-bearing diogenites (Beck and McSween, 2010; Shearer et al., 2010). Recently Mandler and Elkins-Tanton (2013) proposed a new two-step model of magmatic evolution, from a bulk mantle composition based on major and minor elements estimated from earlier studies by Righter and Drake (1997), Dreibus and Wänke (1980), Ruzicka et al. (1997), Lodders (2000), and Boesenberg and Delaney (1997). Their two-step models invoke 60–70% equilibrium crystallization of a magma ocean, followed by fractional crystallization of the residual liquid in shallow magma chambers. This explains the evolution of Vesta in terms of the diverse petrogenesis of HEDs, overcoming the drawbacks in earlier models. An olivine-rich mantle is predicted by all evolution models in which various peridotite (>40% olivine) and olivine–pyroxenites (<40% olivine) forming the upper mantle and the deeper crust (Mandler and Elkins-Tanton, 2013). The Rheasilvia basin (£ 500 km), superimposing the older Veneneia basin (£ 400 km), covers most of the southern hemisphere and is the prime area where exposures of olivine-rich mantle materials were initially expected (Thomas et al., 1997; Tkalcec et al., 2013), but have not been found so far (McSween et al., 2013). Olivine has been primarily found in diogenites, commonly associated with orthopyroxene and some accessory minerals like troilite, chromite, silica, and iron–nickel (e.g., Mittlefehldt, 1994; Bowman et al., 1997; Irving et al., 2009; Beck and McSween, 2010; Beck et al., 2011, 2012, 2013; McSween et al., 2011, 2013). We compiled a list of olivine-bearing HED meteorites (Thangjam et al., 2014b) and found that there are about 30 diogenites with 625 wt.% olivine (mostly less than 10%), 8 harzburgitic diogenites with 40–68% olivine, and 4 dunites with >90% olivine (Floran et al., 1981; Sack et al., 1991; Mittlefehldt, 1994; Bowman et al., 1997; Bunch et al., 2010; Krawczynski et al., 2008; Irving et al., 2009; Beck and McSween, 2010; Beck et al., 2011, 2012, 2013; McSween et al., 2011, 2013). The abundance of olivine in diogenites is heterogeneous, and the observed range is possibly the result of a sampling bias (e.g., Bowman et al., 1997; Irving et al., 2009; Beck et al., 2011, 2012). However, it is interesting to note that the most magnesian olivine is found in peridotitic diogenites and less magnesian olivine is restricted to pyroxenitic diogenites. Eucrites, which are one of the main components of howardites, normally do not contain olivine (Delaney et al., 1980) since eucritic components have been removed from the parental melt upon fractional crystallization (Mason, 1962; Stolper, 1977; Grove and Bence, 1979; Delaney et al., 1980). Mikouchi and Miyamoto (1997) also suggested that eucrites do not contain olivine except for late crystallized Fe-rich olivines. A few eucrites have been reported to contain veinlets with ferroan olivine (Barrat et al., 2011; Zhang et al., 2011). Olivine has also been found in a few howardites, in most cases at the level of less than a few percent (Delaney et al., 1980; Beck et al., 2011, 2012, 2014; Lunning et al., 2014). Olivine-rich impact melts containing 50–75% olivine are observed in PCA 02 and GRO 95574 howardites (Beck et al., 2011, 2014). Also clasts of ordinary chondrites have been identified in howardites (Lorenz et al., 2007). Since some of the LL and H chondrites contain up to 57 vol.% of olivine (Dunn et al., 2014)

one can also test the hypothesis of a primitive source of Vesta’s olivine as conducted by Le Corre et al. (in preparation). Prior to Dawn’s arrival at Vesta (Russell et al., 2012), several attempts were made to detect olivine-rich lithologies on Vesta’s surface by ground-based observations (Larson and Fink, 1975; McFadden et al., 1977; Feierberg et al., 1980; Gaffey, 1997). Binzel et al. (1997) reported olivine-bearing regions based on the observations of four-band spectra (0.43–1.04 lm) from the Hubble Space Telescope (HST). Shestopalov et al. (2010) predicted up to 6.8% olivine by simulating the spectra from Binzel et al. (1997) and the available ground based spectra. However, Li et al. (2010) and Reddy et al. (2010) did not confirm the existence of olivine from their HST color spectra and ground based near-infrared spectra, respectively. Olivine-rich sites have been identified in Dawn’s Vesta data quite late (Ammannito et al., 2013; Nathues et al., 2014a; Ruesch et al., 2014; Thangjam et al., 2014a,b; Zambon et al., 2014) due to their small spatial extent and exceptional distribution, clustering in the northern hemisphere (36–42°N, 46–74°E), around the craters Pomponia, Arruntia and Bellicia. The identification of olivine-rich lithologies in the northern hemisphere raises questions, as well as complexities for understanding the geologic evolution of Vesta, since olivine-rich lithologies were rather expected in the Rheasilvia basin (Thomas et al., 1997). The northern hemisphere is preliminary dominated by howarditic material, while the southern hemisphere, especially in the floor and near the rim of the Rheasilvia basin, is rather diogenitic. In addition, an east– west albedo dichotomy exists, in which the exogenic dark material (Reddy et al., 2012a; McCord et al., 2012; Nathues et al., 2014b) dominates the longitudes between 70°E and 250°E, while the remaining surface is significantly brighter. Several surface areas, for example the Oppia crater, are covered with ‘orange material’, which has been interpreted as impact-melt (Le Corre et al., 2013). Most of the vestan surface shows processed crustal material. The present paper summarizes further results of our extensive mapping efforts (Reddy et al., 2012a, 2012c; Nathues et al., 2014a,b; Thangjam et al., 2013, 2014a,b) of the vestan surface, now with the focus on the global identification of olivine-rich sites, characterized by their spectral properties and their morphology. We compare these sites with local background materials and relate them to their geologic context that allows us to draw conclusions on their origin and processing.

2. Data processing and method The Dawn Framing Camera (FC) has imaged the entire visible surface of Asteroid 4 Vesta from different orbits in 2011/2012. The FC is equipped with a clear (panchromatic) and seven color filters, covering the wavelength range between 0.4 and 1.0 lm (Sierks et al., 2011). Vesta was mapped from Survey, HAMO (High-Altitude Mapping Orbit) and LAMO (Low Altitude Mapping Orbit) orbits at spatial resolutions of 250 m/pixel, 60 m/pixel, and 20 m/pixel, respectively. Framing Camera images exist in three standard levels: 1a, 1b and 1c. The data stream received on ground (level 0) is converted to PDS format images (level 1a) that contain unprocessed, uncalibrated digital values from 0 to 16384 DN (14 bit). Level 1a data is converted to level 1b as radiometrically calibrated PDS-compliant images (physical unit lW cm 2 sr 1 nm 1). All FC color images are affected by a stray light component, the so-called ‘‘in-field’’ stray light (Kovacs et al., 2013). This ghost signal, which leads to absolute errors of up to 10%, needs to be removed from each science image by a deconvolution process, leading to level 1c data, which is the final image data product. In order to correct for the ghost signal, a stray light removal algorithm based on laboratory data has been developed (Kovacs et al.,

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2013), leading to recalibrated data, showing significantly less stray light contamination. The stray light removals are scene- and filterdependent and its residual systematic error is: 0.438 lm filter – 2.0% (11%), 0.555 lm – 2.0% (3.0%), 0.653 lm – 2.5% (5%), 0.749 lm – 3.0% (4.0%), 0.829 lm – 3.0% (9%), 0.917 lm – 2.5% (7%), 0.965 lm – 2.5% (6%); where the numbers in brackets are the typical errors before stray light removal. After stray light removal, level 1c data is converted to reflectance (I/F) by dividing the observed radiance by solar irradiance from a normally solarilluminated Lambertian disk. The stray light corrected level 1c I/F data is used for processing in our ISIS pipeline. ISIS (Integrated Software for Imagers and Spectrometers; Anderson et al., 2004) is a UNIX-based software developed and maintained by USGS. ISIS performs the photometric correction of the FC color data to standard viewing geometry (30° incidence and 0° emission angle) using Hapke functions (e.g., Hapke, 2012). Starting values for the Hapke input data were taken from Li et al. (2013) and Helfenstein and Veverka (1989). They were then optimized for inter-image and spectral consistency. The former is achieved by minimizing photometrically induced seams in color mosaics, and the latter by comparison of the corrected images obtained at different illumination angles. Initially, both conditions can be met by Li et al. (2013) data, but because of local deviations from the global model, they are not fulfilled simultaneously. Therefore a set of parameter values has been adopted which emphasizes the invariability of the radiance data to the range of incidence and emission angles covered by the FC data, and is applied in the photometric correction. For this correction and the visualization of the results, we used the Vesta shape model (gaskell_vesta_20130522_dem.cub) derived from FC clear filter images by Gaskell (2012). The resulting reflectance data are map-projected in several steps, and co-registered to align the color frames, creating color cubes for analysis. All maps are produced in the Claudia coordinate system (Russell et al., 2012, 2013; Reddy et al., 2013). The Planetary Data System (NASA) is providing Dawn data in a longitude system that can be obtained from the Claudia longitude by subtracting 150°. A basic description of the FC data processing pipeline is presented in Reddy et al. (2012c) and Nathues et al. (2014b). Note that the photometric parameters used by Reddy et al. (2012c) have been further improved in our study. For the present analysis, FC color data from HAMO and HAMO_2 orbits were used, whose spatial resolution is about 60 m/pixel. Higher resolution data from LAMO orbits (20 m/pixel) were used to identify details of the morphological context. The color mosaics generated by the ISIS pipeline were analyzed using ENVI and ArcGIS software. In order to assess the uncertainty of the individual color bands, smooth, homogeneous areas on Vesta have been photometrically analyzed. We conclude that for a 4  4 pixel sized area, the relative statistical error is ±0.4% in all bands. This error propagates into the spectral ratios accordingly. Not included are the effects caused by local deviations from the global photometric model connected with the differences in composition. Therefore the total error may be somewhat higher, particularly in areas of steep slopes. HAMO data is taken for the spectral analysis and hence integrated over areas of inhomogeneous material. Since the different materials show non-linear variations in the individual bands, such integration may introduce weight errors.

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composite of three overlapping absorption bands in the 1-lm region due to Fe2+, in which the major broad absorption feature (1.05 lm) is attributed to the M2 site, while the weaker absorption features (0.85 and 1.25 lm) are attributed to the M1 site (Adams, 1974; Burns, 1970, 1993; Hunt, 1977; Singer, 1981; Cloutis et al., 1986; King and Ridley, 1987). The composed spectral feature depends not only on the olivine chemistry, in which the 1lm absorption moves to longer wavelength with increasing Fe content, but also depends on the physical parameters of the regolith, for example grain size and temperature, as well as observational parameters such as phase angle (Adams, 1975; Burns, 1970, 1993; Cloutis et al., 1986; King and Ridley, 1987; Nathues, 2000, 2010). Pyroxenes on the other hand have prominent 1- and often 2-lm absorption features with varying absorption band centers, and their intensities depend on the Fe2+–Ca2+–Mg2+ relations, the asymmetry of the cation distribution on crystallographic sites as well as grain size, temperature and phase angle (Adams, 1974, 1975; Burns, 1993; Singer, 1981; Cloutis and Gaffey, 1991; Klima et al., 2007; Schade et al., 2004). Low-Ca pyroxenes have shorter absorption band centers near 0.9 lm and 1.9 lm compared to high-Ca pyroxenes near 0.98 lm and 2.15 lm (Pieters, 1986). The lack of the 2-lm absorption feature for olivine is commonly used to discriminate olivine from most of the pyroxenes. However, due to FC’s cut-off wavelength at 1 lm, it was required to develop a new set of spectral parameters, which allows us to distinguish olivines from pyroxenes. To achieve this we compiled and measured visible and near-infrared laboratory spectra of various HEDs powders and other potential mixtures, which were resampled to FC filter band passes to enable comparisons between in-flight and laboratory data (Thangjam et al., 2014b). Our olivine identification, i.e. the distinction from the HED lithologies, is based on three spectral parameters: (1) Band Tilt (BT) = (R0.92lm/R0.96lm) that is the most diagnostic parameter, (2) Mid Ratio (MR) = (R0.75lm/R0.83lm)/(R0.83lm/R0.92lm), and (3) Mid Curvature (MC) = (R0.75lm + R0.92lm)/R0.83lm which is the least diagnostic of these parameters; where Rk is the reflectance in the corresponding filter. These three spectral parameters are necessary for discriminating the spectrum-altering effects of olivine from spectral changes associated with changes in pyroxene composition. 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 (Thangjam et al., 2014b). The latter are those HCPs that are consistent with pyroxene compositions among eucrites in HEDs (McSween et al., 2011; Mayne et al., 2009, 2011), in which all other HCPs have been omitted (Thangjam et al., 2014b). According to our lab results FC color data is capable of identifying olivine if the olivine content reaches 40 wt.% (Thangjam et al., 2014b) in an intimate mixture. For mechanical mixtures between eucrites and dunites we estimate the detectability of olivine starting at 60% concentration of dunites which is caused by a masking effect of the olivinefree eucrite. This is reflected in Fig. 3 that shows the overlapping pyroxenitic diogenite (olivine free) and olivine–orthopyroxenitic diogenites (<40% olivine) polygons, which are distinct from the harzburgites (>40% olivine). The depth of the 1-lm absorption band and the visual spectral slope are further spectral parameters that can help to distinguish olivine-rich material from local background material, but these parameters are ambiguous.

3. Identification of olivine Pyroxene and olivine are common rock-forming minerals of mafic and ultramafic terrestrial bodies, which are present in the HED suite as well as on Vesta’s surface. The visible and near-infrared spectral range between 0.4 and 3.0 lm is well suited to infer the mineralogy of these bodies. Olivine spectra generally show a

4. Global distribution, spectral variation and geologic context Inspection of HAMO2 data, covering higher northern latitudes, revealed for the first time the presence of olivine in, and close to, the craters Bellicia and Arruntia (Ammannito et al., 2013;

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Thangjam et al., 2013, 2014a,b) as well as Pomponia (Nathues et al., 2014a). The identified sites do not agree with earlier findings by ground-based observations (e.g., Gaffey, 1997; Binzel et al., 1997), and are in particular much smaller, being undetectable by telescopic techniques from Earth. The olivine-rich exposures identified by us are spectrally and spatially resolved at HAMO resolution (60 m/pixel), with the exception of one exposure at the Rheasilvia central mound, for which we have color information from Survey orbit only. Fig. 1 presents the global distribution of the olivine-rich sites as identified by using Dawn FC color data. As mentioned, most olivine-rich sites cluster in and around the craters Bellicia, Arruntia, and Pomponia. Further sites have been identified near the equator at crater Portia and an isolated spot to its south-east. Interestingly, we have identified olivine-rich units on the inner wall of the Rheasilvia basin at Matronalia Rupes, on its central mound, and possibly at craters Sossia and Tarpeia. All potential olivine-rich exposures associated with the Rheasilvia basin are small-scale, extending not more than 0.5 km2 (Sossia), while exposures in Bellicia cover up to 0.7% of the crater area (Thangjam et al., 2014a,b) and up to 4 km2 per exposure. The spectral variation among the olivine-rich sites is influenced by the quantity of olivine in the regolith, the total area, and by the local background material, either HED’s, orange material, or dark material components. The olivine, as well as its background material, has a steeper visual spectral slope in the northern hemisphere, in particular at sites near the crater Arruntia. The more diogenitic background in and close to the Rheasilvia basin tends to decrease the ratio R0.65lm/R0.44lm in spectra of olivine-rich material. The absolute reflectance at 0.75 lm of the olivine exposures globally varies between 0.1 and 0.4, including sites of apparent mixing with dark material. This variation reflects the albedo of the background material rather than intrinsic spectral properties or grain size effects of the olivine-rich material. However, sites that have been classified as being less contaminated by background material (see Fig. 13) still show a notable absolute reflectance range of 0.1 that is possibly influenced by grain size differences. Also the 1lm absorption band depth varies between different sites. For example, the small olivine-rich spots at Portia show a variation in relative band depth of 10% (0.57–0.67). Many of the olivine-rich

sites are found surrounded by howarditic material. Mixing by downslope motion is often indicated at the transition between the olivine-rich and howarditic contexts, which is reflected in intermediate positions in the spectral parameter space (e.g., Fig. 6B/7B). Because of additional components of ‘orange’ (Le Corre et al., 2013) and exogenous dark carbonaceous chondritic material (Reddy et al., 2012a; Nathues et al., 2014b), which vary from site to site, these mixing trends differ. This is also reflected in the normalized reflectances of the 0.44 lm color band, which clusters between 0.74 and 0.86 within southern locations, but ranges between 0.65 and 0.75 in the northern sites. Since olivine is sensitive to space weathering (Sasaki et al., 2001) and the exposures are located in craters of apparently different age, some spectral differences, like band depth and spectral slope, between different sites may be explained this way. However, data of the Dawn optical instruments suggests that Vesta’s space weathering, acting on the HED lithologies, is different (Pieters et al., 2012) from that of other airless bodies. No evidence was found for accumulation of lunar-like nanophase iron on regolith particles. Instead, the spectroscopic data revealed that locally homogenized upper regolith is generated with time through small-scale mixing of diverse surface components. Most, if not all, detected olivine on Vesta is associated with impact structures, whereby the largest crater in which we identified olivine is Pomponia (£ 59 km, see Section 5.2). However, the presence of olivine is not limited to large craters, but larger craters may show a higher number of olivine-rich exposures. Most of these exposures have been found on the inner crater walls, but also on crater floors (Pomponia and Arruntia), and in three cases olivinerich exposures have been detected in their ejecta blankets (Arruntia, Pomponia, Bellicia). Olivine-rich exposures on Vesta exist in craters with widespread erosion stages (cf. Craddock and Howard, 2000), including fresh impacts (Arruntia) and older craters (Pomponia). The total area of olivine-rich lithologies on Vesta is relatively small and nearly negligible outside its concentration in and around Bellicia–Arruntia–Pomponia. Interestingly, the concentration is located in the southeastern environment of the large Albana crater, centered at 53°E/79°N, a finding which we discuss in Section 6. This concentration is also near the rims of the moderately eroded basins Varonilla (center at 30°E/35°N) and Caesonia (center at 100°E/

Fig. 1. Olivine bearing sites on Vesta identified by using Dawn FC color spectra. The presented global view is a HAMO/HAMO2 mosaic at 0.555 lm in the ‘‘Claudia’’ coordinate system (see text for explanation). Two question marks indicate uncertain detections. The borders of the Veneneia (magenta line) and Rheasilvia (green line) basins are sketched. Legend: blue asterisk – olivine detected on inner wall, floor, and ejecta; red – olivine on inner wall, floor and possibly ejecta; orange – olivine on inner wall and ejecta; green – olivine on inner wall only; yellow – single olivine exposures; purple – unusual olivine exposures (see text); black – possibly olivine at inner crater wall and ejecta (see text). Most of the olivine-rich sites are concentrated in the Bellicia, Arruntia and Pomponia region, while only a few potential sites may be associated with the Rheasilvia basin. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 2. Four craters next to the Arruntia crater, which show exposures of dunitic material (red to orange colors) in this 3D view. Numbers 1–4 mark sites whose spectra are presented in Fig. 3. Spectra of Arruntia and nearby olivine-bearing locations (#5 and #6) are presented in Thangjam et al. (2014a,b). The color scheme in this figure is as follows: R = R0.92lm/R0.96lm, G = 1.0/R0.55lm, B = R0.96lm/R0.92lm; Rk is the reflectance in the filter centered at klm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

30°N), but with the exception of the ambiguous case at Tarpeia, we did not identify any olivine-rich exposure on the floors of the known Vesta basins. The depths and appearance of the olivine-rich outcrops beneath the present surface at the inner crater walls is a key to understand the mechanisms and the history of the material deposition. Their spatial concentrations, mostly close to the rim of the craters, may demonstrate, whether the olivine-rich sites are related to a crater itself or to the pre-impact surface. Thus almost all settings are far from the depth of a presumed olivine mantle, which was assumed to be at a minimum depth of 15–20 km (McSween et al., 2013). In general, the vertical location of the olivine-rich outcrops on the inner walls suggests that this material is embedded in the pre-impact surface as indicated by Osinski et al. (2011) for other planetary bodies. We found that most of the olivine-rich exposures have the same characteristics: (1) outcrops appear within 0–2 km below the crater rim; (2) olivine-rich outcrops are either thin layer(s) or lenses with a maximum lateral extent of 1 km. At Pomponia (Section 5.2), Bellicia, and Arruntia (Thangjam et al., 2014a,b), we found several disconnected outcrops along the inner walls whose depths vary slightly; (3) Pomponia and Arruntia show olivine-rich spots on their crater floors, which possibly originate from downslope movement or avalanches; (4) in many cases the outcrops on the inner crater walls show a material fan caused by downslope movement; (5) fans are composed of dunitic material with increasing addition of background material; (6) in some cases olivine-rich material is part of the ejecta (Arruntia, Pomponia); (7) olivine-rich fans and outcrops are sometimes located close to dark material (e.g., Pomponia); (8) the background material is mostly howarditic. However, some of the identified sites are unique. Those are Matronalia Rupes, Sossia and Portia and all of them are discussed in Section 5.

5. Description of olivine-rich sites 5.1. Craters in the Arruntia vicinity Fig. 2 displays a 3D view of the vicinity of the Arruntia crater in which olivine-rich material is highlighted by reddish colors. Arruntia (#6 in Fig. 2) is a 10 km diameter crater located at 72°E/39°N, whose olivine-rich exposures are described in detail by Thangjam et al. (2014/in press). Its olivine-rich ejecta are unique on Vesta

and can be traced beyond several crater radii. For example, southwest of Arruntia some olivine-rich sites are found in small elongated spots (yellow question mark in Fig. 1), which seem to be remote ejecta from Arruntia, rather than individual outcrops. Further olivine-rich outcrops have been identified by us in four craters next to Arruntia (outcrops #1 to #5 in Fig. 2). These craters are located at 74°E/27°N (#1, #2), 83°E/41°N (#3), 81°E/34°N (#4), and 75°E/40°N (#5). Their diameters are 10, 26, 15, and 8 km, respectively. The westernmost of the four craters (74°E/27°N, outcrops #1 and #2 in Fig. 2) shows a cone-shaped morphology with inner wall slopes of more than 35°. The crater is located in a sloped area, i.e. the elevation difference between the northwestern and southeastern rim is 3 km, while the crater depth measured from the lower rim is 3 km. The crater at 81°E/34°N (outcrop #4 in Fig. 2) is bowl-shaped with a depth of 4 km and a maximum slope of 20°. The crater is more symmetric regarding the elevation of its rim. The easternmost of the three craters (83°E/41°N, outcrop #3 in Fig. 2) is located on the eroded wall of the Caesonia crater, which has an asymmetry of rim height of 2.5 km. Its profile is bowlshaped, but shows a flattened area of 1/3 of its diameter on its floor; the slope of the walls reaches about 30°. The fourth crater 75°E/40°N (outcrop #5 in Fig. 2) is almost overlapping with Arruntia and highly eroded by that impact. In each case the olivine-rich exposures are found on the inner-walls at depths between 300 and 900 m below the rim. The lack of ejecta rays at these four craters indicates that they are likely older than Arruntia. Spectra of olivine-rich sites at these craters are displayed in Fig. 3A, and their locations are presented in Fig. 2. Significant differences in reflectance level and spectral slope are induced by admixed background material that is dominated by howarditic and carbonaceous chondritic material. The spectral parameters BT vs. MR of these spectra (Fig. 3B) lie in the dunite polygon (outcrop #4) and it overlaps the harzburgite polygon. The dimensions of the olivine-rich exposures are small in each case, covering less than 1% of the total inner area of the craters, and olivine-rich ejecta is absent in all four cases. Dark material is present in all craters, especially in the crater at 81°E/34°N. The locations of the olivine-rich exposures on the inner crater walls are comparable to those discussed at Pomponia and Bruttia (see Section 5.2). The four craters have been re-shaped by downslope motion of material, and it becomes difficult to relate the olivine exposures to their pre-impact depth. However, the olivine-rich exposures are all located slightly below the crater rims,

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Fig. 3. (A) FC color spectra of three craters on an arc around Arruntia (cp. Fig. 2) which show exposures of dunitic material. The wide range of observed reflectances and the positions in the ratio parameter space and (B) demonstrates a variable admixture of carbonaceous chondritic material. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

and those are far from the expected depth of mantle material on Vesta. The visible thickness of the outcrops in these craters does not exceed 300 m, but some parts are possibly covered by background material. 5.2. Craters Pomponia and Bruttia Pomponia (112°E/71°N) and Bruttia (86°E/64°N) are located in the north-polar region (see Fig. 1). Pomponia has a diameter of 59 km, shows relatively steep sloped (30°) inner walls, and has a maximum depth of 9 km. Pomponia was identified by Schenk et al. (2013) and Vincent et al. (2013) as belonging to a group of pre-complex craters. They are flatter than the typical bowl shape and some of them present evidence of early modification characteristics of the transition from simple to complex, such as the slumping of crater walls, formation of terraces, or uplift of central material. Pomponia’s overall morphology shows a non-circular rim, with evidence of edge failures and mass wasting. The relatively low depth to diameter ratio of Pomponia (0.16) could indicate a slight uplift of the floor. This is not observable directly, as slumped material erased such topographic features on the crater floor. Due to the illumination conditions Pomponia and Bruttia were only observable during and after the HAMO2 mission phase, and even then, about half of both craters were still shaded. Pomponia is of special interest to us since the shape of the 1-lm absorption band indicates olivine at several sites. Fig. 4A displays a number of FC color spectra, which have been taken at specific Regions of Interest (ROI, see Fig. 5A) at craters Pomponia and Bruttia. The majority of the surface at craters Bruttia and Pomponia is

Fig. 4. (A) FC color spectra of Regions of Interest (ROIs) defined in Fig. 5 at crater Pomponia and Bruttia. While the majority of the surface of both craters is composed of howarditic material (e.g., maroon and magenta ROIs), major spectral variations, induced by admixtures of carbonaceous chondrites (dark material) and dunites (e.g. blue ROI spectrum), are found at exposures on their inner walls. Dunites are located also on the central floor of Pomponia (see Fig. 5A for location of ROI). Subfigure (B) identifies lithologies derived from the color spectra in the parameter space Mid Ratio vs. Band Tilt (see text for explanation). In contrast to the howarditic background material, the olivine-rich sites plot in the dunite (>90% olivine) or harzburgite polygons (>40% olivine). Thus they are quite distinct from the average Vesta spectrum (black dot) and the local background. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

howarditic (magenta and maroon ROIs) with admixtures and outcrops of carbonaceous chondritic and olivine-rich material (green, blue, yellow and red ROIs in Fig. 5A). These olivine-rich outcrops are of dunitic or harzburgitic type as indicated by Fig. 4B. While the smaller crater Bruttia only exhibits one exposure of olivine-rich material, the crater Pomponia shows several exposures similar to the crater Bellicia (Thangjam et al., 2014a,b). Outcrops of dunitic material on the inner wall of Pomponia extend up to 200 m horizontally and up to 600 m vertically along the sloped inner wall, where the exposure along the slope is significantly enlarged by downslope movement. While Fig. 4A/5A display some selected spectra and their spatial locations, Fig. 5B highlights all olivine-rich exposures at Bruttia and Pomponia by reddish colors. The olivinerich exposures on the eastern and western inner wall seem to originate from a depth of 900 m, while the northern outcrops are located about 1200 m deeper, probably due to the surface elevation variations during deposition. The western exposures are located on the upper rim of an older, smaller crater, which has been partly superposed by Pomponia. Some minor olivine exposures can also be found outside Pomponia’s rim, which could be ejecta or impacts into subsurface olivine-rich layer(s). Interestingly, the appearance of dark and olivine-rich material on the inner wall of Pomponia is similar, i.e. both lithologies seem to outcrop at several localities just beneath the crater rim and both materials

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Fig. 5. (A) Projection of ROIs on the photometrically corrected HAMO mosaic of crater Pomponia. Spectra in Fig. 4 have the same color representation. (B) Reddish colors in this 3D view of crater Pomponia (right) and Bruttia (left) indicate the presence of dunite-or harzburgite-rich material. The used RGB color scheme is as in Fig. 2.

move downslope creating fans. Dark material and olivine-rich exposures are sometimes located close to each other and the olivine-rich exposures are found at similar depths as the dark material. We cannot draw a final conclusion on their relative stratigraphic succession, but it seems that both lithologies are of similar age. These findings are in agreement with observations of dark material at Bellicia. Fig. 6C displays a profile across an olivine-rich exposure on the inner wall of Pomponia. We observe transitions from, and to, howarditic background material (spectrum #1, #30), local olivine-rich material (e.g., spectrum #14), and a sharp boundary between dunitic and harzburgitic material (spectra #14 and #15). The transitions between the different materials can be abrupt, i.e. within one or two pixels (100 m) and thus indicate well-defined olivine-rich lenses or layer(s). Curvilinear topographic fine structures cause discontinuous trends in the profile sequence shown in Fig. 6A/C. A further dunitic site is located on the floor (cyan ROI in Fig. 5A) surrounding a small crater of 1 km diameter. The origin of this floor material is of special interest: either it was originally excavated from the deep seated subsurface by the Pomponia impact and then exposed by the small impact, signifying an endogenic origin, or it was deposited by the small impact later on and thus is of exogenic origin (impactor material). However, more likely is that the olivine was deposited on the floor by a process that involved avalanches, i.e. the olivine-rich material was initially located near the surface, like the outcrops on the inner wall of Pomponia, but then, due to a collapse of the southern rim, moved downward and finally ended up at the floor. This scenario is reasonable since, for example, the crater Marcia shows intact lenses of dark material which obviously slid down the inner crater wall. Besides Pomponia, only the crater Arruntia (Thangjam et al., 2014a,b) shows olivine-rich material on its floor. In this case it is obvious that the olivine-rich material has moved downslope from near-rim outcrops. Crater Bruttia is closest to Pomponia among the identified exposures. It is also located at a

similar distance from Albana (see Section 6) as Pomponia. However, the exposure at Bruttia shows a significantly higher reflectance despite nearby dark material in the inner wall. Otherwise the spectral shapes at Pomponia and Bruttia are similar as shown in Fig. 4A. In fact, its position in the parameter plot Fig. 4B is most remote from the HED polygons. In summary, the appearance, location, and size of the olivine-rich outcrops at Pomponia and Bruttia are comparable with those found at Bellicia, Arruntia, and in the vicinity of Arruntia, suggesting a common origin of these sites. 5.3. Crater Portia Located in the equatorial highland (41°E/1°N, see Fig. 1), Portia is a moderately eroded crater (Fig. 7C) of 14 km diameter and 2.5 km depth with a maximum slope of 40°. Its irregular shape appears enlarged by downslope motion. Its northeastern rim shows a number of small spots indicating an olivine signature (Fig. 7A). The band parameters BT vs. MR are similar to those of the exposures near Arruntia, placing them in the overlapping region between the dunite and harzburgite polygons close to the eucrite polygon in a context of howarditic background (Fig. 7B). Although many further impact craters are present in its environment, the olivine spectral signature is only confined to that isolated area. Olivine-rich material is not present on the floor of Portia, where much of the slumped material has accumulated. Instead it is embedded in an area of material with spectral properties differing from the central and southern parts of the crater, as can be identified by the dark-greenish tint in Fig. 7C. The potential olivine-rich patch has basically a circular outline, with a deviation from this shape in the lower inner part of Portia. This suggests the presence of an older crater, whose wall and bowl-shape has been erased by the Portia impact. If this olivine-rich material is not delivered from a remote or exogenous source, intrusion from

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Fig. 6. (A) Spectral variation along Pomponia’s inner crater wall, crossing a prominent olivine-rich outcrop. Transitions from, and to, howarditic background material, local olivine-rich concentrations, and mixed material due to downslope movement can be identified. Subfigure (B) plots the spectral parameters BT vs. MR and associated lithologies. While spectra at the top of the inner wall (#1, brown) are close to the average (howarditic) spectrum of Vesta, subsequent increasing concentrations of dunitic material is observed downwards, reaching a maximum at spectrum #14 (cyan). Then mixing likely causes the material to become apparently harzburgitic like (#15, orange), and finally howarditic again (#30, black). Interestingly, the transition from dunitic to harzburgitic material is abrupt between spectrum #14 and #15, coinciding with a transition from a compact outcrop to a smooth mixed layer of downslope movement. Subfigure (C) displays the locations of the individual spectra (A) along the trace on the inner wall. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. (A) Framing Camera spectra of exposed olivine-rich spots on the northern crater wall of Portia, identified in the 3D view of subfigure (C) – RGB color scheme as in Fig. 2. The red dots in (C) are olivine-rich (dunite/harzburgite, see (B)) and have been excavated by small impacts. It seems that an olivine layer or lens is present in the subsurface. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

greater depth of Vesta remains as an explanation. In each case, this site is unique by the apparent presence of an extended subsurface layer of 50 km2 that could be olivine-rich.

brighter than its environment at LAMO clear filter resolution of 20 m/pixel. Since in this area near the equator, several surface features are visible which follow the east–west direction of the Divalia Fossae, the site may have been influenced by geologic processes related to them. However, a stratigraphic relationship between the exposure and the Fossae is not evident. In summary, we conclude that this site is only potentially rich in olivine, since its spectrum is also consistent with some eucrites.

5.4. Crater at 55°E/5°S

5.5. Matronalia Rupes

At 55°E/5.5°S, 65 km east of Portia (see Figs. 1 and 8C) a crater of 300 m diameter is filled and surrounded by material showing a potential olivine signature (Fig. 8A/B). The spectrum is almost identical with those measured at Portia (cf. Fig. 7A) and thus a similar nature and common origin is feasible. The background material is howarditic as for Portia. Fig. 8C shows the crater, which is clearly

Matronalia Rupes is a huge scarp that marks a major part of the rim and wall of the Rheasilvia basin, extending 80° in longitude. This scarp is mainly composed of diogenitic-rich materials (see spectrum #2 in Fig. 9A). However, we have identified at 70°E/ 52°S three aligned small exposures of dunitic material in its upper slope (Fig. 9C) about 3 km below its rim. They horizontally extend

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Fig. 8. Potential olivine-rich site around a small crater at 5°S, 55°E. The FC color spectrum presented in (A) is an average of some pixels at location 1 in (C). This site plots in the overlapping zone between the dunite, harzburgite, and eucrite polygons.

about 1 km and each is about 100 m high and 200 m wide. Locally, the slope reaches 48° and is relatively constant over more than 10 km perpendicular to the isohypses. This indicates a relaxed flow in downslope motion of the granular background surface material. Surrounded by a relatively smooth surface, the olivine-rich material is found in rather rough outcrops, and interestingly no downslope motion of the olivine-rich material is found, which could be an indication for a relatively high material strength. Fig. 9A compares the average spectrum (#1) of the three olivine-rich outcrops with the surrounding diogenitic material. The reflectance level is among the highest measured for our olivine detections, and the diogenitic background material is bright. Otherwise the spectrum of the olivine-rich exposures is similar to those found near Arruntia, in particular in parameter space (Fig. 8B), i.e. it is located in the transition zone of the dunite and harzburgite polygons. Considering the possible origin, the location of the outcrops 3 km below the upper rim of Matronalia Rupes may be coincident with a pre-Rheasilvia surface, but definitely with a horizon within the upper crust of Vesta. Because of the size of the basin, a much thicker ejecta layer than at the other craters has been deposited, and an exposure of material at the pre-Rheasilvia impact level

Fig. 9. Three exposures in the upper part of the elongated Matronalia Rupes scarp, bordering the Rheasilvia basin, showing spectral signatures of olivine (A, #1). The background material (A, #2) is located at the center of the diogenite polygon in parameter space (B), while the olivine-rich exposures are located in the overlapping area of dunite and harzburgite. The LAMO image (C) identifies the small outcrops, which correspond to spectrum #1. Note the variation in material texture from clumpy to a smooth appearance near these olivine-rich exposures.

becomes seated deeper than elsewhere. The complete lack of similar exposures in a huge nearby outcrop at 70°E/53°S of material with similar compacted or blocky morphology is puzzling, especially in the light of a potential origin or deposition associated with Rheasilvia.

5.6. Crater Sossia Crater Sossia (286°E/37°S) is a young impact crater at the inner wall of the Rheasilvia basin with a diameter of 8 km and a depth of 2.5 km (Fig. 10C). Sossia is located at the southern end of an 80 km long bundle of narrow filaments of dark material, associated with faulty morphology, which crosses the crater Urbinia (276°E/30°S) and almost stretches to crater Drusilia (261°E/15°S).

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Fig. 10. Located at the wall of the Rheasilvia basin, crater Sossia shows a complex distribution of materials including small potential exposures of olivine as indicated by their FC spectra (A). Subfigure (B) shows these three selected exposures in our spectral parameter space, near the transition between dunite, harzburgite, and eucrite. The low reflectances indicate mixing with carbonaceous chondritic material, which is visible in the 3D view (C). This subfigure is two-times elevation-enhanced, emphasizing the strongly sloped terrain. The given numbers correspond to the spectra in (A).

It forms the southern extension of the ‘‘dark ribbon’’ identified by Buczkowski et al. (2014). The crater itself has basically a bowl shape, but from opposite sides in the northwest and southeast, downslope motion has generated a discontinuous contact of the respective slopes at the bottom of the crater. In Sossia’s environment several unusual FC color reflectances are present, which may be caused by exogenic chondritic material. The northwestern inner wall of Sossia (Fig. 10C) has a spectral signature which is compatible with olivine-rich material, as indicated by spectra of the exposures (Fig. 10A) in this well-illuminated area, and its position in parameter space (Fig. 10B). However, large differences in reflectance level and parameter position in the border area between dunites and eucrites indicate influences of dark material, especially on spectrum #2. Some potentially olivine-rich exposures

Fig. 11. (A) Reflectance spectrum of the olivine-rich site #1 on the inner wall of crater Tarpeia that is located on Rheasilvia’s floor. This olivine-rich site plots in the overlapping region between dunites and eucrites (B). On the 3D view of Tarpeia (C) site #1 is marked (north is right).

are found in areas of highest reflectance within the inner crater wall. This material is only found on the northwestern slope, and it may have been enlarged by the downslope motion. Its present extent is 1 km in an elongated area along the slope of about 40°. In this sector, dark material ejecta can be found outside the crater. Their spectra indicate strong variable mixing with the dark background material and cannot be analyzed regarding the lithology of olivine-rich material. Although the location near the top of the wall of an impact structure resembles that of the craters described before, a well-established genetic relationship to the Rheasilvia basin is not supported, since the exposure’s extent is negligible compared with the size of the basin. A stratigraphic conclusion would be further complicated by the local spectral diversity. In summary, Sossia cannot be ranked as a site that is certainly olivine-rich. 5.7. Crater Tarpeia We found one potential olivine-rich exposure in Rheasilvia’s floor far from its outer wall. In the deepest part of the basin, the

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showing different spectral properties populate the whole region. Other nearby spots of similar size are spectrally indistinguishable from their howarditic environment. None of those shows a similar spectrum to the discussed patch. Considering the small size, a connection with the uplift of material on the central mound from deep layers of Vesta seems less plausible.

6. Discussion

Fig. 12. (A) Reflectance spectra of an olivine-rich site (1) on the Rheasilvia central mound and a near-by background area (2). The olivine-rich site (1) is dunitic (B) while the background is howarditic.

crater Tarpeia (£ 38 km) hosts a small site on its inner wall (30°E/ 70°S; see Fig. 11C). The spectrum (#1 in Fig. 11A) plots on the overlapping zone of the dunitic and eucritic polygons (Fig. 11B) and thus the site remains questionable. However, if this site is indeed olivine-rich, then it is the only one detected on the floor of this huge basin. 5.8. Rheasilvia central mound For latitudes south of 85°S no HAMO resolution data is available, but color data from the Survey orbit (250 m/pixel) reveal a potential olivine-rich spot at 10°E/86°S of about 2 by 1 km size (see Fig. 12C). The spectrum of the displayed pixel1 (#1 in Fig. 12A) is indicative of a dunitic site (Fig. 12B), which changes rapidly at the adjacent pixels of the mosaic to a howarditic composition. This potential olivine-rich exposure is located in a shallow depression, possibly a crater, in a setting of intermediate elevation on the mound. The background of the potential olivine-rich spot is rather howarditic (see spectrum #2 in Fig. 12A/B). Sinusoidal flow features 1

All other spectra in this paper are an average of 2  2 pixels.

We are reporting additional sites and expanded evidence for olivine exposures on Vesta in addition to an earlier paper (Thangjam et al., 2014a,b). These newly reported exposures complete the picture between the extended olivine-rich layer(s) at craters Bellicia and Arruntia and newly discovered exposures elsewhere on Vesta. We have determined the mineralogy of the surface material by using a method described by Thangjam et al. (2014b), which allows us to discriminate olivine-rich from HEDdominated lithologies (see also Section 3). Aside from the olivine-rich sites described here, it is unlikely that further exposures larger than 0.02 km2 above our detection limit of 40 wt.% olivine exist.2 In order to find out whether the olivine-rich materials at the various craters are possibly of similar composition and origin, we plot in Fig. 13 the spectral parameters BT vs. MR for those localities at each crater that show the strongest olivine signature. From Fig. 13 it is evident that most outcrops are certainly dunitic, but outcrops at Tarpeia, central mound and Sossia are ambiguous, i.e. their composition could be dunitic or eucritic. We found a trend in which the northern outcrops are remote from the eucrite polygon, while the southern sites are close to or in the overlapping zone between eucrites and dunites and the equatorial sites are between them. This finding strengthens the hypothesis that the northern sites, and possibly also the equatorial ones up to Matronalia Rupes, are of common nature and origin, while the southern ones in Rheasilvia are different. Numerous impacts have redistributed the original stratigraphy of Vesta’s crust. Therefore a distribution due to granular mixing of different components of pure dunite and Vesta-average howardite can be expected to follow a trend stretching from lower left (howardite) to upper right (dunite) in Fig. 13. This is present in our samples, with the northernmost (purest) and southernmost (mixed) spectra at opposite ends. This trend is possibly the result of the radial distribution of the Rheasilvia impact ejecta. While Ammannito et al. (2013) assign the olivine on Vesta to ancient northern basins, Ruesch et al. (2014) favor its origin in Rheasilvia. The anti-correlation of olivine dominance and distance to a presumed source at Rheasilvia is unexpected in the latter case. According to Ruesch et al. (2014), the Rheasilvia event ejected material northward along 30°E, overturned it and thus exposed olivine on the surface. Thus they propose that the olivine just north of the crater rim is of Rheasilvia origin, while the olivine in and around Bellicia and Arruntia is either of Rheasilvia or local origin. Ammannito et al. (2013) favor an endogenic origin in which ancient large impacts excavated and incorporated large blocks of diogenite-rich and olivine-rich material into the eucritic crust, with subsequent impacts exposing this olivine-rich material in Arruntia and Bellicia. These processes would lead to olivine-rich terrains in a howarditic background. Our data do not show diogenite-rich material in Bellicia, Arruntia and Pomponia. Also at Pomponia, where the spectra approximate related properties, the obvious pyroxenitic diogenite signature, which has been found at Antonia in the Rheasilvia basin, is not seen. Marchi et al. (2012) reported several eroded basins in the vicinity of Arruntia and Bel2

Apart from regions which were in shade during HAMO.

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Fig. 13. Spectral parameters BT vs. MR for selected exposures, showing the highest olivine content at several prominent olivine bearing sites. We found a trend in which the olivine content decreases from north to south.

licia, one of which is about 180 km across, with relative excavation depths of about 10–15 km. Excavation of endogenic olivine-rich mantle material by such an impact, followed by re-excavation by further impacts seems feasible, assuming a crustal thickness of 15–20 km (McSween et al., 2013; Mittlefehldt, 2014). However, the fact that olivine-rich material is virtually absent in the Rheasilvia basin suggests that smaller impacts were insufficient to excavate mantle material. A solution to this problem is the assumption of a deeper mantle–crust boundary in the Rheasilvia relative to further north regions. However, this assumption is unrealistic if the exposed materials inside the southern basins originate from depths of about 60–100 km, as modeled by Jutzi et al. (2013) and did not probe olivine-rich material (Clenet et al., 2014). The latter argue that not only the giant Rheasilvia impact, which has created a huge uplifted central mound, but also the combination with the earlier Veneneia impact of similar size and outcome has to be considered. These must have created a multilayer structure in the zone of excavated material in the basin(s). Cheek and Sunshine (2014) suggested that the olivine-rich exposures in Arruntia and Bellicia are of shallow crustal origin, probably signifying a late-stage serial magmatism. The peculiar olivine-rich exposure at crater Portia could represent such a special case. It is distant from the major olivine-rich exposures near Bellicia, Arruntia and Pomponia, and also from the Rheasilvia basin. If this site is explained by an origin on Vesta, local volcanic intrusion may have to be considered. An outstanding result of our survey of olivine-rich exposures is their presence in different geologic settings, all of which are relatively small-scale and related to impact structures of various sizes. Fine detailed properties like grain or particle sizes cannot be tested by our data. Individual differences in context, morphology and spectral properties indicate that a common scenario for all olivine-rich sites may not exist. Thus, the dichotomy between a cluster of significant detections in the northern hemisphere, and ambiguous exposures in the largest impact basin Rheasilvia is striking. Our global map of olivine-rich exposures shows all of them related to impact craters, and the majority of them have been found in their inner walls near their rims. It can be said that the appearance of most olivine-rich sites on Vesta is similar to that of the dark material sites (Reddy et al., 2012a; Nathues et al., 2014b), although the olivine-rich sites are less frequent. The outcrops as well as the ejecta imply that the olivine-rich layer(s) or lenses are no more than 200 m thick. The position in the inner crater walls near the rims and at a similar level as the near sub-surface of the apparent pre-impact level clearly indicate that the cratering is the cause for the present visibility of the layer (Jin et al., 2013; Shankar et al.,

2013). The association with impacts implies major alteration to the original layer and its composition: the material must have been scattered over a larger area than the pre-impact layer, and consequently mixed and diluted. Much of this original layer has become undetectable, and the original volume was much larger than what is presently visible. In fact, the estimated volume of a projectile which has created one of the large northern craters is of a similar order as the one which can be deduced from the present exposures in the group of northern olivine-rich exposures. The potential endogenic origin raises the question if known large impacts on Vesta have been deep enough to excavate and expose material of the mantle. In these cases, the ages of the exposed layers must be older than the associated deep impact. Likely none of the impact structures showing olivine, except the Rheasilvia basin, are deep enough to probe Vesta’s mantle, and even that one seems to have failed to reach it. In this case, a significant exposure of olivine-rich material can be expected in its central mound, which, however, we do not observe. Instead our data reveals a howarditic composition that is in agreement with McSween et al. (2013). We do not know with certainty if the figure of Vesta’s mantle has reached equilibrium (Fu et al., 2014), or if local variations of crustal depths exist. We neither see significant support for such conclusions by Dawn’s gravity data (Park et al., 2014) nor by the compositional evidence from neutron backscatter observations by the GRaND instrument (Prettyman et al., 2012). Furthermore, if the detected olivine on Vesta is of endogenic origin then one could expect that a relation to the determined material density exists. However, we do not see a preferential distribution relative to outstanding gravity anomalies (Park et al., 2014). There is a region of higher density to the west of the northern olivine sites, which may be caused by a broad ejecta layer emerging at the Rheasilvia basin. But this region only partly coincides with the concentration area of olivine, and Pomponia is located in an area of lower density. Previous attempts to explain the olivine on Vesta (Ammannito et al., 2013; Ruesch et al., 2014) neglect an important finding: almost all olivine-rich sites in and around Bellicia, Arruntia and Pomponia are located in outcrops just a few hundreds of meters below the surface in an almost horizontal arrangement, which is very similar to the appearance of the dark material on Vesta. As a consequence, the alternative to an endogenic origin, the delivery by exogenous projectiles, needs to be discussed, as for the dark material (Reddy et al., 2012; Nathues et al., 2014b). The survival of impactor material, particularly in oblique impacts, coupled with low velocity, has been demonstrated for terrestrial and lunar cases (Pierazzo and Melosh, 2000; Bland et al., 2008; Yue et al., 2013). This sets constraints on their source regions in the main-belt, since only low impact velocities preserve significant amounts of projectile material, i.e. the source must be dynamically close to the orbit of Vesta. Objects with this property must have a composition matching the spectral properties of the olivine-rich outcrops, which are characterized by a high enrichment, reaching up to 90% olivine. While the numerous S-type asteroids have spectral signatures of olivine and pyroxene, only the rare A-type asteroids show an almost pure olivine composition. However, S-type asteroids of the sub-classes S(I) to S(IV) (Gaffey et al., 1993; Sanchez et al., 2014) show spectra that exhibit a clear olivine signature. The number of olivine-rich exposures outside the Arruntia– Pomponia–Bellicia area is low and only a few impacts of small Sor A-type impactors would be needed to explain them. The craters Arruntia–Pomponia–Bellicia roughly form an arc which has its center in the large Albana crater (diameter 90 km). Fig. 14 demonstrates that this area has slightly different spectral appearance when combining the following ratios of reflectances: R and G = 1/ 3 (1 R0.92 lm/R0.55lm), B = R0.75lm/(R0.65lm + R0.83lm). Except for some poorly illuminated areas in craters, a gradient of the yellow

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Fig. 14. False color mosaic of the background of the olivine-rich areas. The image covers the longitudes from 0°E to 135°E and the latitudes from 66°S to 85°N. For explanation see text.

color intensity is visible toward the south-eastern rim of the crater Albana. The channels R and G describe the correlation between the reflectance at 0.55 lm and the depth of the 1-lm absorption band, while the B channel describes the start of that band with smaller values indicating a start at shorter wavelengths. Except for the Albana context, the G value globally varies ±0.0015 around an average of 1.009. The G value rises to more than 1.02 at the rim of Albana, while the value of the B channel decreases from 0.56 to 0.52. The higher R and G values indicate a comparably shallow 1lm absorption band, consistent with the delivery and admixture of projectile material by an S- or A-type asteroid. This distribution supports a connection with the Albana impact, which may have deposited this material. The degree of erosion of Albana indicates an origin prior to the Rheasilvia impact. Thus the depths of the locally exposed outcrops in the younger craters are consistent with a subsequent covering by ejecta from those. Very rough estimates of the volume of today’s olivine-rich material, and that deposited at Albana, can also be accommodated (of the order of 100 km3). Inside Albana or even farther north, no further olivine-rich exposures have been found, but most of this surface is not illuminated by the Sun in the Dawn data. In order to test an alternative to an evolved A- or S-type impactor Le Corre et al. (in preparation) investigated the possibility of an L or H-chondritic impactor which could contain a significant amount of olivine (up to 57 vol.%; Dunn et al., 2014). With the exception of the sites at crater Tarpeia, Sossia and Matronalia Rupes, all of them are embedded in a howarditic background. Diogenites are virtually absent in the vicinity of the olivine-rich outcrops in and around Bellicia, Arruntia and Pomponia. This is astonishing since one would expect that endogenic olivine is rather associated with diogenites than with howardites. Whether the potential olivine-rich exposures in the Rheasilvia

basin are of endogenic origin is difficult to fathom. If these are indeed olivine-rich outcrops, their low number and small size are further indication for an exogenic origin, rather than for uplifted or unearthed mantle material, which itself should show large-scale structures concurrent with the geologic context. Despite the potential delivery of olivine-rich asteroid material, the absence of obvious mantle material on Vesta’s surface may be linked with its general rarity among asteroids. This is not only true for the specific olivine-rich taxonomic A-type in the main belt (Bus and Binzel, 2002). The ‘missing mantle problem’, i.e. the scarcity of asteroidal bodies having a composition similar to mantle materials of differentiated and disrupted bodies (Burbine et al., 1996), is a wellknown dilemma. Recently, the composition of HED meteorites (Kiefer and Mittlefehldt, 2014) and the Nice model of early dynamical evolution in the asteroid region (Morbidelli et al., 2010) indicated a rapid differentiation of Vesta, which implies a persistent stripping and accretion at the surface, possibly even a fragmentation and re-accretion of the whole body (Consolmagno et al., 2014). Such a scenario of counteracting constructive and destructive processes does not only result in a much deeper-seated mantle, but also in a late accretion of both differentiated and primitive projectiles. This is in agreement with the high porosity and great depth of the regolith observed even in the oldest areas of the surface (Denevi et al., 2012) and the low compaction at impact sites in these areas (Hoffmann et al., 2012). The absence of a detectable olivine-rich mantle raises also the question whether the material from which Vesta accreted was indeed of CI or of H + CM composition as proposed by Toplis et al. (2013). We are currently testing petrological models based on various E + H mixtures to further constrain the composition of Vesta’s restitic mantle material. If the contribution of enstatite chondrite material is higher in bulk Vesta than previously

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assumed, a restitic mantle would result in lower modal olivine and higher orthopyroxene contents. In this case, even large impacts excavating rocks from 60 or 80 km may predominantly deposit olivine–orthopyroxenite instead of peridotite material. This would explain the vast regions covered by pyroxenitic diogenites and the scarcity of diogenites with more than 40% olivine. Other compositions like, for example, of enstatite chondrites, as proposed for the Earth, are reasonable explanations for the lack of olivine on Vesta (Javoy et al., 2010). We found only two potential arguments against an exogenic origin of all olivine-rich material on Vesta: (1) the olivine-rich subsurface layer at Portia that is possibly a remnant of an intrusion; (2) plastically deformed peridotitic diogenites show similar patterns of lattice preferred orientation (LPO) as Earth’s upper mantle peridotite (Tkalcec et al., 2013; Tkalcec and Brenker, 2014). However, at this stage it is not clear if such peridotitic diogenites with distinct LPO are derived from Vesta’s mantle or from another differentiated body of the main belt. Consequently, the majority of the identified olivine-rich sites on Vesta must have exogenic sources, and thus it seems we have only minor evidence for endogenic olivine-rich material. If this is correct, then one needs to question the correctness of many evolution models (e.g., Ruzicka et al., 1997; Righter and Drake, 1997) regarding the predicted depth of the olivine-rich mantle. 7. Conclusions/summary Based on the previous results and discussions we conclude the following:  A discontinuous dunitic subsurface layer exists in the Bellicia– Arruntia–Pomponia region that is exposed in several medium to large impact craters. It is not found outside the craters, except for obvious fresh ejecta, neither is it found with increasing abundance at progressively greater subsurface depths as would be expected for mantle material.  This layer is up to 200 m thick and has an extension of about 60,000 km2. The total volume is consistent with the ejection of projectile material from a crater of more than about 50 km diameter. The Albana impactor is a possible source of the Bellicia–Arruntia–Pomponia olivine. Its heavily gardened ejecta blanket is apparently unilateral, consistent with an oblique impact, which is more likely to preserve its projectile material.  Olivine-rich subsurface lenses exist further south up to the rim of the Rheasilvia basin at Matronalia Rupes. These exposures are distant from the northern outcrops.  Morphological affinity and similar stratigraphic age of dark and olivine-rich material are strong arguments for an exogenic origin of both.  The presence of olivine on Vesta is not necessarily related to achondritic material, but may be linked to pristine, chondritic origin (Le Corre et al., in preparation).  The olivine-rich subsurface layer at crater Portia could be alternatively of endogenic origin, if the morphology and size are considered.  The identified olivine-rich sites at craters Tarpeia and Sossia can be alternatively eucritic. The latter is located in a geologically complicated area involving a wide range of reflectances.  The central mound of the Rheasilvia basin shows one olivinerich spot at 10°E/86°S. But this location has no further characteristics to assign a specific origin.  Summarizing, endogenic olivine on Vesta does not or only marginally exist on the surface of Vesta. This means, we do not see unique signs of mantle material on Vesta’s present surface.

 Thus, most, if not all, of the detected olivine-rich sites are of exogenic origin, i.e. due to the infall of olivine-rich impactors.  A direct conclusion of this evidence is that the Vesta mantle must be at a greater depth than the excavation depth of the Veneneia and Rheasilvia impactor, alternatively, the upper mantle contains less than 40% olivine and thus cannot be detected by FC data. Both conclusions suggest that Vesta’s parent material was already depleted in olivine compared to CI meteorites.

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