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Vesta, vestoids, and the HED meteorites: Interconnections and differences based on Dawn Framing Camera observations

B. J. Buratti,1P. A. Dalba,1M. D. Hicks,1V. Reddy,2M. V. Sykes,2T. B. McCord,3 D. P. O’Brien,2C. M. Pieters,4T. H. Prettyman,2L. A. McFadden,5Andreas Nathues,6 Lucille Le Corre,6S. Marchi,7Carol Raymond,1and Chris Russell8

Received 10 January 2013; revised 2 August 2013; accepted 1 September 2013; published 3 October 2013.

[1] The Framing Camera (FC) on the Dawn spacecraft provided thefirst view of 4 Vesta at sufficiently high spatial resolution to enable a detailed correlation of the asteroid’s spectral properties with geologic features and with the vestoid (V-type) asteroids and the Howardite-Eucrite-Diogenite (HED) class of meteorites, both of which are believed to originate on Vesta. We combine a spectral analysis of the basin with visible and near-IR spectroscopy of vestoids and with archived data over the same spectral range for HED meteorites. The vestoids are only slightly more akin to the Rheasilvia basin than to Vesta as a whole, suggesting that the crustal material ejected is a well-mixed collection of eucritic and diogenitic materials. The basin itself is more diogenitic, implying Vesta is differentiated and the impact that created Rheasilvia uncovered a mineralogically distinct layer. The Rheasilvia basin exhibits a larger range in pyroxene band strengths than Vesta as a whole, further implying that the basin offers a view into a complex, differentiated protoplanet. The

discrepancy between the spectral properties of the HED meteorites and Vesta, in particular the meteorites’ deeper pyroxene absorption band and the redder color of the vestoids, can be explained by the abundance of smaller particles on Vesta and by the addition of low-albedo exogenous particles to its surface, which in turn are due to its larger gravity and longer exposure time to impact processing. Solar phase effects are slight and do not explain the spectral discrepancies between the HEDs, Vesta, and the vestoids.

Citation: Buratti, B. J., et al. (2013), Vesta, vestoids, and the HED meteorites: Interconnections and differences based on Dawn Framing Camera observations, J. Geophys. Res. Planets, 118, 1991–2003, doi:10.1002/jgre.20152.

1. Introduction

[2] The largest asteroids in the Main Belt are miniplanets mirroring on a smaller scale the history and geologic evolution of the terrestrial planets. Chief among these is 4 Vesta, a differentiated body holding 9% of the mass of the asteroid belt. A common class of terrestrial meteorites, the Howardite-Eucrite-Diogenite (HED) assemblage, presents a similar—although diverse—mineralogy to that of Vesta (as inferred from reflection spectra), and it is believed to

originate on the asteroid [McCord et al., 1970]. Another piece of the picture is provided by the existence of the V- type, or vestoid, class of asteroids that appear to be dynami- cally related to Vesta [Zappala et al., 1990] and that present similar reflection spectra to that of Vesta [McCord et al., 1970; Binzel and Xu, 1993; Moskovitz et al., 2010]. These vestoids also span a range of orbital space between powerful resonances that can deliver them to near-Earth space [Wisdom, 1985], with possible assistance from the Yarkovsky effect [Bottke et al., 2006]. (We use the terms

“V-type asteroid” and “vestoid” interchangeably.) The dy- namical properties of the bulk of vestoids are consistent with their origin on 4 Vesta, including some dynamical outliers such as 809 Lundia and 956 Elisa, in an impact occurring at least 1 BY ago [Nesvorny et al., 2008]. Finally, a large im- pact basin was discovered in the south pole of Vesta [Thomas et al., 1997; Li et al., 2010; Marchi et al., 2012; Schenk et al., 2012]: this large basin, named Rheasilvia by the Dawn team and adopted by the IAU, implies an energetic impact that could have propelled a large number of fragments into escape orbits, some of which could evolve onto near-Earth orbits.

Geologic analysis and crater age dating of the basin suggest that it is the youngest large basin on Vesta, at only ~1 Gyr old [Marchi et al., 2012; Schenk et al., 2012], which is in gen- eral agreement with estimates of the age of the Vesta family from collisional evolution modeling [Marzari et al., 1996,

1California Institute of Technology, Jet Propulsion Laboratory, Pasadena, California, USA.

2Planetary Science Institute, Tucson, Arizona, USA.

3Bear Fight Institute, Winthrop, Washington, USA.

4Department of Geological Sciences, Brown University, Providence, RI, USA.

5NASA Goddard Space Flight Center, Greenbelt, MD.

6Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany.

7NASA Lunar Science Institute, Boulder, Colorado, USA.

8Department of Earth and Space Sciences and the Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA, USA.

Corresponding author: B. J. Buratti, California Institute of Technology, JPL-Jet Propulsion Laboratory, 4800 Oak Grove Dr. 183-401, Pasadena, CA 91109, USA. (bonnie.buratti@jpl.nasa.gov)

©2013. American Geophysical Union. All Rights Reserved.

2169-9097/13/10.1002/jgre.20152

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1999] and dynamical models [Nesvorny et al., 2008]. Although this picture presents a consistent story, many details remain to be worked out and several inconsistencies need to be resolved.

The open issues include the generally stronger band strengths and redder visible spectral slopes in vestoids and HEDs [Burbine et al., 2001; Pieters et al., 2006]; the seeming absence or minor role of lunar-type space weathering on Vesta [Pieters et al., 2012] and possibly vestoids; the correlation of the HEDs and vestoids with specific locations—most importantly the basin—and depths on Vesta; the lack of correlation of the

age of the HEDs and the age of the basin [Marchi et al., 2012; Schenk et al., 2012]; whether all HEDs and vestoids originated on Vesta; and more generally, the identity of other differentiated asteroids in the Main Belt and their possible identity with asteroidal fragments and terrestrial meteorites [Lazzaro et al., 2000; Reddy et al., 2011]. Answers to these questions will tie the evolution of protoplanets such as Vesta to that of the terrestrial planets, and shed new light on both impact processes and the mechanisms for the injection of asteroids into the near Earth environment.

Figure 1. Clear-filter images obtained with the Framing Camera showing four views of Vesta covering non-Rheasilvia regions (a); one image with both basin and nonbasin terrains (b), and an image including only the Rheasilvia southern impact basin. See Table 1 for a list of the images used.

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[3] In this paper we investigate the connections between Vesta, vestoids, and the HED meteorites with two new data sets: images from the multispectral Framing Camera (FC) on the Dawn spacecraft [Sierks et al., 2011; Reddy et al., 2012a] and recently acquired spectra over the same

wavelength range of vestoids that are Near Earth Objects (NEOs) (M. D. Hicks et al., Spectral Diversity and Photometric Behavior of Near-Earth and Main-Belt V-type Asteroids. sub- mitted to Icarus, 2013). These data sets are augmented with the existing measurements of HEDs in the RELAB data archive Table 1. Summary of Dawn FC Observations

R.S. in Image? Central Latitude Central Longitude Date (UTC) Spatial Resolution (km/pixel) Solar Phase Angle

No 284° 2011-07-24 0.484 43°

No 196° 2011-07-24 0.484 40°

No 107° 2011-07-24 0.484 37°

No 16° 2011-07-24 0.484 34°

Yes 72° 72° 2011-08-12 0.258 46°

Partially 17° 109° 2011-07-25 0.487

In each case, we used images taken in channels 2, 5, and 7, which correspond to central wavelengths of 0.43μm, 0.75 μm, and 0.92 μm, respectively.

Figure 2. The same sequence of images as Figure 1, but with color ratios of images obtained with the 0.42μm and 0.75 μm filters, which we call the “blue/red” ratio. The images used are listed in Table 1.

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(http://www.planetary.brown.edu/relab), providing measure- ments for the third main piece of the puzzle, as well as previ- ously published spectra of NEO vestoids that span the spectral range of the FC camera [Binzel et al., 2004]. The FC spacecraft data enable thefirst detailed visible disk-resolved study of the spectral properties of Vesta, including the correlation between geologic units and spectroscopic mea- surements, which in turn will provide the clues for under- standing the connections between HEDs, vestoids, the Rheasilvia basin, and Vesta as a whole.

2. Data Analysis

[4] The Framing Camera (FC) on Dawn was designed to study the geology, morphology, and spectral properties of the surface of Vesta and 1 Ceres [Sierks et al., 2011;

Le Corre et al., 2011]. The FC is a 5.5° square CCD with 1024 × 1024 individual detectors sensitive over the visible and near-IR range (0.40 to 1.05μm). Seven narrow band (40 nm bandwidth) filters at 0.43, 0.55, 0.65, 0.75, 0.83, 0.92, and 0.98μm span this range; in addition, a clear broad- bandfilter spanning the 0.45 to 0.92 μm spectral range was also included. Although detailed compositional analyses can- not be done with only a few filters and a limited spectral range, two of the most important spectral features of Vesta can be studied with the FC: the slope between 0.4 and 0.7μm and the depth of the pyroxene absorption band at 0.92μm. The position of this latter band is a sensitive indica- tor of pyroxene geochemistry: the minimum occurs at shorter wavelengths for magnesium-rich diogenites than for more iron-rich eucrites. Howardites are brecciated mixtures of these two types of meteorites, and provide a continuum in Figure 3. The same sequence of images as Figure 1, but with color ratios of images obtained with the

0.92μm and 0.75 μm filters, which we call the “IR/Red” ratio. The images used are listed in Table 1.

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spectral properties between the two end-members [Pieters et al., 2006]. The slope between 0.4 and 0.75μm is due to charge transfer transitions in pyroxene [Burns, 1993], and for S-type asteroids it is a marker for the degree of space weathering [Pieters et al., 2000; Hapke, 2001]. Threefilters are well placed for studying the visible slope and the depth of the pyroxene band: the filter bands at 0.43 μm and 0.75μm for the slope and those at 0.75 μm and 0.92 μm for the band depth. For the FC data, our strategy has been to obtain ratios of images at these filters for comparison to vestoid and HED spectra.

[5] From the Dawn approach and survey phases, we chose a series of global images in the threefilters that show a view of Vesta without Rheasilvia, a global view with Rheasilvia, and a view of Rheasilvia only (see Figure 1 and Table 1). Three images in each of the key wavelengths (0.43μm, 0.75 μm, 0.92μm) were calibrated with the Dawn calibration proce- dures available with the ISIS-3 software that is available from the United States Geologic Survey (http://isis.astrogeology.

usgs.gov/; All images are available on the NASA Planetary Data System). These procedures included geometric correc- tions, flatfielding, bias subtraction, radiometric corrections, and removal of the solar spectrum. For each of the three views, a spectral ratio of 0.43μm/0.75 μm (“Blue/Red”) and 0.92μm/0.75 μm (“IR/Red”) was obtained (see Figures 2 and 3). Thefirst ratio represents the visible slope caused by the broad charge transfer absorption band, with a smaller ratio representing a steeper slope, while the second ratio represents the depth of the pyroxene bands, with a smaller number representing a deeper band. Figure 2 shows that the slope

between 0.43 and 0.75 in the visible spectrum of Vesta appears to be uniform across the face of Vesta, in spite of the large albedo variegations on the asteroid [Reddy et al., 2012a]. The one region with a lower ratio signifying a red visible color is just south of the equator near 100° longitude and is associated with the ejecta blanket of the crater Oppia Figure 4. The values for each pixel from the color ratio images (Figures 2 and 3) shown as a color-color

plot for the four images off the Rheasilvia basin.

Figure 5. The values for each pixel from the color ratio im- ages (Figures 2 and 3) shown as a color-color plot for the im- age on the Rheasilvia basin.

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(see panel 2b). This same region was noted by Reddy et al.

[2012a]. In contrast, the IR/Red mosaic shows a much broader range, especially in the Rheasilvia basin. While the FC does not include the full range of the pyroxene absorption band, the slope between 0.75μm and 0.92 μm is a proxy for band location because the slope decreases as the band position moves to longer wavelengths, if the band depth does not change (see the next section). The ratios for each pixel in the images are shown in a scatter plot in Figures 4 and 5, with IR/red on the x axis and blue/red on the y axis. Figure 4 shows the ratios from non-Rheasilvia regions while Figure 5 shows the ratios for the Rheasilvia regions.

[6] For the equivalent colors from vestoid spectra, we extracted reflectances at 0.43 μm, 0.75 μm, 0.92 μm from two data sets: our own measurements obtained with the Double Spectrometer and the 200 inch telescope on Mount Palomar (Hicks et al., submitted manuscript, 2013), and spectra previously obtained by Binzel et al. [2004]. Because of the rare appearances and short apparition times for a spe- cific NEO, there was virtually no overlap between the two data sets. Relative reflectances at the same wavelengths as those of the FC filters were extracted from the spectra for each asteroid. These spectra were calibrated to photometric standard stars, and a spectrum of Vesta obtained with the same instruments and the same technique shows good agree- ment with previous ground-based observations of Vesta (see

Hicks et al., submitted manuscript, 2013). We limited our data set to the Near Earth population, as it is this subset of vestoids that is the immediate source of the HED meteorites (although ultimately of course, the NEOs come from the Main Belt). This set of asteroids has a large range in solar phase angles, so a correction was required for viewing geom- etry [Reddy et al., 2012b]. (Hicks et al., submitted manu- script, 2013) derived a correction for the effects of solar phase angle for both the visible slope (B-R) and the band depth (R-I), and we applied their values of 0.002 mag/° and 0.0016 mag/°, respectively, to both the FC data and the vestoid data. Because the RELAB data were obtained at a

“solar” phase angle of 30°, the Dawn and Palomar data were both corrected to 30°. The Blue/Red and IR/Red ratios uncorrected for solar phase angle are shown in Figure 6, while the corrected ratios are shown in Figure 7 (The ranges plotted for Vesta and the basin represent all the values within 2σ). The small change between the two figures illustrates that the corrections for phase reddening are not great and cannot explain the spectral discrepancies between the vestoids and Vesta. For comparison we also show spectra of other stony asteroids from Binzel et al. [2004]: the Q-type, the S-type, and the A-type. For the S-type, we plot the 10 lowest-numbered asteroids with error bars to express the range in color. No solar phase angle corrections are done for the non-V-type bodies, as (Hicks et al., submitted manuscript, 2013) derived Figure 6. A color-color plot showing the visible slopes (y axis) and the band depth (x axis) of vestoids

(black dots), Vesta off the Rheasilvia basin (pastel orange), and on the Rheasilvia basin (pink). The dotted oval is the position of the diogenite meteorites from the RELAB archive, and the solid oval represents the eucrite samples. (The howardite class of meteorites dwells intermediate between the diogenites and eucrite end-members.) For comparison, other stony asteroids are shown, illustrating that only the vestoids are near the spectral properties of Vesta. The“dark terrain” on Rheasilvia is the material with the smallest visible slope and deepest pyroxene band: it is represented primarily by the ejecta material surrounding the Oppia crater. It is outside of the 2σ range of the basin’s colors.

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the corrections only for V-type asteroids. The spectrum de- rived of Vesta from the Double Spectrometer at the Hale Telescope on Mount Palomar at the time of the Dawn orbit insertion (Hicks et al., submitted manuscript, 2013) is shown;

the sub-Earth latitude of ~40° closely matched the Dawn ap- proach trajectory and included views of substantial amounts of the Rheasilvia basin. The lack of precise agreement be- tween this well-calibrated spectrum and the FC colors shows that additional corrections could be made to the FC absolute camera calibration based on ground-based measurements.

[7] Over 100 spectra of the Howardite-Eucrite-Diogenite (HED) assemblage are archived in the RELAB database at Brown University (http://www.planetary.brown.edu/relab).

Most of the spectra were obtained from particulate samples with particle sizes<25 μm, but there is a sufficiently large number of samples with particle sizes between 25 and 250μm to study the effect of particle size on spectral proper- ties. For simplicity, we focus on the eucrite and diogenite end-members as many previous studies have shown the howardites to be intermediate between these two classes in spectral properties [Pieters et al., 2006; De Sanctis et al., 2012a, 2012b]. But the integrated spectrum of Vesta most closely resembles Howardites; as Vesta, it is composed of both eucritic and diogenitic material [Pieters et al., 2011;

De Sanctis et al., 2012a, 2012b]. Figures 8 and 9 show the eucrite and diogenite spectra obtained from the RELAB archive in their original units and normalized to 0.75μm for easy comparison with the FC and vestoid measurements.

[8] Although the blue to red ratio offers the widest spec- tral range to intercompare the spectral properties of Vesta,

V-type asteroids, and the HED meteorites, we performed a similar analysis between the 0.55μm filter, which we call the VIS band pass, as it is close to the astronomical visible filter, and the 0.75 μm filter. The results are shown in Figure 10. The solar phase angle correction for the V-R colors was 0.0086 mag/°, based on (Hicks et al., submitted manuscript, 2013).

3. Results

[9] The results plotted in Figure 7 show that the spectral properties of the vestoids are the closest match among the stony asteroids to the spectral properties of Vesta. Even if there were other differentiated asteroids that provided source material for some V-type asteroids, the dynamical arguments strongly connect the vast majority of them to Vesta [Bottke et al., 2006; Nesvorny et al., 2008]. Figure 7 also shows that the Rheasilvia basin and the regions outside of Rheasilvia have distinctly different spectral properties. First, the Rheasilvia basin exhibits a wider range of pyroxene band depths than the off-Rheasilvia Regions. This difference could also be due to different band positions, as both the FC and our own vestoid data from Palomar do not give a definitive minimum for this spectral band; a band at larger wave- lengths would be exhibited as a smaller slope between 0.75μm and 0.92 μm. This finding is consistent with the view that the impact that caused the Rheasilvia basin, as well as a second basin (Veneneia), excavated large volumes of compositionally diverse material that spanned the range of HED mineralogy. Second, Vesta as a whole could have Figure 7. The same as Figure 6, except the vestoids and the FC camera data have been corrected for the

effects of viewing geometry (solar phase angle): 0.002 mag/° for the B-R ratio, and 0.0016 mag/° for the I-R ratio (Hicks et al., submitted manuscript, 2013). The effects of viewing geometry are slight, and do not explain the spectral discrepancies between the HEDs, Vesta, and the vestoids.

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been gardened—and possibly space weathered—into a more compositionally homogeneous surface than the basin area. Geologic evidence and cratering studies point to both basins at Vesta’s south pole to be 1–2 billion years old [Schenk et al., 2012; Marchi et al., 2012]. Chemical analysis of the products of the short-lived radionuclides

26Al and60Fe provides evidence that Vesta differentiated within a few million years of the CAl’s formation [McSween and Russ, 2010]. Thus, the nonbasin regions of Vesta have been subject to the effects of gardening and weathering for more than twice as long as the basins, resulting in a more homogeneous composition. In addition, the northern hemisphere has not been excavated by large basins, which expose deeper crustal material that is different in composition to the upper part of the crust, so the compo- sition is necessarily more uniform.

[10] Previous studies showed that the vestoids are redder in the visible than both HEDs and Vesta [Burbine et al., 2001;

Pieters et al., 2011; Marchi et al., 2010]. Our sample of NEO vestoids shows a slighter trend in this direction, as NEO vestoids are closer to the HEDs (particularly the eucrites) than Main Belt vestoids, which is expected as the

NEO objects are the reservoir for terrestrial meteorites.

However, the range of IR/Red colors of the eucrites is more like the Rheasilvia basin rather than Vesta as a whole, while the Blue/Red color of the diogenites is closer to the basin.

This result suggests that the material excavated from the impact and subsequently forming the vestoids and HED meteorites included a wider range of pyroxene mineralogies comprising both the surface eucrite-howardite mixture and more diogenitic material from deeper inside Vesta. The lower layer exposed by the impact is more diogenitic, as the Rheasilvia basin’s color is more consistent with that type of meteorite. Indeed, the estimated depth of the basin is twice that of the estimated depth of Vesta’s crust (H. Y. McSween et al., Composition of the Rheasilvia basin: A window into Vesta’s interior, submitted to Journal Geophysics and Research, 2013): this combination implies that the mantle should be exposed. The lack of significant color variations in the visible (except for the one or two craters with fresh ejecta blankets) implies that the upper crust of Vesta may be well mixed as the result of impact gardening. One possi- bility to be investigated with future observations is that vestoids are rubble piles.

Figure 8. The available spectra from the RELAB’s eucrite collection are plotted on the lower cell; four particle sizes are represented. On the top, each spectrum is normalized to unity at 0.75μm. The spectra obtained at smaller particle sizes tend to have shallower pyroxene absorption bands at 0.92μm, although the visible slope seems to not be correlated with particle size. The spectrum of Vesta (Hicks et al., submitted manuscript, 2013) is shown for comparison.

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[11] Although the HED class of meteorites shows a strong correlation to Vesta, it has been known for some time that they differ in important details [Pieters et al., 2011]. The most significant difference is that they tend to have deeper spectral bands. Figure 7 shows that we confirm that finding, as well as the result from an analysis of VIR observations on Dawn that the Rheasilvia basin is more diogenitic than other regions of Vesta [De Sanctis et al., 2012a, 2012b].

However, the HED samples from the RELAB archive, partic- ularly the eucrites, are redder than the surface of Vesta. This discrepancy could be due to differences in particle sizes, composition, or the degree of space weathering.

4. Discussion

[12] The Howardite-Eucrite-Diogenite group of achon- dritic meteorites shares a common mineralogy, but their range of lithologies implies that they originated in a differen- tiated body. The three subclasses are thought to occupy different regimes of a layered body, with an iron core and a diogenitic mantle overlain by a eucrite-rich crust [Takeda, 1997; Russell et al., 2012]. This scenario is in general

agreement with the spectral properties derived for the Rheasilvia basin from the Dawn framing camera (Figure 7), with the basin more diogenitic than the bulk asteroid. Our results are in accord with the view that the vestoids come from Vesta, as their visible colors are not consistent with any other type of stony asteroid. Another possibility is that the vestoids came from another differentiated asteroid that was completely disintegrated. The existence of both M-type asteroids in the Main Belt and iron meteorites on the Earth does suggest that there were an array of such bodies that had been disrupted and reduced to their metallic cores. However, there is abundant dynamic evidence that the vestoids came from Vesta, except for a few cases [Nesvorny et al., 2008; Bottke et al., 2006].

[13] We also find that the vestoids are systematically redder than Vesta, especially in the VIS/Red ratio, in agree- ment with many previous studies [Burbine et al., 2001;

Florczak et al., 2002; Moskovitz et al., 2010; Marchi et al., 2010]. On the surface, our results appear to be more consis- tent with the view that the vestoids were ejected from Vesta in many episodes dating from the period of the Late Heavy Bombardment [Binzel et al., 1997], as the vestoids do not Figure 9. The available spectra from the RELAB’s diogenite collection are plotted on the lower cell; four

particle sizes are represented. On the top, each spectrum is normalized to unity at 0.75μm. The spectra obtained at smaller particle sizes tend to have shallower pyroxene absorption bands at 0.92μm, although the visible slope seems to not be correlated with particle size. The spectrum of Vesta (Hicks et al., submitted manuscript, 2013) is shown for comparison.

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share the color of the Rheasilvia basin. Indeed, radioisotope dating implies that the HEDs were last experienced shock heating and the formation of impact melts 3.5–4 billion years ago [McSween and Russ, 2010], although they were most likely ejected without being reset again, as ages are generally reset by sitting in a warm basin or ejecta layer. Another problem with accepting a recent age for Rheasilvia is that very few impacts of this size occurred after the period of the LHB, although this statement may be less true for the asteroid belt. In any case, the volume of material contained in the vestoid class of asteroids, as well as their great number, implies a large impact event that excavated huge amounts of crustal material. Other large asteroids in the Main Belt have no comparable family of associated asteroids: if the vestoids were formed by impact events throughout Vesta’s history, these other large asteroids (Ceres, Pallas, etc.) would have kindred asteroid families as well. Finally, the HED class of meteorites comprises about 6% of terrestrial falls, and the majority of achondrites [McSween and Russ, 2010].

Vesta is unique as a producer of fragments, and its large impact basins [Rheasilvia and Veneneia], which are also unique based on our limited reconnaissance of major asteroids, provide the most plausible origin for both the vestoids and the HEDs.

[14] Both the vestoids and the diogenite meteorites are more closely matched to the color of the Rheasilvia basin than to that of a global Vesta. This result suggests not only that the fragments came from the basin but that the material below the crust is more diogenitic than the surface; our re- sults provide strong evidence that Vesta is differentiated.

We note that the similarity between the vestoids and the basin

compared to the global Vesta is slight, a result that suggests the crust of Vesta is a well-mixed admixture of eucrites and diogenites, in agreement with the observation of Pieters et al. [2011] that the global spectrum of Vesta most closely matches the laboratory spectrum of the howardites. The exposedfloor of the basin is more diogenitic than the whole of Vesta and provides a glimpse into the interior of this differentiated protoplanet. There are several regions of the Rheasilvia basin that exhibit especially deep pyroxene bands and blue visible slopes (these are noted as“dark terrain” in Figure 7 that lie outside the 2σ distribution; the largest region is near 0° longitude and 60° latitude). These regions are the most diogenitic and they may represent the deepest layer of Vesta visible on the surface; indeed, the most extensive region appears to be connected to ejecta features from two large impact craters within the Rheasilvia basin. The Dawn VIR maps also show this region to be anomalously diogenitic [De Sanctis et al., 2012].

[15] What could cause the discrepancy in the systematic differences between the HEDs and Vesta? Plausible reasons include different lithologies, different particle sizes, or space weathering. Another view is that the mantle should be rich in olivine, analogous to that mineral’s presence in the Earth’s mantle [Gaffey, 1997; Reddy et al., 2010]. The addition of a small amount of this mineral to Vesta’s spectrum brings it more in line with the spectra of the HEDs [McSween and Russ, 2010]. Although the identification of specific minerals requires the higher resolution of Dawn’s Visual and Infrared spectrometer (VIR), this mineral’s presence on the surface of Vesta has proven elusive so far [De Sanctis et al., 2012a, 2012b], with a recent possible detection [De Sanctis et al., Figure 10. The same analysis as that represented by Figure 7, except with the VISfilter on the FC, with

corresponding measurements of the vestoids and the RELAB data. The phase angle effect for the V-Rfilter is 0.00086 mag/°.

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2013]. Up to 25% of olivine in the mineralogical mixture might not be detectable at all [Beck et al., 2012; McSween et al., submitted manuscript, 2013]. There appears to be no clear pathway to explain the discrepancy in terms of mineralogy.

[16] Shestopalov et al. [2008] noted several faint absorp- tion bands in the visible spectra of vestoids that they attribute to ferrous iron in low-calcium pyroxene. Although we did not see these bands in our Palomar spectra (Hicks et al., submitted manuscript, 2013), the spectrum of Vesta obtained on the same night exhibits the band at 0.5μm, as do the HED spectra (Figures 8 and 9).

[17] For S-type asteroids (as well as for the lunar surface), the effects of space weathering are to redden the spectrum in the visible and mute the absorption bands [Binzel et al., 2004]. The mechanism is believed to be the enrichment of nanophase metallic Fe formed from impact melting [Pieters et al., 2000; Hapke, 2001]. For Vesta, no such situation exists; in fact, the brighter (fresher) regions are redder.

Based on Dawn results, a completely different mechanism of space weathering is caused by micron-sized opaque parti- cles reducing the spectral contrast and generally darkening the surface [Pieters et al., 2012]. In the lower impact velocity regime of Vesta, brecciation is more important than impact volatization: this form of space weathering is essentially mechanical rather than chemical. (“Space weathering” may even be a misnomer for this process.) In this scenario, the HEDs are in the correct place in Figure 7 for a“fresher,” less weathered surface. The redder color of the vestoids is also explained by space weathering: their surfaces are“fresher”

than that of Vesta. In the VIS/Red graph (Figure 10), this effect is also evident, particularly regarding the comparison between the vestoids and Vesta.

[18] Another alteration mechanism is the addition of spectrally neutral, low-albedo material to Vesta’s surface. A range of techniques enabled by instruments on Dawn, including infrared spectroscopy with VIR [McCord et al., 2012;

De Sanctis et al., 2012a, 2012b] and elemental mapping [Prettyman et al., 2012], as well as theoretical considerations [Reddy et al., 2012c], suggest that this unexpected low-albedo substance on Vesta consists of exogenous hydrated carbona- ceous material. This material should be less prevalent on vestoids, as they possess smaller gravitational wells and thus cannot attract and retain as much exogenous infall. Because the material is spectrally neutral, the vestoids would be redder than Vesta.

[19] Another important factor for explaining the spectral differences may be the particle size of grains in the regolith of Vesta. Pieters et al. [2011] showed that the spectrum of Vesta between 0.3 and 2.6μm fits a typical howardite meteorite if the particle sizes are less than 25μm.

Figures 8 and 9 show that the band depth in both diogenites and eucrites (especially) is less for samples composed of smaller particles. For the eucrites, the band depth is about 20% less for thefine particles. This is about the size of the discrepancy between the meteorites and the FC data for Vesta in Figures 7 and 10. This argument is partly circular because the meteorite data in Figure 7 includes all sizes of particles; it is most likely that a combination of the effects of space weathering (i.e., the addition of spectrally neutral, low-albedo particles) and particle sizes causes the discrep- ancy. Other lines of evidence point to the existence of small

particles on the surface of Vesta. The V-type asteroids are very rough at RADAR wavelengths [Benner et al., 2008], but at visible wavelengths Vesta appears to be smoother than the typical asteroid [Buratti et al., 2012]. This condi- tion is explained by the infilling of facets and asperities on the surface byfine particles, similar to the “ponding” seen on Itakawa and Eros, but on a smaller scale. With the larger gravityfield of Vesta, small particles would be more likely to be retained than on the smaller vestoids. The high albedo of Vesta [Reddy et al., 2012a] also suggests an abundance of fine particles. Ironically, the redder slope of vestoids compared with Vesta wasfirst explained by larger particles [Burbine et al., 2001];

[20] The existence of M-type asteroids in the Main Belt and the presence of iron meteorites on Earth both imply that there were many differentiated asteroids: these bodies must be the core of battered, differentiated protoplanets. Thus, it is possible that some of the vestoids may come from other differentiated asteroids. The surface of 1459 Magnya is basaltic and it is not dynamically tied to Vesta, but it may be the fragment of a larger body [Lazzaro et al., 2000].

Figure 6 shows that the spectral properties of Magnya over- lap those of the vestoids; many of the vestoids, especially the outliers, may be fragments of other differentiated aster- oids that underwent breakup. Clearly, additional dynamical modeling is called for, both to answer the question of whether any vestoids could have been expelled from Vesta in impact events other than the event that formed Rheasilvia, and whether many of the vestoids could have come from other differentiated asteroids. Nesvorny et al.

[2008] showed that the great bulk of the vestoids are consis- tent with an origin on Vesta, including some dynamical out- liers. A small number of vestoids (e.g., 4796 Lewis and 5379 Abehiroshi) have inclinations sufficiently low to preclude their origin in the main Rheasilvia impact event. They most likely originate from another differentiated asteroid, or they were ejected from Vesta during an impact event prior to Rheasilvia’s formation, possibly during the period of the LHB. Is there a possible dynamical pathway for large numbers of vestoids to be ejected in events other than the one that formed the Rheasilvia basin? The Veneneia basin is a likely candidate, since it is older than Rheasilvia and also large [Schenk et al., 2012].

[21] This work has bearing on the cratering mechanics of the Rheasilvia basin and the interior structure of Vesta. The interior could consist of either a well-defined layered crust and mantle, or a conglomerated model [Pieters et al., 2011]. In thefirst case, the central peak should show a highly diogenitic composition, while in the second the composition of the impact region would be more jumbled. The second case is very clearly the right model: if anything, the peak is not particularly diogenitic, particularly in comparison to the region near 0° longitude that does show a high concentration of this mineral [see also De Sanctis et al., 2012a, 2012b]. The diogenite distribution and implications for interior structure are discussed in more detail in (McSween et al., submitted manuscript, 2013).

5. Conclusions

[22] A comparison between three data sets—disk resolved multispectral measurements of Vesta, ground-based spectra

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of vestoids from the NEO population, and laboratory mea- surements of the Howardite-Eucrite-Diogenite population— reveals important interrelationships among these three classes of bodies. The vestoids are the class of asteroids that is most similar to Vesta, based on measurements from the Framing Camera on Dawn and on ground-based measurements of stony near-Earth asteroids (V-type, A-type, S-type, and Q-type).

Together with dynamical studies, these results strongly sug- gest that the vestoids originate on Vesta. The deeper pyrox- ene band of the HEDs (especially the diogenites) can be explained by the presence of smaller particles on Vesta.

The redder color of vestoids and HEDs (in particular the eucrites) can be explained by the presence of dark, spectrally neutral carbonaceous material on Vesta. The Rheasilvia basin shows a wider spectral diversity than Vesta as a whole, imply- ing that Vesta is a complex, differentiated body. In general, the basin is more diogenitic, which implies a differentiated asteroid with a distinct layer that was uncovered by the impact which created the basin. The effects of viewing geometry (solar phase angle in particular) cannot explain the differences between the three classes of bodies.

[23] Acknowledgments. This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology under contract to the National Aeronautics and Space Administration. We acknowledge support from the Dawn Participating Scientist program. This research utilizes spectra acquired by the NASA RELAB facility at Brown University.

We thank Dr. Deborah Domingue and an anonymous referee for their detailed reviews.

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