Icarus 256 (2015) 22–29

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Spectral slope variations for OSIRIS-REx target Asteroid (101955) Bennu:Possible evidence for a fine-grained regolith equatorial ridge

http://dx.doi.org/10.1016/j.icarus.2015.04.0110019-1035/� 2015 Elsevier Inc. All rights reserved.

⇑ Corresponding author.E-mail address: rpb@mit.edu (R.P. Binzel).

Richard P. Binzel a,⇑, Francesca E. DeMeo a, Brian J. Burt a, Edward A. Cloutis b, Ben Rozitis c,Thomas H. Burbine d, Humberto Campins e, Beth Ellen Clark f, Joshua P. Emery c, Carl W. Hergenrother g,Ellen S. Howell h, Dante S. Lauretta g, Michael C. Nolan h, Megan Mansfield a, Valerie Pietrasz i,David Polishook a, Daniel J. Scheeres j

a Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USAb Department of Geography, University of Winnipeg, Winnipeg, Manitoba R3B 2E9, Canadac Department of Earth & Planetary Sciences, University of Tennessee, Knoxville, TN 37996, USAd Department of Astronomy, Mount Holyoke College, South Hadley, MA 01075, USAe Department of Physics & Astronomy, University of Central Florida, Orlando, FL 32816, USAf Department of Physics and Astronomy, Ithaca College, Ithaca, NY 14850, USAg Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USAh Arecibo Observatory/USRA, Arecibo, PR 00612, USAi Division of Geological and Planetary Science, California Institute of Technology, Pasadena, CA 91125, USAj Department of Aerospace Engineering Sciences, University of Colorado, Boulder, CO 80309, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 26 January 2015Revised 4 April 2015Accepted 6 April 2015Available online 16 April 2015

Keywords:AsteroidsAsteroids, surfacesRegolithSpectroscopy

Ongoing spectroscopic reconnaissance of the OSIRIS-REx target Asteroid (101955) Bennu was performedin July 2011 and May 2012. Near-infrared spectra taken during these apparitions display slightly morepositive (‘‘redder’’) spectral slopes than most previously reported measurements. While observationalsystematic effects can produce such slope changes, and these effects cannot be ruled out, we entertainthe hypothesis that the measurements are correct. Under this assumption, we present laboratorymeasurements investigating a plausible explanation that positive spectral slopes indicate a finer grainsize for the most directly observed sub-Earth region on the asteroid. In all cases, the positive spectralslopes correspond to sub-Earth latitudes nearest to the equatorial ridge of Bennu. If confirmed byOSIRIS-REx in situ observations, one possible physical implication is that if the equatorial ridge is createdby regolith migration during episodes of rapid rotation, that migration is most strongly dominated byfiner grain material. Alternatively, after formation of the ridge (by regolith of any size distribution),larger-sized equatorial material may be more subject to loss due to centrifugal acceleration relativeto finer grain material, where cohesive forces can preferentially retain the finest fraction (Rozitis, B.,Maclennan, E., Emery, J.P. [2014]. Nature 512, 174–176).

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1. Introduction

While telescopic measurements are attainable (in principle) forthousands of Solar System small bodies, ‘‘ground truth’’ can beobtained for only a select few. Thus, in situ spacecraft measure-ments and returned sample analysis reside as high level goals inplanetary science (Planetary Science Decadal Survey, 2011).Achieving a direct link between telescopic reconnaissanceand ground truth is one of the primary objectives for NASA’s

OSIRIS-REx sample return mission (Lauretta et al., 2012, 2014)whose target is the near-Earth Asteroid (101955) Bennu (provi-sional designation 1999 RQ36; hereafter Bennu). Upon selectionas a spacecraft target, the �500 m diameter (Nolan et al., 2007,2013) Bennu has been the focus of a wide range of Earth-basedtelescopic measurements (e.g. Clark et al., 2011; Müller et al.,2012; Hergenrother et al., 2013; Nolan et al., 2013).

In this work we follow-up the investigation of Bennu’s visibleand near-infrared reflectance spectral properties presented byClark et al. (2011). Clark et al. (2011) found that Bennu had a rela-tively featureless blue-sloped spectrum and classified it as a B-typeobject. They offered that the best compositional analog was a CM1

R.P. Binzel et al. / Icarus 256 (2015) 22–29 23

chondrite based on Bennu’s spectral properties and low albedo(�0.05) (Emery et al., 2014). Hergenrother et al. (2013) also con-firmed the B-type designation with filter photometry obtained inSeptember, 2005. Fig. 1 illustrates the opportunities for astronom-ical observations of Bennu, which had several favorable apparitionswithin a few years following its discovery. Beyond 2005 however,Earth-based measurements have been considerably more challeng-ing in terms of faint apparent magnitudes, with Bennu rarely beingbrighter than V = 20 prior to the anticipated 2018 arrival of OSIRIS-REx. We present (Section 2) newly obtained spectral measure-ments that show some differences in spectral slope relative tothe results of Clark et al. (2011) and Hergenrother et al. (2013),but we note that we cannot conclusively rule out these differencesas being observational systematic effects. Circumspectly treatingthe new observations at face value, in Section 3 we consider geo-metric aspects of the asteroid to ascertain possible causes for theslope variation. In Section 4, we consider the laboratory measuredscattering properties of the postulated analog meteorite class (car-bonaceous chondrite) to explore additional causes for slope varia-tion, where fine-grained particle sizes appear to exert the mostdominant effects. In Section 5 we make a brief examination ofthe possible variability of Bennu in the context of possible physicalprocesses, followed by a brief summary of conclusions (Section 6).

2. Observations

Asteroid Bennu was discovered September 11, 1999 by theLINEAR Survey (Stokes et al., 2000) during a particularly favorableapparition (Fig. 1). Over the next few weeks, ‘‘1999 RQ36’’ was well-placed as an excellent target of opportunity object for physicalstudy. Visible wavelength spectra were obtained at McDonaldObservatory by E. Howell and reported by Clark et al. (2011).Similarly, near-infrared spectroscopy measurements at the NASAInfrared Telescope Facility (IRTF) using the SpeX instrument(Rayner et al., 2003) were obtained and subsequently reported byDavies et al. (2007). Binzel et al. (2004) noted ‘‘1999 RQ36’’ as anexcellent low Delta-V mission candidate with one imminent

Fig. 1. Earth-based observing circumstances for (101955) Bennu, beginning with its discothe tick marks, highlighting the challenging circumstances for the 2011 and 2012 data

favorable apparition (V16.0) in September 2005 that would notbe exceeded until September 2060 (V12.2). Near-infrared spectralmeasurements obtained at the IRTF during that 2005 opportunitywere reported by Clark et al. (2011) and filter photometry mea-surements obtained using the Kuiper 1.5 m telescope werereported by Hergenrother et al. (2013). Dates and circumstancesfor all of these previous observations are presented in Table 1.

Motivated by NASA’s 2011 selection of the OSIRIS-REx missionwith Bennu as its target, additional observations of Bennu wereundertaken during its challenging July 2011 and May 2012apparitions (Table 1). We utilized the 6.5 m Baade Telescope atthe Magellan Observatory in Las Campanas, Chile, obtainingnear-infrared spectral measurements using the FIRE (Folded-portInfraRed Echellette) instrument described by Simcoe et al.(2013). Our processing procedures for the spectral images are fullydescribed by DeMeo and Carry (2014). In brief, we used FIRE inlow-resolution prism mode with a slit width of 0.8 arcsec orientedalong the parallactic angle. Exposures of 95.1 and 126.8 s wereused to avoid saturation due to thermal emission beyond 2.2 lmfrom the instrument and telescope. The solar analog stars usedfor calibration were L105-56 (G5 V) and L102-1081 (G5 IV)(Drilling and Landolt, 1979), two stars whose excellent calibrationrelative to Hyades 64 (spectral classification of G8 V; Hardorp,1978) is known through our asteroid spectral reconnaissance pro-gram on the IRTF (Binzel et al., 2011). Neon-argon lamp spectrawere taken for wavelength calibration. Quartz lamp dome flatswere taken for flat field corrections. Within our Spextool(Cushing et al., 2004) pipeline settings, we used the automaticaperture routine (boxcar = 0) and local sky subtraction (nolo-cal = 0). Our use of the long slit (50 arcsec) allowed ample samplingof the background sky for our sky subtraction.

Our resulting reflection spectra from the Magellan FIRE obser-vations are shown in Fig. 2, where we compare these spectra withthose presented by Clark et al. (2011). Within the much poorersignal-to-noise limits owing to Bennu’s faint magnitude, theMagellan spectra from 2011 and 2012 are consistent with thefeatureless nature of Bennu’s spectrum. These lower signal-to-noisespectra, when formally classified over their near-infrared spectral

very in 1999 (arrow). Epochs for each of the spectral measurements are denoted bysets. An asterisk denotes the OSIRIS-REx projected arrival in 2018.

Table 1Observational circumstances for astronomical spectra of (101955) Bennu and spectral slope measurements for both Bennu and the Murchison CM2 carbonaceous chondrite. Theastronomical measurements (see Section 2) are ordered from near-equatorial to increasingly polar sub-Earth latitudes (/), determined using JPL Horizons and the pole solution ofNolan et al. (2013). Spectral slope (change in relative reflectance per micron) is calculated over the wavelength range 0.48- to 0.95-lm for the visible (VIS) data and over 1.0- to2.20-lm for the near-infrared (NIR) data. Negative spectral slopes for Bennu circumstantially coincide with mid-latitudes away from the equator (shaded entries). Heliocentricand geocentric distances are denoted by r, D; a is the solar phase angle; K describes the sub-Earth meridian. All dates are for the mid-point of the measurements, where theKuiper 1.5 m entry is the mean across multiple nights. Laboratory measurements of the Murchison meteorite are described in Section 4. Published references for the spectralmeasurements are: 1 = This work; 2 = Clark et al. (2011); 3 = Davies et al. (2007); 4 = Hergenrother et al. (2013).

Fig. 2. New spectral measurements of Bennu (Magellan data) relative to previousmeasurements presented by Clark et al. (2011) and Davies et al. (2007). All spectrafall within the broad category of ‘‘C-complex’’ asteroids, with the negative spectralslope falling within the sub-class denoted as ‘‘B-type.’’ All spectra are alsocompatible with the range of spectral characteristics for carbonaceous chondritemeteorites.

24 R.P. Binzel et al. / Icarus 256 (2015) 22–29

range, fall within the C-complex of asteroid taxonomy (DeMeoet al., 2009). This is not in disagreement with Bennu’s B-type clas-sification from Clark et al. (2011), as we note that a B-type is a sub-set within the broader C classification (Tholen, 1984). Nor is thereany disagreement with the Clark et al. (2011) CM-chondrite mete-orite analog conclusion: the spectra presented here fully fall withinthe realm measured for carbonaceous chondrite meteorites, asdemonstrated in Section 4.

Spectral slope alone characterizes the distinction between theMagellan spectra and the measurements of Clark et al. (2011).Spectral slope variations can and do arise from systematic observa-tional effects, often related to seeing conditions and choice of spec-trographic slit and atmospheric dispersion (where somewavelengths might be more refracted than others and lost fromthe slit). However, as well demonstrated by Filippenko (1982),these effects are minimal at near-infrared wavelengths and areotherwise mitigated by slit orientation along the parallactic angle,as was done for the Magellan measurements. We further evaluated

possible systematic effects, by thoroughly cross-checking ourBennu measurements using different standard stars across multi-ple nights. Through all of these various alternatives, the Bennuresults persisted with their slightly positive slope while otherknown asteroids measured in the same manner produced results(spectral slopes) matching measurements obtained in other pro-grams on other telescopes. Thus we are unable to identify anyspecific systematic error to attribute the otherwise unexpectedpositive spectral slope for Bennu that results from our newMagellan measurements. In Table 1 we tabulate least squares spec-tral slopes for all available measurements. A wavelength range of0.48- to 0.95-lm is used for the visible (VIS) data and 1.0- to2.20-lm for the near-infrared (NIR) data.

3. Geometric considerations

While the possibility of unaccounted systematic effects cannotbe completely ruled out, we proceed to take at face value the morepositive (‘‘redder’’) spectral slope characteristics of the Magellanmeasurements for Bennu. We proceed in this manner so as to fullyexplore the possibilities for what the OSIRIS-REx spacecraft mayreveal in situ, and as noted above, to inform the limits for interpre-tation of groundbased spectroscopic reconnaissance.

We first attempt to understand physical differences in theobservational circumstances that could be a factor in the spectralvariations. An examination of Table 1 shows that neither phaseangle nor sub-Earth meridian (preferentially looking at morningor afternoon side) is unique to the Magellan measurements. Therotation period of Bennu is not known with enough precision topinpoint the longitudes of the observations for measurementsseparated by several years. Hergenrother et al. (2013) reports asynodic rotation period of 4.2905 ± 0.0065 h, where theuncertainty corresponds to about 0.1% of the rotation period.Thus knowledge of the sub-Earth longitude (corresponding toone full rotation) is lost after �1000 rotations, corresponding toabout six months; our measurements are separated by years.Thus we have as an unknown whether these measurements sam-pled a completely different rotational hemisphere from all others.However, we consider this unlikely due to the long integrationtimes of the Magellan measurements, each of which sample about180 degrees of longitude within the relatively short 4.3 h rotationperiod of Bennu, making it likely that some longitude overlapoccurred during the two observation intervals. In addition the

Fig. 3. Reflectance spectra of different spots on a saw-cut face of the MurchisonCM2 carbonaceous chondrite (all measured at i = 30�, e = 0�).

R.P. Binzel et al. / Icarus 256 (2015) 22–29 25

visible wavelength spectra obtained at McDonald Observatory byE. Howell (reported by Clark et al., 2011) were specifically investi-gated for rotational variation. While a few percent variation occursin the spectral slope, this is within the range of normal spectropho-tometric calibration and atmospheric variability. Most impor-tantly, all of the McDonald visible spectra across all rotationalphases maintain a negative slope.

From Table 1, the only geometric variable that might beaccountable, under the assumption that the Magellan positiveslopes are real, is the sub-Earth latitude. We note the Magellanmeasurements and those by Davies et al. (2007) all show positiveslopes and happen to sample Bennu at nearly equatorial sub-Earthlatitudes. A strong latitude dependence is an intriguing speculationfor Bennu as its overall spherical shape is dominated by an equato-rial ridge, as revealed through radar imaging (Nolan et al., 2013).Such equatorial ridges are a known archetype among near-Earthasteroids, previously observed, for example, on (66391) 1999KW4 (Ostro et al., 2006). Walsh et al. (2008, 2012) present a modelin which such a ridge is created through spin-up due to theYarkovsky–O’Keefe–Radzievsky–Paddack (YORP) effect, a torqueapplied on the asteroid by the momentum of sunlight’s photons(Rubincam, 2000). Assuming a strength-less ‘‘rubble-pile’’ struc-ture, the model of Walsh et al. (2008, 2012) shows how theincreasing rotation rate causes the transport of material frommid-latitudes towards the equator, gradually forming an equato-rial ridge. Whether finer grain material or coarser grain materialor boulders dominate the flow in creating the ridge is not under-stood; thus OSIRIS-REx has the potential to yield measurementsthat strongly influence our understanding of YORP and regolithmigration processes – particularly if those processes might natu-rally lead to spectral variations as purported by the groundbasedspectra.

4. Spectral variation investigations for carbonaceous chondrites

We begin this section by briefly summarizing the currentlyknown scope of physical processes that have possible relevanceto variations in spectral slopes for carbonaceous chondrites. Forexample, space weathering is somewhat of a catch-all term thatencompasses various processes that can affect planetary surfaces(Hapke, 2001). Various space weathering relevant laboratoryexperiments conducted on carbonaceous chondrites show bothspectral reddening and bluing (e.g., Lazzarin et al., 2007; Morozet al., 2004; Vernazza et al., 2013; Brunetto et al., 2014). Of mostprobable to relevance to Bennu are space weathering studies ofthe more primitive, aqueously-altered carbonaceous chondrites,which show spectral reddening in the case of Mighei, likely dueto melting and recrystallization of olivine (Moroz et al., 2004),and reddening or bluing in the case of the Tagish Lake ungroupedcarbonaceous chondrite (Vernazza et al., 2013). Space weatheringtrends inferred from spectroscopic observations of dark asteroidssuggest that space weathering can produce spectral reddening(Lazzarin et al., 2007) or bluing (Nesvorny et al., 2005; Lantzet al., 2013); for the case of carbonaceous chondrites we believethe expected outcome from weathering is too ambiguous to beconclusive as an attributable cause. While space weathering isrelated to irradiation, Hiroi et al. (1993) additionally found thatheating (particularly the maximum lifetime temperature experi-enced) alters carbonaceous chondrite sample properties in a waythat yields bluer slopes; but no mechanism for why such a long-term process should have a latitude dependence is readilyapparent.

To further illuminate possible reasons why spectral slopes ofdifferent latitudes of Bennu may vary, we have chosen to investi-gate in more detail how various parameters can affect the spectralproperties of carbonaceous chondrites, using the Murchison CM2

chondrite as a spectral proxy for Bennu, even though Murchisonmay possess weak absorption bands not seen in the spectra ofBennu. Reflectance spectra of Murchison were newly acquired atthe University of Winnipeg’s Planetary SpectrophotometerFacility using an ASD FieldSpec Pro HR instrument that acquiresdata from 350 to 2500 nm, with a spectral channel step sizebetween 2 and 7 nm. The data are internally re-sampled by theinstrument to output data ultimately at 1 nm intervals. Data belowapproximately 400 nm may be affected by low signal levels possi-bly leading to spurious spectral slopes. Unless otherwise indicated,spectra were measured at a viewing geometry whereby the lightsource was placed at an incidence angle i = 30� and the flux wasmeasured at an emission angle e = 0�. Incident light was providedby an in-house 50 W quartz–tungsten–halogen collimated lightsource. Sample spectra were measured relative to a Spectralon(Labsphere, North Sutton, NH) standard and corrected for minor(less than �2%) irregularities in its absolute reflectance. Between200 and 1000 spectra of the dark current, standard, and samplewere acquired and averaged, to provide sufficient signal-to-noisefor subsequent interpretation.

Fig. 3 shows reflectance spectra measured on different spots ofsaw-cut faces of Murchison (all measured at a constant phase angleof 30�), which have variable amounts of roughness. It can be seenthat variations in surface roughness, assuming that the composi-tion of the slab is horizontally homogeneous at the area scale ofspot measurements (�5 mm), can have a large effect on overallspectral slope, causing it to range from overall blue to stronglyred. Thus, the differences in slope of the Bennu spectra could beattributed to surface roughness effects if a solid, regolith-free sur-face is present.

To investigate whether differences in surface texture of a pow-dered regolith across Bennu could account for spectral slope differ-ences, we measured reflectance spectra of a powdered sample ofMurchison (<150 lm grain size) with different densities and tex-tures: as a powder poured into a sample cup with a smooth surfacemade by drawing the edge of a glass slide across the sample to pro-vide a flat surface, the same sample with a light dusting of airfallMurchison powder, and the same sample pressed into the samplecup with the face of a glass slide to provide a denser sample with aflat surface. While all three spectra are red-sloped beyond 700 nm(Fig. 4), the ‘‘dusted’’ sample is both darker and more red-slopedthan the other two spectra: it has a 2.4/0.6 lm reflectance ratioof 1.5 vs. �1.25 for the other two spectra, consistent with otherstudies (e.g., Schröder et al., 2014). While we are not able to effecta change from red- to blue-sloped, our results indicate that surfacetexture ± particle density can change overall slopes of carbona-ceous chondrites.

Fig. 4. Reflectance spectra of a <150 lm powdered sample of the Murchison CM2carbonaceous chondrite for three different packing densities (see text for details; allspectra measured at i = 30�, e = 0�).

Fig. 6. Reflectance spectra of a single subsample of the Murchison CM2 carbona-ceous chondrite measured during progressive crushing (see text for details; allspectra measured at i = 30�, e = 0�).

26 R.P. Binzel et al. / Icarus 256 (2015) 22–29

For completeness, we also explore phase angle as a possiblecause of spectral variability on carbonaceous chondrites. Fig. 5shows reflectance spectra of a sample with particle size <90 lmsample of Murchison measured at various phase angles relativeto a Spectralon standard measured at i = 15� and e = 0�. It can beseen that phase angle appears to have a large effect on overallreflectance, particularly at large emission angles, consistent withprevious studies (Cloutis et al., 2011a; Johnson et al., 2013).While these spectra do not change from being red-sloped tobecoming blue-sloped, their spectral slopes do vary across a widerange: 2.4/0.6 lm reflectance ratio varies between �1.2 and 3.However as noted in Section 3, the observations of Bennu whichshow changes in spectral slope differ only modestly in phase angle,suggesting that phase angle effects are not a likely cause of theslope differences.

In part because the above processes do not appear to give a con-sistent explanation, but mainly because the shape of Bennu impliesregolith migration forming the equatorial ridge, we focus most par-ticularly on particle size effects on spectral slope. Variations in thegrain size of carbonaceous chondrites can cause spectral changes,specifically increasing average grain size leads to darker and moreblue-sloped spectra (e.g., Johnson and Fanale, 1973; Cloutis et al.,2011b, 2013; Gillis-Davis et al., 2013). To better isolate theseeffects from possible sample heterogeneities, we progressivelycrushed a subsample of Murchison, crushing it enough to enablethe entire subsample to pass through a sieve, beginning with the

Fig. 5. Reflectance spectra of a <90 lm powdered fraction of the Murchison CM2carbonaceous chondrite measured relative to Spectralon at i = 15� and e = 0�.

coarsest sieve (<1000 lm), measuring its reflectance spectrumafter it all passed through a sieve, then further crushing the sampleto enable it to pass through the next finest sieve, measuring itsreflectance spectrum, and repeating the process. The reflectancespectra of the progressive crushing series are shown in Fig. 6.Consistent with previous results, it can be seen that the samplebecomes progressively brighter and more red-sloped with decreas-ing grain size. Variations in average grain size could plausiblyexplain the latitudinal variations in spectral slope seen on Bennu.Generally smaller particles along the equatorial ridge are consis-tent with the results of Nolan et al. (2013) which suggests thatthe regolith is smoother at decimeter scales at the equator thanat the poles.

Available images of small near-Earth asteroids, such as Itokawa(Saito et al., 2006), show a variety of surface morphologies, rangingfrom boulder fields to areas that appear smooth at the resolution ofthe imaging systems. To determine the importance of smallamounts of fine-grained material on the spectral properties ofcoarser-grained carbonaceous chondrites, we measured reflectancespectra of mixtures of <45 lm size powders with 500–1000 lm sizepowders of Murchison Fig. 8. The 100% 500–1000 lm Murchisonpowder is blue-sloped, while the <45 lm powder is brighter andred-sloped, as expected. What is notable is that even 5% of

Fig. 7. Reflectance spectra of the Murchison CM2 carbonaceous chondrite underthree different preparations that progressively change from negative to positiveslopes: (i) A ‘‘coarse’’ sample where 100% of the grains are in the size range of 500–1000 lm. (ii) A mixed distribution where 5% of the grains are ‘‘fine’’ powder <45 lmwhile 95% are in the coarse 500–1000 lm range. (iii) A ‘‘fine’’ sample where 100% ofthe grains have sizes <45 lm. All spectra were measured at i = 30�, e = 0�.

R.P. Binzel et al. / Icarus 256 (2015) 22–29 27

<45 lm powder added to the coarse fraction changes the slope fromblue to red, indicating that the finest fraction has a major effect onspectral slope, with fine-grained powders introducing spectral red-dening (Schröder et al., 2014).

5. Asteroid spectral variations: A broader context view

Our working hypothesis, if we consider the variability of Bennu’sspectral slope to be real and not from undiagnosed systematiceffects in the measurements, is that the equatorial ridge of Bennuhas a redder spectral slope than the higher latitudes. A finer grainsize for the equatorial regolith, relative to grain size at higherlatitudes, is our preferred interpretation for this presumptive slopedifference. While we cannot rule out an overall pole-to-equatorcompositional change (rather than a particle size effect), we do notprefer this hypothesis as the YORP-induced migration of materialmust induce some mixing that is likely to homogenize any differ-ence of compositional units that could possibly be present. Thusstark compositional contrasts, if present, would likely be mutedby regolith migration (especially considering that telescopic viewsintegrate an entire hemisphere). Similarly, we cannot rule out anoverall redder northern hemisphere on Bennu; all of our Earth-based views are looking most directly ‘down on’ latitudes belowBennu’s equator. In other words, we cannot rule out that what weinterpret as an equatorial effect could be a hemispheric dichotomy.All viewing geometries on Bennu that present a more equatorialaspect also allow more surface area from the northern hemisphereto come into view. However, for similar reasons to the regolith mix-ing argument above, we prefer to limit our speculation just to theproperties of the equatorial ridge owing to it being a specific distin-guishing and dynamically created feature of Bennu.

Here we give some brief consideration, physically, as to howfine-grained material might predominate along the equatorialridge. As a starting point, the presence of the ridge itself is evidenceof a previous era involving a rotation rate higher than that forBennu’s current 4.3 h period. (As noted previously, the workinghypothesis is YORP induced spin-up formed the ridge through

0.0 0.5 1.0 1.5 2.0 2.5

Wavelength (µm)

0.4

0.6

0.8

1.0

1.2

1.4

1.6

RelativeReflectance

Grayscale: Bennu astronomical data

Murchison: 100% Fine, < 45µmMurchison: 5% Fine, 95% CoarseMurchison: 100% Coarse, 500-1000µm

Fig. 8. Same Murchison CM2 carbonaceous chondrite spectra from Fig. 7, plotted asrelative reflectance (normalized to unity at 0.55 lm) and directly compared to theastronomical measurements of Fig. 2. The astronomical measurements display arange of slopes falling within a small change in the grain size distribution.Modifying the ‘‘coarse’’ sample (100% grains in the 500–1000 lm size distribution)by ‘‘fine’’ powdering just 5% of the grains into <45 lm sizes, increases the spectralslope from negative to positive and transitions through the range displayed by theastronomical measurements. This high sensitivity of spectral slope to changinggrain size distribution supports the hypothesis, that if the astronomical variability isreliable, grain size variations that are latitude dependent provides a viableexplanation.

pole-to-equator migration of material by centrifugal forces, e.g.Walsh et al., 2008, 2012.) Some evidence supporting the idea ofthe preferential migration of fine-grained particles comes fromScheeres and Sanchez (2014) and Scheeres (2015). These authorsnote that the equator of a rapidly spinning spheroid is the lowestpoint of the asteroid from a gravitational point of view. Hayabusaresults (Miyamoto et al., 2007) indeed reveal that finer-grainedmaterials do tend to pool in both the local and global geopotentiallows of Itokawa. If applicable to Bennu, the lower geopotentialequatorial ridge of Bennu could indeed be covered with finer grainscompared to higher latitude areas.

It is also interesting to consider whether coarser-grained equa-torial material might be preferentially lost due to rapid rotationand the resulting geopotential low. For Bennu’s current shape, arotation period faster than 3.24 h at any time during its spin rateevolution produces centrifugal forces that exceed self-gravity atits equator. Even under this condition of ‘‘negative gravity,’’ equa-torial material can be retained by cohesive forces (in the form ofvan der Waals forces) within Bennu’s regolith (Scheeres et al.,2010; Rozitis et al., 2014). However, these cohesive forces areinversely proportional to the square of the grain diameter, whichwould result in fine-grained material being preferentially keptand coarser-grained material being preferentially lost fromBennu’s equator. A fine-grained ridge with �10% of Bennu’s totalsurface area could have been formed if a spin period of 3 h orshorter was reached at any point during its YORP-driven rotationalevolution (Fig. 9). A rotation period of 3 h or shorter is well withinthe plausible range of models for YORP spin-up (often reaching sig-nificantly shorter periods) and spin-down cycles (Bottke et al.,2006). Studies of the thermal inertia properties of binary asteroids(Delbo et al., 2011) further suggests the migration of regolithtoward the equator, to the extreme outcome of substantial regolithdepletion with the shedding of the mass to form a satellite. Even inthis extreme case, Polishook et al. (2014) present evidence thatfine-grained regolith may be retained by the primary.

In the context of other asteroids, spectral variations on asteroidsare not unknown, but with few exceptions, are not often con-firmed. The most reliable, of course, occur for (4) Vesta, whosedichotomous surface as deduced through rotational spectral varia-tions has been known for decades (e.g., Bobrovnikoff, 1929; Gaffey,1983, 1997; Binzel et al., 1997), and verified in situ by the Dawn

Fig. 9. Effective surface gravity map of (101955) Bennu at a rotation period of 3 h.Map calculations were performed using a polyhedral gravity model (Werner andScheeres, 1997) with the radar-derived shape (Nolan et al., 2013) and theYarkovsky-derived bulk density of 1.26 g cm�3 (Chesley et al., 2014). The equatorialridge (�10% of the total surface area) experiences negative effective gravity becauserotational centrifugal forces exceed self-gravity within this region.

28 R.P. Binzel et al. / Icarus 256 (2015) 22–29

spacecraft (Reddy et al., 2013). Gaffey (1984) reported spectralvariations on (8) Flora and noted that the interpreted mineralogicalvariations were not consistent with a chondritic assemblage.Sasaki et al. (2006) observed S-type (832) Karin as having rota-tional spectral variations that varied from ordinary chondrite-like(unreddened) to typical S-type-like (reddened); however, thesevariations were not observed by subsequent researchers(Chapman et al., 2007; Vernazza et al., 2007). Among near-Earthobjects, Murchie and Pieters (1996) interpreted that (433) Eroshad distinct rotational variations with a pyroxene-rich side andan olivine-rich side. However, these variations for Eros were notconfirmed by in situ measurements by the NEAR-Shoemaker space-craft. A curious, and as yet unsettled case involves C-type Asteroid(175706) 1996 FG3 for which multiple observers (Binzel et al.,2001, 2012; de Léon et al., 2011, 2013) confirm variations in thethermal flux that may be more dependent on morning/eveningmeridian viewing geometry than hemispheric variations(Moskovitz et al., submitted for publication).

In spite of the unsettled history over the reliability for manycases of variable spectra, one solid case appears particularly rele-vant to support the possibility of a real variation on Bennu.Shepard et al. (2008) compiled a six-year multi-wavelength studyof the B-type Asteroid (2100) Ra-Shalom, finding a range of spec-tral slope variations analogous to the range for Bennu that wereport here. For Ra-Shalom, the spectral variations are repeatedover multiple rotational cycles and show a strong correlation withlongitude. Grain-size variation, from coarse regolith dominatingsome longitudes on Ra-Shalom, with fine-grain (presumablysmooth) plains dominating others, is the preferred interpretation.A proposed example for this Ra-Shalom scenario is the in situknowledge of (25143) Itokawa, where Hayabusa mission results(Fujiwara et al., 2006) clearly reveal this range of terrains.

Finally we note some possible implications for tracing the originof Bennu. If the spectral variations on Bennu are real, positive spec-tral slopes exposed on some regions of Bennu could be consistentwith its suspected source region in the main asteroid belt. Morespecifically, Bennu’s origin has been proposed as being part ofthe Polana family or one of its sub-families, including the recentlyidentified Eulalia family (Campins et al., 2010; Walsh et al., 2013;Bottke et al., 2015). All members of the Polana and Eulalia familieshave similar positively sloped near-infrared spectra (Pinilla-Alonsoet al., submitted for publication). Hence, if at least part of Bennu’ssurface has a near-infrared spectrum consistent with those of itsproposed relatives, it would strengthen its connection with thissource region.

6. Conclusions

Ongoing spectral reconnaissance of OSIRIS-REx mission target(101955) Bennu, made under the challenging observationalcircumstances of the 2011 and 2012 apparitions, show a possiblydifferent spectral slope than found by Clark et al. (2011). The mostsubstantially different geometric variable is the sub-Earth latitude,where the redder spectral slopes correspond to aspects most nearly‘‘overhead’’ to the known equatorial ridge feature on Bennu.Laboratory studies of the most likely carbonaceous chondritemeteorite analogs show that finer grain sizes yield redder spectralslopes – thus leading to the hypothesis of finer grained regolith inthe vicinity of Bennu’s equator relative to the poles. While weemphasize that telescopically derived slope differences are notrobust to systematic effects, the possible spectral variability pre-sents a testable hypothesis for OSIRIS-REx and its in situ measure-ments. Particularly interesting will be the revelation of the natureof Bennu’s equatorial ridge and its regolith size distribution rela-tive to higher latitudes, where an important outcome will be

insights on regolith migration processes induced by YORP spin-up and possible regolith cohesion and depletion effects (maxi-mized at the equator) arising from rapid rotation (Scheeres et al.,2010; Rozitis et al., 2014). If real, such spectral slope variationsare not likely unique to Bennu and may be observable on otherasteroids.

Acknowledgments

This paper includes data gathered with the 6.5 meter MagellanTelescopes located at Las Campanas Observatory, Chile. Thanks toPeter Sullivan for observing during the May 2012 run. This workwas supported by NASA Contract NNM10AA11C (D.S. Lauretta,PI). FED acknowledges support provided by NASA throughHubble Fellowship Grant HST-HF-51319.01-A awarded by theSpace Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., forNASA, under Contract NAS 5-26555. EAC thanks Kim Tait of theRoyal Ontario Museum and Jim Bell of Arizona State Universityfor providing samples of Murchison for this study, and the CSA,NSERC, MRIF, and the University of Winnipeg for supporting theestablishment and operation of the University of Winnipeg’sPlanetary Spectrophotometer Facility. Partial support for ESH wasprovided by NAG5-8070.

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