Near-IR Reflectance Spectroscopy of 433 Eros from the NIS

Bell et al. Eros Low Phase Spectra 1

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Near-IR Reflectance Spectroscopy of 433 Eros from the NIS Instrumenton the NEAR Mission. 1. Low Phase Angle Observations

J.F. Bell III1, N.I. Izenberg2, P.G. Lucey3, B.E. Clark1, C. Peterson1, M.J. Gaffey4,J. Joseph1, B. Carcich1, A. Harch1, M.E. Bell1, J. Warren2, P.D. Martin1,

L.A. McFadden5, D. Wellnitz5, S. Murchie2, M. Winter3, J. Veverka1, P. Thomas1,M.S. Robinson6, M. Malin7, A. Cheng2

1Cornell University, Department of Astronomy2Johns Hopkins University Applied Physics Laboratory

3University of Hawaii at Manoa4Rensselear Polytechnic Institute

5University of Maryland6Northwestern University

7Malin Space Science Systems, Inc.

Submitted to IcarusSubmitted 6 November 2000

Revised 18 July 2001Accepted 29 July 2001

Manuscript Pages: 49Tables: 4

Figures: 24

Please address all correspondence to:Jim Bell

Cornell UniversityDepartment of Astronomy

402 Space Sciences BuildingIthaca, NY 14853-6801Phone: 607-255-5911

Fax: 607-255-5907Email: [email protected]


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Abstract

From February 13 to May 13, 2000, the Near Infrared Spectrometer (NIS) instrument on the

Near Earth Asteroid Rendezvous (NEAR) mission obtained more than 200,000 spatially-

resolved 800 to 2500 nm reflectance spectra of the S-type asteroid 433 Eros. An important subset

of the spectra were obtained during a unique opportunity on February 13 and 14, when the

NEAR spacecraft flew directly through the 0° phase angle point between Eros and the Sun just

prior to the orbital insertion maneuver. This low phase flyby (LPF) dataset consists of ~2000

spectra of the northern hemisphere of Eros, obtained from 1° to 47° phase angle and at spatial

resolutions of between 6x12 km to 1.25x2.50 km per spectrum. The spectra were calibrated to

radiance factor (I/F, where I = observed radiance and πF = solar input radiance) and then

photometrically corrected to normal albedo. The average northern hemisphere spectrum of Eros

is similar to the asteroid's unresolved telescopic spectrum, and exhibits absorption features near

1000 nm (Band I) and 2000 nm (Band II) consistent with an orthopyroxene to

orthopyroxene+olivine mixing ratio of approximately 0.38±0.08. The ensemble of NIS LPF

spectra fall primarily within the S(IV) to upper S(III) fields of the Gaffey et al. (1993) S-asteroid

classification scheme and exhibit Band I and Band II properties similar to those of ordinary

chondrite meteorites. While some small spatially-coherent spectral variations have been

detected, neither the opx/(opx+ol) mixing ratio, nor other spectral parameters, vary spatially by

more than ~1σ across the entire northern hemisphere of the asteroid, suggesting a remarkable

homogeneity of the composition and mineralogy of the uppermost regolith. Spectral mixture

modeling suggests that the presence of glass and/or a reddening agent like nanophase iron, likely

formed from exposure of the regolith to the space environment, is a component of the surface of

Eros. Reddening and darkening components could also explain the dissimilarity in overall


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spectral slope and albedo between Eros and other S(IV) asteroids and ordinary chondrite

meteorites. The largest (but still weak) spectral variations across the surface are seen in the

depths of Band I and Band II, which are greatest in and around the largest craters and at the 0°

longitude "nose" of the asteroid, and in the Band II/Band I area ratio (BAR) between the large

impact craters Psyche and Himeros. These subtle NIS spectral variations are usually associated

with albedo variations seen in NEAR imaging and topographic data to be related to downslope

movement of regolith materials.

Keywords: Asteroids, Surface Composition, Spectroscopy, Space Missions


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Introduction

The Near Earth Asteroid Rendezvous (NEAR) spacecraft went into orbit around the S-type

asteroid 433 Eros on February 14, 2000, beginning a year-long close reconnaissance of this large

near-Earth object. NEAR's instrument suite was designed to reveal details about the asteroid's

geology, mineralogy, composition, and interior structure, and to provide new insights on the

relationship between asteroids and meteorites (e.g., Cheng et al., 1997; Veverka et al., 1997).

One of the fundamental goals of the NEAR mission was to understand whether the most

common asteroid class (the S types, of which Eros is a member) are directly related to the most

common meteorite types (the ordinary chondrites) or to some other class of meteorites, thus

providing constraints on the source regions for meteorites as well as the evolutionary histories of

their parent bodies (e.g., Chapman, 1996).

One of the key instruments on NEAR for addressing this goal is the Near-Infrared

Spectrometer (NIS). NIS is a 64-channel diffraction grating spectrometer that measures reflected

sunlight in the 800 to 2500 nm spectral region. The instrument is mounted on one corner of the

NEAR science deck, has an onboard calibration target, and has an internal scanning mirror that

allows it to measure the spectrum of the asteroid over a wide range of viewing geometries. Other

instruments, including the MultiSpectral Imager (MSI), have fixed boresights. Additional details

about the NIS instrument can be found in Warren et al. (1997).

Silicate minerals that have been found to occur within meteorites, and that have been

presumed to occur on asteroids, exhibit diagnostic solid state absorption features in the 800 to

2500 nm wavelength region. Specifically, olivine and pyroxene are known to be important


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components of asteroid and meteorite spectra, and both exhibit diagnostic Fe2+ electronic

transition absorption bands within the NIS wavelength range (e.g., Adams, 1974; Cloutis et al.,

1986; King and Ridley, 1987; Gaffey et al., 1993). NIS has a maximum spectral resolution of 22

nm from 800 to 1500 nm, and 44 nm from 1500 nm to 2500 nm, providing adequate spectral

sampling for the characterization of the positions, strengths, and areas of these relatively broad

solid state mineralogic absorptions. NIS went through an extensive pre-flight calibration and

characterization program, and also observed both the Earth and the Moon for calibration during

the ~4 year cruise phase before Eros orbital operations. The instrument carries an on-board

diffuse gold calibration target for periodic monitoring of its stability. Additional details on the

preflight and inflight calibration and performance of the NIS can be found in Warren et al.

(1997) and Izenberg et al. (2000a).

One of the most important phases of the NIS experiment occurred just prior to NEAR

orbital insertion. On February 13 and 14, 2000, the spacecraft flew past Eros at a range of ~177

km in order to determine accurately the asteroid's mass prior to orbital operations and to provide

an opportunity to obtain low phase angle spectroscopic measurements of the surface with NIS.

Because of spacecraft power requirements that dictated a near-normal orientation of the

solar panels to the Sun and dynamical stability constraints involving orbits around small bodies,

most of the NEAR orbital mission was conducted at a 90° phase angle viewing geometry. This

geometry is ideal for morphologic (imaging) studies but is poor for spectroscopic investigations

because of low light levels and extensive (and rapidly changing) shadows. Therefore, the

opportunity to obtain low phase angle measurements with NIS, down to 0° phase, represented

some of the most critical measurements that the instrument would obtain during the mission.

These measurements would not only provide good lighting conditions, but obtaining spectra over


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a wide range of phase angles during the ~24 hour flyby period would provide the data required to

construct a photometric model of the asteroid's surface that could be used to correct all NIS

orbital observations to a common viewing geometry, as well as to derive physical properties of

the asteroid's surface (e.g., Clark et al., 2001).

Veverka et al. (1999, 2000) described the initial results of the NEAR 1998 flyby and 2000

rendezvous with Eros, including information on the asteroid's size, shape, density, and overall

geology. They also presented initial NIS average spectral data and showed that Eros has a strong

opposition surge at low phase angles and that it exhibits phase reddening at higher phase angles.

Here, we expand on that initial report and provide new details and information about the LPF

phase of the NIS investigation. The first part of this paper describes the often complex and

elegant observations that were obtained by NIS as well as our motivation for performing them.

Next we briefly describe the reduction, calibration, and photometric correction of the NIS data,

deferring details to other papers as appropriate. We then describe the average spectral properties

of Eros as derived during the NIS LPF and compare those properties to previous telescopic

observations. Spectral variations on Eros are then examined in three ways. First, we use a

statistical analysis of all the data without regard for geographic context to explore correlations

among derived spectral parameters. Second, we map the data and spectral parameters onto a

shape model of Eros, in order to explore possible relationships between the spectra and the

geology of the asteroid. And third, we explore the possibility of "unmixing" Eros spectra to

determine the presence of and constrain the abundance of putative endmember compositional

components. The final section presents our interpretations of the NIS LPF spectra. We focus on

the use of the Band Area Ratio (BAR) parameter to provide information about: (1) olivine and

pyroxene on Eros and the relationship of the Eros spectra to spectra of common meteorite types;


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(2) the implications of spatial variations in band depth and spectral slope for the alteration

history of the asteroid's uppermost regolith; and (3) the specific postulated relationship between

Eros and the ordinary chondrite meteorites, especially in terms of explaining how these objects

can exhibit similar absorption band characteristics but quite dissimilar albedo and spectral slope.


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Observations

Figure 1 shows a schematic of the NEAR low phase flyby. Because of solar panel pointing

requirements, the spacecraft was not able to point the MultiSpectral Imager (MSI) camera to

view Eros during most of the initial flyby phase between February 13 and 14, 2000. From that

time until just after the Orbit Insertion Maneuver (OIM), NIS was the only instrument able to

make direct observations of Eros, by using its internal scan mirror to follow Eros as NEAR

passed through the zero phase point between the Sun and the asteroid.

NIS is nominally boresighted with NEAR's science camera, the MSI, and the other fixed

instruments on the spacecraft at a mirror position 90° from the spacecraft +Z axis (+Z is the

pointing direction of the solar panels; Figure 2). The spectrometer scan mirror can scan 30°

towards the +Z direction to observe sunlight obliquely reflected off a gold calibration plaque,

and 110° in the opposite direction to scan beyond the -Z (0° phase) direction. LPF observations

were obtained from ~60° opposite the pointing of the solar panels to ~90° past the boresight of

the other NEAR instruments (Figure 2).

Table 1 presents a summary of the NIS observations performed during the LPF. The

measurements focused on using a combination of mirror scans, spacecraft slews along the axis

perpendicular to the mirror scans, and spacecraft twists around that axis, to build a series of

nearly-constant phase angle 2-dimensional "global spectral maps" of the asteroid. Near closest

approach and during the short period around the actual zero phase point, mapping was restricted

to a smaller part of the asteroid where the phase angle was predicted to be lowest, based on the

best available Eros shape model and predictions of the expected flyby trajectory at the time the

sequences were uploaded to the spacecraft.

Four main observation types were utilized during the LPF: "global mosaics", "latitude


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scans", the "full nixel test", and "minimum phase point" (MPP) observations. Each type

addressed specific data gathering goals.

Global Mosaics. These observations employed a combination of mirror scans and

spacecraft slews and rotations to create a rectangular mosaic of spectra laid down across Eros

(Figure 3). Early global mosaics (above 30° phase angle, 450-km distance, ~3-km resolution)

were performed with a simple slew of the spacecraft virtual boresight down the long axis (X-

axis) of the asteroid, with the mirror scanning orthogonally. Rotation of Eros during the ~15

minute observation skews the mosaic slightly. In Figure 3a's frame of reference (Eros-fixed

point-of-view) the slew begins at the near 0° longitude "nose" of Eros, and slews down to the

180° "nose." As the spacecraft slewed, the mirror repeatedly scanned from right to left across

the asteroid. These higher phase angle global maps were taken during sequences GA, GC, GF,

and after the minimum phase during GL (Table 1). The slew lengths and number of mirror scan

positions increased in length as the spacecraft approached the asteroid, to ensure coverage of the

entire body.

As the spacecraft got closer to Eros, phase angle decreased and resolution increased, and it

took longer to make a full mosaic. Rotation of the asteroid made slewing down a fixed line

inefficient. In order to create the global mosaic, the spacecraft spun with the asteroid in what

was termed the "2001 maneuver" (named because the spacecraft motion relative to the asteroid

was similar to that between the shuttle and the Earth-orbiting space-station in the famous

rendezvous scene in the movie "2001: A Space Odyssey"). The same virtual boresight slew and

mirror scan of earlier mosaics was performed while the spacecraft spun with the asteroid. The

maneuver was designed to keep the spacecraft stable with respect to the asteroid so the slew

would be straight down the body's X-axis. The arrows in Figure 3b describe the slew and scan of


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the mosaic pattern, which was performed in two separate parts. In both Figures 3a and 3b,

spacecraft motion is roughly parallel to the direction of the mirror scan. The lower phase, two-

part global mosaics (Figure 3b) were obtained in sequences GH and GJ (Table 1).

Latitude scans. These sequences were the converse of the global maps in complexity, and

were designed to guarantee observing all of the illuminated hemisphere of Eros despite possible

minor navigation and pointing errors, while keeping the NEAR fanbeam antenna pointed at the

Earth so that the required Doppler data could be obtained for mission navigation purposes.

Latitude scans fixed the target point on asteroid nadir, and scanned the mirror back and forth

across nadir while the asteroid rotated underneath the spacecraft (Figure 4). A subset of latitude

scans were "InGaAs optimized" by setting the integration time and the dark current sampling

frequency at each mirror position to increased values, to improve signal-to-noise ratio (SNR) for

the less sensitive 1500-2500 nm Indium-Gallium-Arsenide (InGaAs) detectors of the NIS.

Nominal latitude scan sequences were performed in sequences GB and GD, and InGaAs

optimized latitude scans were performed in sequences GG and GI (Table 1).

Full Nixel Test. "Nixel" is the informal term for a single spectral observation footprint of the

NIS (a "NIS pixel"). The full nixel test was designed to observe a single spot on Eros many

times. LPF was the first time since Earth swingby over two years prior that any field-filling

target could be observed by NIS, and the first time the instrument could observe the actual target

body as designed. With the target point fixed on one spot on Eros, a series of spectra were taken

(1753 in all) with varying integration times and dark current interleave steps. In addition to

providing a detailed look at a small region of Eros (northwest of Himeros, Figure 5), these data,

combined with earlier SNR predictions (Warren et al., 1997; Izenberg et al., 2000a), constrained

optimum integration times for spectral observations during the orbital mission. Full nixel tests


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were all in LPF sequence GE (Table 1).

MPP observations. This set of small mosaics focused on the region of Eros that was

predicted to pass through zero phase angle (the "minimum phase point" or MPP). The zero

phase point actually occurred along a curving path across the surface of the asteroid (much like

the path of the Moon's shadow during a total solar eclipse on the Earth). The MPP observations

were targeted to a region where the predicted zero phase track was as far as possible from the

limbs of the asteroid, to avoid missing the zero phase measurements because of trajectory or

pointing uncertainties. The observations followed a mosaic pattern similar to the early global

maps, though both the slews and mirror scans were shorter, resulting in a 5x3 mosaic of nixels,

almost all fully illuminated (Figure 6). Exceptions occurred with the first and last MPP

sequences, which did not fall on the asteroid because of slight trajectory timing uncertainties.

MPP observations began in sequence GJ and were the only observation type executed in

sequence GK (Table 1). The actual 0° phase point was observed by NIS around 05:15 UTC on

February 14, 2000, but because of the non-zero angular diameter of the Sun, the 0.38°x0.76° size

of the NIS aperture, small uncertainties in the shape model of the asteroid, and the likely

existence of slight variations in topography within the NIS footprint, the actual average phase

angle within the NIS measurement containing the 0° phase point is estimated to be about 1°.

LPF Timeline and Coverage. The observation timeline (Table 1) followed a pattern of

switching between global mapping observations and latitude scans with the full nixel test and

MPP observations inserted at key times. Figure 7 shows the coverage of Eros obtained by NIS

during the LPF. Using the 22,540 plate shape model of Thomas et al. (2001), we determined

that approximately 45% of the surface area of the asteroid was covered by at least one spectrum,

and approximately 33% of the surface was covered by at least 5 spectra. LPF coverage was


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limited to the northern hemisphere, and the spatial resolution (NIS footprint size) of the

rectangular narrow slit measurements ranged from about 6x12 km at the start of the LPF to

about 1.25x2.50 km near the end. Additional examples of the typical NIS footprint size are

shown in Figure 8. The maximum phase angle of the LPF coverage was typically around 30°, but

coverage in some areas extended to about 50°. Figure 9 shows the minimum phase angle of the

NIS LPF observations for each plate in the shape model. A large part of the northern hemisphere

between 270° to 360° longitude (the MPP region) was covered at very low phase angles (< 3°).

Data from that region span a wide range of phase angles (Table 1), and provide unique and

important input data for the photometric modeling work reported by Clark et al. (2001).

Data Reduction and Analysis

Spectral Calibration. NIS data calibration is described by Izenberg et al. (2000a) and

Warren et al. (1997). Raw analog voltages are measured by the NIS detectors and then converted

to digital data numbers (DNs) for each channel within the NIS electronics. DNs are then

converted in the onground NIS calibration software to radiance units (e.g., W m-2 sr-1 µm -1) using

the equation

(1)

where DN ,t,T,M,s,i is the raw analog-to-digital count level at each detector channel of wavelength

at time t, instrument temperature T, mirror position M, slit position s, illumination angle i, ; Obs

is the number of one-second observations summed to make a single spectral measurement;

Dark ,t,T is the one-second detector dark signal, at time t close to the time of the data spectra,


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including possible bias induced by instrument temperature T; Gain corrects high-gain (10x)

Germanium channel data to low (1x); Xtalk is the correction applied to the first five Ge

channels from second order light; Mir ,M,s is the correction factor for scan-mirror position M at

slit position s, Phot ,i is a photometric correction for the NIS calibration target observations only

at solar illumination angle i; and Resp ,s is the per-channel DN-to-radiance conversion factor

determined on the ground, using slit s (narrow or wide). Age ,t is the change in detector response

over the operational lifetime of the instrument. The resulting radiance data is then converted to

Radiance Factor (Iλ/Fλ) by dividing radiance at instrument (Iλ) by the specific solar radiance

(solar irradiance Fλ/π) convolved over the NIS bandpasses (Wehrli, 1986). See Izenberg et al.

(2000a) for additional NIS calibration details as well as a discussion of calibration uncertainties.

NIS data are taken in a series of discrete sequences defined by the total number of

observations per sequence, the number of spectra to acquire per observation, the number of 1-

second integrations to co-add per spectrum, the dark spectrum interleave step and duration, the

number of mirror steps between observations, and the number of times the entire sequence is to

be repeated (Izenberg et al., 2000a). Spectra are calibrated sequence by sequence, and are saved

in FITS format files along with ancillary spacecraft and instrument telemetry data.

Spectral Database. Individual sequences can cover periods of a few minutes to a few hours,

and can contain hundreds to thousands of individual spectra. Individual spectra and short

sequences can be examined on a step by step basis (e.g., McFadden et al., 2001), but large scale,

global analyses, such as those used to produce coverage maps like Figure 7 and the parameter

maps to be discussed later are simplified by merging all or a subset of the NIS data into a single

large database. NIS databases contain individual NIS I/F spectra, their derived uncertainties, and

viewing geometry information based on knowledge of spacecraft position and pointing and the


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most up-to-date shape model of Eros (Thomas et al., 2001). Additional database entries can

store the results of spectral parameter determinations (see below) and other analysis results.

Photometric Correction. Because the NIS data were obtained at widely varying viewing

geometries, each spectrum must be corrected photometrically before comparisons of spectral

properties such as albedo can be made among spectra. To photometrically correct a spectrum

obtained at a specific incidence, emission and phase angle, we: (1) derived a photometric model

for each wavelength that predicts the brightness of average Eros surface materials as a function

of incidence, emission, and phase angle (Clark et al., 2001); (2) calculated the brightness of Eros

at 0° incidence, emission and phase angles (normal reflectance) at each wavelength; (3) divided

the normal reflectance by the model brightness at the input incidence, emission and phase angle

for each wavelength; and (4) multiplied the input spectrum by the quantity in step 3. This

spectral photometric correction procedure was found to be accurate to within ±5% on average.

The accuracy was estimated by comparing spectra obtained at 0°-5° and 90°-100° phase angles

for each of 38 locations on Eros. Additional details on the photometric modeling are found in

Clark et al. (2001).

Mapping. NIS data were mapped onto the asteroid in two ways. The software that creates

NIS database files uses the best-available spacecraft trajectory (SPICE data), instrument pointing

information derived from pre-flight calibration data, and the Eros shape model of Thomas et al.

(2001) to determine the latitude, longitude, and viewing angles (incidence angle i, emission angle

e, and phase angle ) and their ranges for each NIS spectrum. This information is stored in the

database file. The first mapping routine queries a database file and uses the latitude and longitude

information to determine which plate in the shape model each spectrum covers. The pointing

and trajectory uncertainties in this process are typically less than about 20% of the NIS field of


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view, or roughly equivalent to 2°of positional uncertainty along the equator as viewed from a

distance of 200 km. Any entry in the database (I/F, viewing angle, spectral parameters) can be

mapped onto plates in the shape model. The software keeps track of how many spectra fall on

each plate and allows the user to display the minimum, maximum, mean, or median of the

desired database values for each plate. The output map displays the value for each plate on a

regular-grid map-projected view of the asteroid (e.g., Figure 7). A second mapping routine

allows the user to "drape" a flat map projection onto the shape model and to view it from an

arbitrary direction (e.g., Figure 18 below). Because Eros has a highly irregular shape, this

presentation of NIS spectral parameters provides a more intuitive assessment of the relative

surface area of different features observed on the asteroid.

Spectral Parameters. Rock forming minerals exhibit characteristic, compositionally- and

mineralogically-dependent absorption features in the near-infrared. For example, variations in

the strength and position of broad solid state absorption features near 1000 nm (termed "Band I")

and 2000 nm (termed "Band II") have been shown to be caused by Fe2+ electronic transition

absorption features that are diagnostic of variations in olivine and pyroxene chemistry and

mineralogy (e.g., Adams, 1974; Cloutis et al., 1986; King and Ridley, 1987; Cloutis and Gaffey,

1991). Variations in the depths and integrated areas of Band I and Band II have been shown to be

related to changes in pyroxene composition, grain size, olivine/pyroxene ratio, and/or viewing

geometry effects (e.g., Gradie and Veverka, 1986; Cloutis et al., 1986; Cloutis and Gaffey,

1991). Additionally, the slope of the spectrum in the near-IR has sometimes been used as a

measure of either the abundance of Ni-Fe metal on an asteroid surface (e.g., Cloutis et al., 1990;

Clark, 1995; Gaffey and Gilbert, 1998), or a proxy for the degree of modification of the surface

by exposure to space via processes collectively known as "space weathering" (see below and,


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e.g., Hapke et al., 1975; Allen et al., 1993; Pieters et al., 2000).

Spectral parameters that are derived for each NIS spectrum are summarized in Table 2.

Normal albedo in NIS channel 5 (902.7 nm) or channel 7 (945.9 nm) is used for correlation with

MSI filter 5 (900 nm) or filter 4 (950 nm) image data, as appropriate. The other parameters are

used to constrain the mineralogy and composition of the asteroid based on calibrations derived

from analyses of laboratory minerals, mineral mixtures, and meteorites as discussed above.

Determination of band parameters follows the methods standardized by Adams (1974),

Cloutis et al. (1986), Gaffey et al. (1993), and others. First, a linear continuum is fitted across

points on the spectral curve outside the absorption band being characterized (Clark and Roush,

1984). Separate continua are chosen for Band I and Band II. The short wavelength Band I

continuum point is chosen as the maximum of NIS channels 1 and 2 (centered at 816 and 838

nm respectively), and the long wavelength Band I continuum point is chosen as the maximum of

NIS channels 30, 31, and 32 (1443, 1465, and 1486 nm). The short wavelength Band II

continuum point is the same as the long wavelength Band I continuum point, and the long

wavelength Band II continuum point is chosen as the maximum of NIS channels 54 and 55

(2277 and 2320 nm). The spectral slope is the slope of the continuum [∆(I/F) / ∆ λ] across the

band of interest, for spectra scaled to 1.0 at NIS channel 1 (816.2 nm). Band center is defined as

the wavelength of the minimum of an nth order (typically n=3-5; Gaffey et al. 1993) polynomial

fit to the spectrum, after dividing out the linear continuum (e.g., McCord et al., 1981; Singer,

1981). Band depth is calculated as in Clark and Roush (1984) as 1.0 - (Rb/Rc), where Rc is the

linear continuum value at the band center wavelength and Rb is the value of the NIS spectrum at

the band center wavelength. Band areas are calculated by integrating the shoulder-to-shoulder

area encompassed by a continuum-removed absorption band, using a five-point Newton-Cotes


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integration formula between the wavelengths of the continuum endpoints. The Band Area Ratio

(BAR) is defined as Band II area divided by Band I area. The NIS spectral parameters were

derived using both photometrically corrected (Clark et al., 2001) and uncorrected I/F data, to

investigate the influence of viewing geometry on NIS spectral parameters and to validate the

effectiveness of the photometric correction.

NIS spectra do not cover the full extent of the Band I Fe2+ electronic transition absorption

feature, which extends down to 600-750 nm for many iron-bearing silicate minerals (e.g.,

Adams, 1974). NIS parameters calculated using the first or second NIS channel as the short

wavelength continuum point of Band I result in under-estimation of Band I depth and area, and a

skewing of the band center towards longer wavelengths. We overcome this limitation in two

ways. First, we derived a correction factor to compare NIS Band I center position directly to

Band I center reported in the literature for laboratory samples measured over the full extent of

Band I. This was achieved by comparing the Band I center derived from polynomial fitting of

NIS spectra to the Band I center derived from polynomial fitting of merged MSI+NIS spectra of

the same regions of the asteroid (Izenberg et al., 2000b), which extends the wavelength coverage

across nearly the entire range of Band I. We found that the Band I center position is

approximately 57 nm longer using NIS-only partial Band I wavelengths than using the full

MSI+NIS Band I wavelength range, and that there is a slight correlation between the spectral

slope of the NIS data and the derived Band I center correction. Our analysis resulted in a Band I

center correction that is a function of both the NIS-only Band I center and the NIS spectral slope:

BI_cenC = BI_cenN + [18.368 - 161.109 • SN] (2)


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where BI_cenN is the Band I center wavelength determined from the NIS spectra over the 800 to

2400 nm wavelength region, SN is the NIS spectral slope across Band I, in units of I/F per

micron, and BI_cenC is the corrected Band I center wavelength that can be compared directly to

laboratory and telescopic spectra. We found that this Band I center correction adds an additional

9.3 nm (1σ) uncertainty to the derived Band I center wavelength. Similarly, we found that Band I

is systematically shallower using NIS-only partial Band I wavelengths than using the full

MSI+NIS Band I wavelength range, and that there is a slight correlation between the spectral

slope of the NIS data and the derived Band I depth correction. Our analysis resulted in a Band I

depth correction that is a function of both the NIS-only Band I depth and the NIS spectral slope:

BI_depC = BI_depN + [0.0047 + 0.1533 • SN] (3)

where BI_depN is the Band I depth determined from the NIS spectra over the 800 to 2400 nm

wavelength region, SN is the NIS spectral slope across Band I, in units of I/F per micron, and

BI_depC is the corrected Band I depth that can be compared directly to laboratory and telescopic

spectra. We found that this Band I depth correction adds an additional 0.0075 (1σ) uncertainty to

the derived Band I depth. And finally, the restricted coverage of Band I by NIS results in a

systematically higher value of BAR than would be measured over the full laboratory wavelength

range (because some of the area of Band I is "missing" in NIS measurements). Our analysis

uncovered a simple linear relationship between BAR derived from NIS-only wavelengths and

BAR over the full wavelength range:

BARC = [0.041 + 0.493 • BARN] (4)


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where BARN is the Band Area Ratio determined from the NIS spectra over the 800 to 2400 nm

wavelength region and BARC is the corrected Band Area Ratio that can be compared directly to

laboratory and telescopic spectra. We found that this correction adds an additional 0.186 (1σ)

uncertainty to the derived BAR values.

All of the NIS spectral parameters analyzed here have had the corrections in Equations (2)

through (4) applied to them prior to analysis.

Second, we found that a self-consistent data set can be generated with NIS data alone if the

same wavelength restriction is applied to other spectra used for comparison. For example,

McFadden et al. (2001) use a suite of laboratory olivine and pyroxene spectra to derive a smooth

monotonic conversion equation between BAR determined using NIS-only partial Band I

wavelengths to that determined using full sampling of Band I. A similar set of data was used to

re-derive the Cloutis et al. (1986) calibration between BAR and the olivine to pyroxene ratio

inferred by NIS spectra (McFadden et al., 2001). Each of these conversions introduces

additional uncertainties, but are necessary to perform comparisons between NIS-derived results

and full-wavelength laboratory, asteroid, and meteorite spectral results reported in the literature.

Results

Average Spectral Properties

Figure 10 shows the average photometrically-calibrated NIS spectrum from the low phase

flyby, compared to previous telescopic spectral measurements of Eros and to the MSI average

spectrum obtained during the late 1998 Eros flyby (Veverka et al., 2000). The NIS

measurements are generally consistent with MSI data and previous telescopic observations,


20

except that the average telescopic spectrum of Murchie and Pieters (1996) appears less red at the

longest wavelengths than either the NIS average spectrum or the K band photometry of Chapman

and Morrison (1976). Re-examination of the original Larson et al. (1976) FTS measurements

used in the Murchie and Pieters (1996) average spectrum indicates that the slope at the longest

wavelengths may have been artificially supressed in the Larson et al. (1976) measurements as an

artifact of their data reduction process (McFadden et al., 2001). However, it is also important to

note that the uncertainties in both the NIS spectra and in the photometric correction are largest at

these longer NIS InGaAs wavelengths.

The band parameters for the NIS LPF average spectrum are listed in Table 3. Even though

MSI and NIS average spectral data for the northern hemisphere of Eros can be merged into a

composite spectrum that provides more complete coverage of the 1 µm region for Band I

calculations, the resulting spectral sampling is still quite coarse shortward of 800 nm. For this

reason, we chose not to perform band fitting on the merged spectrum but instead to use the NIS-

only spectrum (sampled at fine wavelength intervals) and the empirical corrections described

above to relate NIS spectral parameters to laboratory spectral parameters derived over the full

range of visible to near-IR wavelengths. Thus, the Band I center wavelength, Band I depth, and

BAR values reported in Table 3 include the corrections described in Equations (2) through (4)

above.

Spectral Variations

Statistical trends. Calibrated NIS data from the LPF exhibit large increases in I/F with

decreasing phase angle (Figure 11a). These changes are due to the well-characterized

relationship between scattering geometry and observed radiance (e.g., Hapke, 1963; 1981;


21

Bowell and Lumme, 1979). The data in Fig. 11a formed the basis for developing a photometric

model of the surface of Eros that is used both to correct the data to standard viewing geometry

circumstances and to investigate aspects of the physical properties of the asteroid's particulate

surface (e.g., Clark et al., 2001). When variations in viewing geometry are properly accounted

for, most of the dependence of radiance on phase angle disappears (Figure 11b). Residual high

frequency variations remain even in the photometrically-corrected I/F values, because the model

is not able to accurately correct NIS spectra obtained at very high incidence or emission angles

(i, e > 70°). These residual variations provide an estimate of the overall accuracy of the

photometrically-corrected data.

We examined the entire LPF dataset for other correlations among spectroscopic parameters

by generating correlation plots (also known as 2-D histograms) of parameters versus each other

and versus viewing geometry and geographic coverage. Analyses were performed on data both

photometrically corrected to normal albedo (Clark et al., 2001) and on the original uncorrected

measurements of I/F.

Figure 12 shows a plot of Band I center vs. Band II center over the full range of spectral

variability observed among laboratory orthopyroxene and clinopyroxene samples (e.g., Adams,

1974; Cloutis et al., 1986). The LPF data cluster near one region of the plot, near the

orthopyroxene to low-calcium clinopyroxene end of the Adams (1974) band correlation diagram.

The vertical separation of the data cloud in the not photometrically corrected plot (Fig. 12a)

correlates with phase angle: the lowest phase angle (1° to 3°) spectra have a slightly longer Band

I center wavelength than spectra at higher phase angles. This behavior is consistent with the

observed phase reddening of Eros reported by Veverka et al. (2000) and Clark et al. (2001).

After application of the Clark et al. (2001) photometric correction, the spectra cluster into a


22

much smaller region of the plot, and indicate no statistically significant variation in Band I or

Band II center within the LPF data set.

Figure 13 shows a plot of Band I depth vs. Band II depth. Both bands vary by less than

±2% in absolute band depth. Much of this observed variability may be related to viewing

geometry effects rather than compositional or mineralogic variations, because the depths of Band

I and Band II both vary by a few percent as a function of phase angle (Figure 14). These small

changes in spectral reflectance properties as a function of phase angle observed on Eros are

consistent with the theoretical predictions of Gradie and Veverka (1981, 1986) for spectral

changes on small irregular objects. These variations are caused by changes in the scattering

geometry of the observations, and can be modeled as resulting from the wavelength-dependence

of the photometric parameters that describe the phase function of the surface (Gradie and

Veverka, 1981; 1986; Clark et al., 2001). Except for the variations due to phase angle, our

analysis does not reveal any statistically significant variations in the strength of either Band I or

Band II in the NIS low phase flyby dataset.

A plot of the variations in the slopes of the continua across Band I and Band II is shown in

Figure 15a. In both photometrically uncorrected and corrected datasets there is a high

correlation between Band I slope and Band II slope, consistent with the lack of any discontinuity

in the "redness" of the Eros spectrum across the NIS wavelength region. In Figures 15b and 15c

the slope of the continuum across Band I shows no statistically significant correlation with

albedo or Band I depth within the photometrically-corrected dataset.

Spatial Variations. The statistical approach to spectral variations in the previous section is

useful for understanding the overall or average level of variability in the NIS LPF dataset, but


23

does not provide detailed insight on the relationship (if any) of the observed variability to the

underlying geology or morphology of the asteroid. To study the spatial variability of spectral

variations, we mapped the derived spectral parameters onto the Thomas et al. (2001) 22,540

plate shape model of Eros.

In Figure 11b it was shown that the photometric correction of Clark et al. (2001) removed

the overall increase in I/F with decreasing phase angle seen in Figure 11a. However, small

residual variations in albedo remained even after photometric correction. Figure 16 is a map of

these residual albedo variations, overlaid onto a sketch map of major geologic features (Veverka

et al., 2000; Thomas et al., 2001). Most of the northern hemisphere of Eros has a relatively

uniform near-infrared albedo of 0.233±0.033 at 900 nm (see Fig. 15b and Clark et al., 2001).

However, spatially-coherent variations at the 1σ level and less can be seen in the northern

hemisphere, and primarily at low latitudes. The brightest regions (900 nm albedo ~ 0.25 ) are

primarily associated with the large craters Psyche and Himeros; the darkest (900 nm albedo ~

0.21) occur near the 0° longitude "nose" of the asteroid.

In Figure 13b it was shown that variations in Band I depth and Band II depth do not appear

to exceed the statistical uncertainties in the band depth determinations. Spatial variations in the

depths of Band I and Band II are shown in Figure 17. Both bands are shown at the same relative

depth contrast (±1.2% band depth range approximately around the mean). There do appear to be

correlations between band depth and surface features. For example, parts of the rims of Psyche

and Himeros have slightly enhanced or diminished band depths relative to the average, and the

equatorial region near the 0° nose of the asteroid exhibits the deepest bands overall. A more

intuitive view of these band depth variations can be seen in the data projected back onto the Eros

shape model in Figure 18. While spatial variations are seen in association with some geologic


24

features, we do not see evidence for the hemispheric-scale differences in spectral properties

reported by Murchie and Pieters (1996) from telescopic data.

Finally, Figure 15 showed that the slope of the continua across Band I and Band II exhibited

little or no evidence of variation or statistically significant correlation with albedo. Figure 19

shows a map of the continuum slope across Band I. As in the previous maps, there is evidence

for weak correlation between this parameter and features on the surface of Eros. However, there

is no consistent correlation between the spectral slope and other parameters like Band depth (Fig.

17). For example, the western rim and plains west of Psyche exhibit elevated band depths and

spectral slope; however, parts of the rim and plains surrounding Himeros exhibit low Band I

depth but high spectral slope.

Spatial mapping of NIS spectral parameters demonstrates that correlations between the

spectral properties of Eros and the asteroid's surface geology can be detected, despite the fact that

spectral variability on Eros is inherently weak and difficult to detect in straightforward statistical

analyses like those in Figures 14 and 15. The reasons that correlations can be observed in the

mapped data include the high fidelity and stability of the NIS and its calibration, the high overlap

and large number of spectra acquired during the low phase flyby period, the availability of

excellent ancillary information on the NIS viewing geometry determined from interrogation of

the shape model of Eros, and the spatially contiguous nature of the subtle variations detected.

Interpretations and Discussion

Mineralogy of Eros: The Band Area Ratio Approach

Cloutis et al. (1986) and Gaffey et al. (1993) defined and derived methods to relate

measured near-IR asteroid and meteorite spectral parameters to estimated mineralogic


25

compositions, based on mixing studies of well-characterized laboratory samples. Specifically,

they determined that the ratio of Band II to Band I area is proportional to the olivine (ol) to

orthopyroxene (opx) ratio within a sample, assuming that ol and opx are present in comparable

fine-grained particle sizes. McFadden et al. (2001) have re-derived the Cloutis et al. (1986) BAR

calibration for the specific wavelengths of the NIS instrument.

Figure 20 shows plots of BAR for the NIS LPF dataset both with and without photometric

correction, overlaid onto the Gaffey et al. (1993) BAR plot that shows the expected fields of

S(IV) asteroids (defined by the limits of ordinary chondrite spectral parameters), ol- and opx-rich

meteorites, and the ol-opx mixing line of Cloutis et al. (1986). All of the NIS LPF data fall

within a relatively small part of the BAR plot, especially after photometric correction (Clark et

al., 2001). Overall, the surface of Eros appears remarkably (and, to many of us, disappointingly)

spectrally homogeneous. Within the uncertainties, the NIS data fall within the S(IV) to upper

S(III) fields of the Gaffey et al. (1993) S-asteroid classification scheme and exhibit spectral

similarities to ordinary chondrite meteorites. Specifically, Eros LPF spectra are most consistent

with the spectra of L ordinary chondrites and appear statistically distinct on average from both H

and LL chondrites according to the parameterizations of Gaffey et al. (1993) and Gaffey and

Gilbert (1998). A small number of spectra fall closer to the very low iron LL chondrite field than

the higher iron L chondrite field, but these spectra tend to occur at very high emission angles

along the edge of the LPF coverage, and thus are subject to larger photometric correction

uncertainties. Additional distinction by metamorphic grade is not possible at the current level of

NIS data calibration and spectral parameterization accuracy.

Using the Cloutis et al. (1986) calibration of BAR for mixtures of orthopyroxene and

olivine, modified for the NIS wavelength range by McFadden et al. (2001), the BAR value of the


26

average photometrically-corrected northern hemisphere spectrum of Eros of ~0.79±0.19 is

consistent with an opx wt.% of approximately 38±8% (opx/(opx+ol) = 0.38±0.08), irrespective

of the grain sizes of the two components (but assuming that the grain sizes are similar in both).

This estimate of the olivine to pyroxene ratio assumes that there is little or no clinopyroxene

(cpx) in the mixture. Cloutis et al. (1986) and Gaffey et al. (1993) point out that the addition of

cpx would cause substantial deviations vertically away from the ol/opx mixing line in Figure 20.

In fact, the aggregate data cloud for the NIS LPF BAR data is vertically offset from the canonical

ol/opx mixing line, arguing for the possible spectral influence of a second pyroxene (cpx) in the

observed spectra. This possibility is explored further by McFadden et al. (2001). Further, the

very small distribution of BAR values, irrespective of the level of photometric calibration, argues

that variations in the ol:opx ratio are only on the order of ±5% or less across the half of Eros

studied carefully during the NIS low phase flyby.

As discussed above, it is instructive to also examine spatial variations in BAR to determine

if there is any even weak correlation with geologic features or units. Figure 21 shows a map of

BAR values derived from photometrically corrected data and the shape model of Thomas et al.

(2001). This figure shows that small (roughly ±1σ) variations in BAR values are associated with

the floor and southeastern rim of Psyche, the 0° longitude nose, and the high latitude northern

plains (slightly lower than average BAR values), as well as the western rim and plains west of

Psyche and part of the southeastern rim of Himeros (slightly higher than average BAR values).

The correlation between BAR variations and geologic features and the relative insensitivity of

this correlation to changes in viewing geometry argue that the variations may be related to subtle

compositional/mineralogic differences. If true, the inferred differences in the ol:opx ratio, based

on the Cloutis et al. (1986) and McFadden et al. (2001) calibrations, would be on the order of


27

±5% for the most extreme BAR variability, and more typically less than ±2% for the typical

level of variation seen in Figure 21. However, because the variations are close to the SNR limit

of the calibrated NIS data, and because they typically are strongest closest to the edges of the

coverage map (highest emission angles), it is not possible to completely rule out calibration or

other viewing geometry effects as the source of some of these observed BAR variations.

Mineralogy of Eros: Simple Linear Mixture Modeling

Another approach to mapping the distribution of composition and/or mineralogy across the

surface of Eros is to try to spectrally "unmix" the NIS data into fractional abundances of pure

endmember materials. For the Moon and Mars, simple linear mixing models have been shown to

model adequately the reflectances observed by coarsely-sampled multispectral imaging systems

(e.g., Adams et al., 1986; Bell and Hawke, 1995). This approach has also been used for some

studies of spatial and spectral variability on asteroids (e.g., Vilas et al., 1997; Gaffey and Gilbert,

1998), and it has been shown that some S-asteroid spectra can be mimicked with mixtures of

spectra of nickel-iron meteorites and primitive and basaltic achondrites (Hiroi et al., 1993). The

latter work did not have the benefit of accurate albedos like those which have been measured by

NIS, and so the ability of macroscopic mixtures of metal and rock to reproduce all of the spectral

properties of Eros or other S-asteroids has not been tested. High spatial resolution color imaging

from NEAR (Murchie et al., 2000) reveals that the proposed endmember components required to

match the spectrum of Eros (likely including olivine, pyroxene, and NiFe metal; Murchie and

Pieters, 1996) do not occur in "checkerboard" deposits on the asteroid's surface. Instead, they are

likely mixed intimately within a fine-grained regolith. We attempted some simple linear

unmixing models on the average Eros spectrum using a variety of grain sizes and compositions


28

of olivines, pyroxenes, and metals. While we found some mixes that could reasonably

approximate the average NIS LPF spectrum (Figure 22), in general we found that the spectrum

could not be adequately fit using simple linear combinations of typical asteroidal/meteoritic

components. In particular, we could not match both the smooth, long-wavelength side of Band I

and the overall red slope of the NIS spectrum with any of the laboratory mineral and mineral

mixture spectra that we tried. The closest matches we found from linear mixing models included

anomalously large relative abundances of darkening/reddening agents like troilite, and usually

still required additional artificial reddening of the spectrum.

Mineralogy of Eros: Nonlinear Mixture Modeling and the Influence of "Space Weathering"

While the connection between S-asteroids and specific meteorite classes remains

controversial, our BAR analysis above and that of Murchie and Pieters (1996) suggest that the

relative abundance of olivine and pyroxene on Eros is similar to that of ordinary chondrites.

However, the spectrum of Eros, like those of other S-asteroids, is dissimilar to ordinary

chondrites in terms of spectral slope and contrast of the absorption bands. The NIS data also

show that the average albedo of Eros is lower than that of average ordinary chondrites.

The difference in spectral properties between S(IV) asteroids and ordinary chondrites (and

other meteorites) has been attributed to the alteration of the optical surface by exposure in space

to processes now collectively called "space weathering" (e.g., Chapman, 1996; Hapke, 2001).

The archetypical example of space weathering occurs on the Moon, where exposure to space

causes lunar soils to darken, redden, and lose spectral contrast. On the Moon, the optical effects

of space weathering have been attributed to a combination of the generation of nanophase iron

metal, probably via reduction of ferrous iron during micrometeorite vaporization (Hapke et al.,


29

1975; Keller and McKay, 1993) and possibly aided by the presence of solar wind protons,

accumulation of dark impact glass, or evolution of grain size distributions. The relationship

between NIS spectra of Eros and spectra of ordinary chondrites is superficially similar to the

relationship between spectra of lunar rocks and soils, in that Eros is redder and exhibits weaker

absorption bands.

Nonlinear mixing models, especially those based on quantitative radiative transfer models

that account for the details of both scattering and absorption, have been shown to be successful at

matching the behavior of intimately-mixed laboratory, lunar, and asteroidal surfaces (e.g.,

Hapke, 1981, 1993; Mustard and Pieters, 1989; Clark, 1995; Lucey, 1998). Quantitative

modeling of the reflectance spectra of S-asteroids in particular has shown that in many cases

these spectra can be represented within the precision of astronomical measurements by mixtures

of minerals or meteorites with macroscopic nickel-iron metal (Hiroi et al., 1993; Clark, 1995).

More recently, Pieters et al. (2000) and Hapke (2000, 2001) have argued that the red nature of S-

asteroid spectra relative to meteorites and simple mineral mixtures can be explained by a

variation of lunar-type space weathering which includes the effects of extremely fine-grained

iron metal. They presented arguments and data showing that objections previously raised against

this hypothesis can be overcome.

Here we model the average spectrum of Eros as a mixture of common meteoritic minerals,

and use the Hapke (2001) formulation of the effects of submicroscopic iron metal as a reddening

agent principally because this formulation is better constrained than the effects of macroscopic

iron, which does not provide good matches to the Eros spectra in our linear or nonlinear mixing

models (e.g., Fig. 22).

We begin our modeling exercise with an attempt to model the spectrum of an LL-chondrite.


30

Our mixing model, based on equations presented in Hapke (1993), enables adjustment of the

modal abundance of the mineral endmembers, their grain sizes, their degree of alteration by

nanophase iron particles, and their photometric geometry. The model uses mineral optical

constants computed by Lucey (1998), which are then altered according the amount of nanophase

iron by the methods of Hapke (2001). Given the assumed mineral grain sizes, the single

scattering albedo of each altered component is then computed, then mixed according to the

modal abundances and densities. Bidirectional reflectance of the mixture is then computed.

Figure 23 shows a model fit to the spectrum of the LL6 chondrite Manbhoom. The modal

abundance used in this model is given in Table 4 and is consistent with the general modal

abundances of LL6 chondrites, though the mode of this meteorite has not been measured. The

model reproduces most of the features of the meteorite spectrum, with a mismatch near 2300 nm

that may be due to the presence of phyllosilicate-bearing alteration minerals in the meteorite. In

particular, note the structure of the spectrum between 900 and 1500 nm due to the overlapping

absorptions of olivine and pyroxene.

Modeling the additional effects of nanophase iron alone on the modeled chondrite spectrum

cannot reproduce the spectrum of Eros (Figure 24a). For a given reflectance, the model altered

spectra are too red, suggesting the necessity for a darkening agent. Such a darkening agent might

be the result of a process with gives rise to black chondrites; that is, darkened via shock

implantation of nickel-iron grains much larger than those represented by Hapke's formulation,

though still submicroscopic (e.g., Keil et al., 1992). In the absence of a published quantitative

model for shock-darkening effects, we represent this agent by a fine-grained spectrally neutral

component with 5% bidirectional reflectance at i=30°, e=0°. We assume this component has a

single scattering albedo which is independent of grain size, and is not affected by alteration.


31

Addition of the darkening agent alone creates spectra which are less red than Eros at

comparable reflectances (Figure 24b); however, combining the darkening agent with model

alteration due to nanophase Fe can produce the proper slope and albedo (Figure 24c). In this

case the spectral contrast (band depths) of the modeled spectrum is considerably lower than that

of Eros. To arrive at similar contrasts, we increased the grain size of the model silicates to 200

µm prior to alteration, which gives a reasonable match to the albedo, slope and spectral contrast

of Eros (Figure 24d). The large grain size used in the model is worrisome, but given the many

simplifications inherent in this model, we argue that these results indicate that the Hapke (2001)

model for the presence of nanophase iron particles applied to ordinary chondrites is a

quantitatively plausible explanation for the general spectral properties of Eros (albedo, slope, and

spectral contrast). The combination of a darkening agent plus the reddening effects of space

weathering could also quantitatively account for the very small spectral differences between

areas on Eros that show relatively large albedo differences (Murchie et al., 2001).

Our initial mixing model results using an LL-like assemblage and the Hapke (2001)

treatment of nanophase iron should not be interpreted as evidence for a unique solution. Our

nonlinear mixing model results show that the spectrum of Eros is generally consistent with a

mixture of olivine and pyroxene in proportion of ~5:1, reddened by nanophase iron, and

darkened by an additional unidentified component. These characteristics are plausible for an LL-

chondrite parent body where space weathering has provided the nanophase iron, and shock has

produced larger iron grains (~ 1 µm or larger) imbedded in mineral grains to provide a neutral

darkening component. However, olivine-rich meteorites other than LL-chondrites can be

invoked with equal plausibility. Additionally, spectra of Eros lack the detailed structure between

900 to 1500 nm typically seen in spectra of ordinary chondrites and other mixtures of olivine and


32

pyroxene (e.g., Figs. 22, 23). This structure is caused by multiple overlapping Fe2+ crystal field

absorption features in well-ordered mineral lattice structures; the lack of this structure in spectra

of Eros may indicate the presence of a glassy component on the surface, or the possible presence

of heterogeneous or zoned silicates (e.g., Bennett and McSween, 1993). Iron-bearing silicate

glass exhibits both Band I and Band II absorptions, but is generally darker than crystalline

silicates with comparable iron contents (e.g., Bell et al., 1976; Basu et al., 1981; Pieters et al.,

1991). Iron-bearing silicate glass also shows a very smooth spectrum consistent with its disorder.

Without much more detailed modeling of the NIS spectra it is not possible to assess

quantitatively the possible abundance or nature of glass on Eros. However, it is an intriguing

possibility that cannot be ruled out at this time, and it has important potential implications for the

study of asteroidal regoliths in general.

The implication of our NIS measurements that Eros has near-IR spectral properties

consistent with ordinary chondrite meteorites is supported by the initial results of the NEAR X-

ray spectrometer investigation (Trombka et al., 2000). X-ray data of a portion of the northern

hemisphere reveal ratios among Al, Mg, Si, and Fe that are similar to those found in L and LL

chondrite meteorites and which are unambiguously distinct from those found in H chondrites,

pallasites, or achondrites. X-ray results, particularly the possible evidence for substantial

depletion of elemental sulfur in the Eros regolith, could also be consistent with a subset of

primitive achondrite meteorites. While information on the near-IR spectral properties of

primitive achondrites is presently inadequate to assess their consistency with NIS measurements

(McCoy et al., 2000), initial studies of the spectral properties of some ground and seived

primitive achondrite powders by Burbine et al. (2001) have shown them to have BAR values

inconsistent with those of the NIS measurements, since they are either too pyroxene-rich or


33

pyroxene-poor.

Implications of Band Depth and Spectral Slope Variations

There is generally poor agreement between the spatial variations of the strengths of Band I

and Band II (Figs. 13, 17), suggesting that weak variations in a surface mineralogic component

exhibiting just Band I (e.g., olivine) may be responsible for the observed variations. Possible

evidence for subtle variations in the olivine to pyroxene ratio across Eros is further supported by

weak spatially-coherent variations in BAR (Fig. 21). One possible interpretation of Fig. 21 is

that areas of low BAR value like Psyche may be regions where the olivine abundance is

enhanced by a few percent relative to the surface average. However, the spectral influence of

possible minor amounts of clinopyroxene on the surface (e.g., Fig. 20 and related discussion)

may preclude the accurate interpretation of BAR variations in terms of olivine and

orthopyroxene alone (e.g., McFadden et al., 2001). This possibility is also examined further in

analysis of the highest-resolution NIS data from low orbit (Bell et al., 2001).

The large depth of both Band I and Band II near the 0° longitude nose of Eros relative to the

Band depths on the rest of the asteroid (Figures 17 and 18) deserve special mention.

Gravitationally, this region experiences the greatest centrifugal forces (Veverka et al., 2000;

Thomas et al., 2001) and so could contain either the most loosely-bound or most tidally-

disrupted (Bottke et al., 1999) regolith deposits. Figure 19 shows that this part of the asteroid has

a generally lower near-infrared spectral slope than average. Combined with the increased band

depth, these observations could indicate a spectrally "fresher" and/or coarser-grained regolith

along the noses of this irregularly-shaped object compared to the rest of the surface. However,

this region is near the edge of the LPF coverage, and the 180° longitude nose is not as well


34

covered in the dataset. Thus the data could be subject to extreme emission angle artifacts

introduced (or uncorrected) by the photometric correction procedure. This effect is also

examined further in analysis of the highest-resolution NIS data from low orbit (Bell et al., 2001).

Conclusions

Initial calibration, analysis, and interpretation of near-IR reflectance spectra of Eros

obtained with the NIS instrument during the low phase flyby of NEAR just prior to orbital

insertion has led to the following results and conclusions:

• More than 2000 spatially-resolved spectra of the northern hemisphere of Eros were

obtained by NIS, covering phase angles from ~47° down to ~1°. The data have been calibrated to

I/F and normal albedo with absolute accuracies of 5% to 10%.

• NIS northern hemisphere spectra of Eros are remarkably spectrally homogeneous. The

spectra are consistent with telescopically-derived whole disk average spectra, and show both the

1000 nm (Band I) and the 2000 nm (Band II) absorption features consistent with an olivine to

pyroxene mixing ratio of opx/(opx+ol) = 0.38±0.08.

• NIS low phase flyby spectra fall primarily within the S(IV) to upper S(III) fields of the

Gaffey et al. (1993) S-asteroid classification scheme and exhibit Band I and Band II properties

most similar to those of petrographic type L ordinary chondrite meteorites. Data calibration and

spectral parameterization uncertainties preclude a unique petrographic identification, however,

and both LL and H type ordinary chondrites could still be considered statistically possible

matches.

• Spectral modeling of NIS spectra shows that removal of typical spectral signatures

associated with only small amounts of darkening/reddening agents like glass or metal produces


35

"unweathered" Eros spectra that are similar to the spectra of ordinary chondrite meteorites.

• Spectral variations across the northern hemisphere of Eros are extremely weak, suggesting

a strikingly compositionally and/or mineralogically homogeneous uppermost regolith. This lack

of variability may be indicative of a homogeneous composition throughout the asteroid, and/or it

could indicate the presence of a pervasive darkening/reddening agent like nanophase Fe, formed

from exposure of the regolith to the space environment.

• The subtle spectral variability that can be detected includes stronger Band I and Band II

absorptions in association with large topographic slopes in and near large craters and along one

"nose" of the asteroid, and weak variations in the Band II/Band I area ratio (BAR) between the

large impact craters Psyche and Himeros. The band depth variations may suggest that there are

different surface exposure ages on Eros associated with downslope movement of materials, and

the BAR variations could be indicative of very subtle (few %) variations in the olivine to

pyroxene ratio of surface materials.

Acknowledgements. The NIS low phase flyby involved many complex, elegant, and

somewhat risky spacecraft maneuvers and instrument sequences, and we are indebted to the APL

Mission Operations staff and JPL Navigation Team for helping us to make it such a success. In

particular, we thank Bob Farquhar, Mark Holdridge, Karl Whittenburg, Gene Heyler, David

Dunham, Jim McAdams, Bobbie Williams, and Bill Owen for many hours of calculating,

checking, and double checking to make sure the science team got exactly what we wanted during

the flyby. We also thank Keith Peacock and Hugo Darlington for their critical assistance with

NIS calibration and data reduction, and R. Clinite for his assistance with NIS spectral mapping.

We are grateful to Ed Cloutis for providing us with some of his olivine, pyroxene, and metal


36

mixture spectra for use in our linear mixing analysis. We also thank Ed Cloutis and Rick Binzel

for detailed reviews and critical comments on an earlier version of this paper.


37

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44

Table 1. February 13-14 2000 NIS Low Phase Flyby Observation Timeline

Time Sequence Observation phasea Int. time Range Resolution Mirror(UTC) METb Start Name Description (°) (sec) (km) (km/0.38°) Start

2000-044:07:03 125835667 GA Global Map Narrow 49.40 16 905.15 6.00 1732000-044:07:19 125836559 GB Latitude Scan 47.16 16 768.02 5.09 1542000-044:14:53 125863799 GC Global Map Narrow 44.42 16 633.16 4.20 1852000-044:15:14 125865059 GD Latitude Scan 43.07 16 591.85 3.93 1682000-044:17:09 125871959 GD Latitude Scan 41.21 16 528.72 3.51 1682000-044:19:09 125879159 GE Full Nixel Test 38.77 varies 475.41 3.15 1722000-044:20:04 125882459 GF Global Map Narrow 37.94 16 457.39 3.03 1972000-044:20:26 125882459 GG InGaAs Optimized Lat Scan 34.60 64 403.53 2.68 1752000-044:23:24 125895179 GH Global Map Narrow 30.22 16 349.30 2.32 2962000-044:23:54 125896379 GI InGaAs Optimized Lat Scan 25.51 64 309.66 2.05 1992000-045:01:49 125903159 GJ Global Map Narrow 19.73 16 272.75 1.81 2952000-045:02:35 125905919 GJ 5x3 MPP Observationc n.a.d 16 260.93 1.73 2982000-045:02:47 125906639 GJ Global Map Narrow 14.89 16 247.68 1.64 2952000-045:03:33 125909399 GJ 5x3 MPP Observation 11.99 16 236.66 1.57 2982000-045:03:45 125910119 GJ Global Map Narrow 8.85 16 225.23 1.49 2942000-045:04:38 125913359 GJ 5x3 MPP Observation 5.66 16 212.52 1.41 2982000-045:04:50 125914019 GK 5x3 MPP Observation 4.10 16 208.95 1.39 2982000-045:04:54 125914277 GK 5x3 MPP Observation 3.74 16 207.59 1.38 2982000-045:04:59 125914535 GK 5x3 MPP Observation 2.90 16 206.26 1.37 2982000-045:05:03 125914793 GK 5x3 MPP Observation 2.54 16 204.95 1.36 2982000-045:05:07 125915051 GK 5x3 MPP Observation 2.07 16 203.66 1.35 2982000-045:05:12 125915309 GK 5x3 MPP Observation 1.89 16 202.40 1.34 2982000-045:05:17 125915639 GK 5x3 MPP Observation 2.46 16 200.82 1.33 2982000-045:05:21 125915897 GK 5x3 MPP Observation 2.78 16 199.62 1.32 2982000-045:05:26 125916155 GK 5x3 MPP Observation 3.79 16 198.44 1.32 2982000-045:05:30 125916413 GK 5x3 MPP Observation 4.44 16 197.28 1.31 2982000-045:05:34 125916671 GK 5x3 MPP Observation 5.45 16 196.16 1.30 2982000-045:05:39 125916929 GK 5x3 MPP Observation n.a. 16 195.06 1.29 2982000-045:05:49 125917559 GL Global Map Wide 11.07 16 187.78 1.25 292

aPhase angle quoted is for the middle of the observation.bMET = Mission Elapsed Time. MET = 0 is defined as 17 Feb. 1996 20:43:30.628 UT.cMPP = Minimum Phase Point observation, focused on lowest phase angle regional coverage.dn.a. = not applicable because NIS slit pointed off the asteroid.


45

Table 2. NIS Spectral Parameters

Parameter Name Description1 Normal Albedo (An) Calibrated I/F data, photometrically-corrected to i=e=α=0° using the

model of Clark et al. (2001)2 Band I center Minimum of 3rd to 5th order polynomial fit of linear continuum-

removed spectrum in the 1000 nm region3 Band I depth 1.0 minus the value of the continuum-removed spectrum at the

wavelength of the Band I center4 Band II center Minimum of 3rd to 5th order polynomial fit of linear continuum-

removed spectra in the 2000 nm region5 Band II depth 1.0 minus value the of the continuum-removed spectrum at the

wavelength of the Band II center6 Near-IR Spectral Slope Change in I/F divided by change in wavelength for spectra that have

been normalized to 1.0 at NIS channel 1 (816 nm)7 Band Area Ratio (BAR) Ratio of integrated area of Band II between its continuum points to

integrated area of Band I between its continuum points

Table 3. Eros Average±1σ Spectral ParametersParametera This work Murchie & Pieters (1996) Average Normal Albedo at 902 nm 0.234±0.034 ---Band I Continuum slopea 0.58±0.15 0.368±0.015Band I center (nm) 971±13 969±15Band I depth (%) 17.2±0.8 19.0±1.0Band II Continuum slopeb 0.55±0.16 ---Band II center (nm) 1994±19 2000±100Band II depth (%) 11.3±0.5 ---Band II/Band I area ratio 0.79±0.19 0.62±0.12opx / (opx + ol)c 0.38±0.08 0.3±0.1 aParameters calculated using NIS-only data corrected to the full laboratory wavelength range using the empirical

corrections from Equations (2) to (4) described above.bDefined here as ∆An/∆λ (with λ in µm) in the merged MSI+NIS spectrum in Fig. 10 scaled to 1.0 at 560 nm.cUsing the Cloutis et al. (1986) olivine to pyroxene ratio mixing calibration, as modified by McFadden et al. (2001).

Table 4. Mineral Abundances and Composition Input to Model Shown in Figure 23.Olivine (Fo70) 62 wt .%Orthopyroxene (En77) 15 wt .%Anorthite 7 wt .%Troilite 10 wt .%Fe-Ni metal 5 wt .%Grain size 70 microns


46

Figure 1. Schematic of the NEAR/NIS Low Phase Flyby observation circumstances. Thespacecraft passed through the Eros-Sun line at approximately 05:15 UTC on 14 February 2000.Not to scale.


47

Figure 2. Schematic of the position of the NIS instrument on the NEAR spacecraft. NIS ismounted on one corner of the science deck of the spacecraft. The +Z axis is defined as thepointing direction of the NEAR solar panels, seen at top and viewed edge-on. The NIS scanmirror is able to direct the instrument's field of regard over a 140° span. Scan mirror position 75,corresponding to a 30° mirror angle, is directed along the +X' axis, which is the nominalboresight for the instruments when nadir pointed. Mirror position 0 is used for measurements ofthe NIS calibration target, which reflects sunlight obliquely into the spectrometer. The shadedsection of the NIS mirror scan range show the mirror positions used during the Low Phase Flybyperiod, with mirror position ~300 corresponding to observations at ~0° phase angle.


48

Figure 3. (A) Example of a NIS global mosaic from sequence GF (Table 1). Figure is fixed inthe frame of reference of the asteroid, so the relative rotation between NEAR and Eros during thetime of the mosaic acquisition makes the mosaic look skewed. Arrows show 1) spacecraft slewdirection, 2) mirror scan direction, and 3) Eros rotation direction. (B) Global mosaic fromsequence GH obtained during the "2001 maneuver" (see text). Arrows the same as in (A).


49

Figure 4. Example of NIS fields of view obtained during a Latitude Scan from sequence GD(Table 1).

Figure 5. NIS aperture positions during the full Nixel test sequence.


50

Figure 6. Example of NIS fields of view obtained during the minimum phase point (MPP) scanin sequence GK.


51

Figure 7. Simple cylindrical projection map showing NIS spectral coverage of Eros during theLow Phase Flyby part of the NEAR mission on February 13 and 14, 2000. Coverage was limitedprimarily to the northern hemisphere because of the flyby trajectory and Eros season at the timeof the encounter. This map shows the total number of spectra occurring within each of the22,540 plates of the shape model of Thomas et al. (2001). Maximum coverage (~140 spectra perplate) is concentrated at high northern latitudes. Coverage near the equator is limited to only asmall number of spectra per plate, and those are typically at very high incidence and emissionangles. NIS coverage is overlaid onto a sketch map of major geologic features (Thomas et al.,2001) and the MSI photometrically-normalized base map of Bussey et al. (2001). Gray areassouth of the equator and elsewhere indicate regions of no NIS coverage.


52

Figure 8. Examples of NIS LPF footprints from (a) Sequence GA, near 49° phase angle; (b)Sequence GJ, near 20° phase angle, and (c) sequence GK, near 2° phase angle. The Eros shapemodel is from Thomas et al. (2001).


53

Figure 9. Simple cylindrical projection map showing minimum phase angle for coverage duringthe LPF on February 13 and 14, 2000. This map shows the lowest phase angle measured withineach of the 22,540 plates of the shape model of Thomas et al. (2001). The Minimum Phase Point(MPP) region can be identified here as the large region of phase angle coverage below 3° atlongitudes between 270° to 360° and latitudes between +15° to +80°. Nearly all of the northernhemisphere was covered by NIS at phase angles below 30°. NIS coverage is overlaid onto asketch map of major geologic features (Thomas et al., 2001) and the MSI photometrically-normalized base map of Bussey et al. (2001). Gray areas south of the equator and elsewhereindicate regions of no NIS coverage.


54

Figure 10. Average NIS spectrum from the low phase flyby dataset. 722 spectra of the northernhemisphere of Eros were photometrically corrected to normal albedo (Clark et al., 2001) andaveraged (the average excluded the sequence GE "full nixel test" spectra all obtained from thesame region of the asteroid). The dashed line labeled "M&P Telescopic Average" is the averageEros telescopic spectrum from Murchie and Pieters (1996). The solid boxes are the average MSIspectrum obtained during the late 1998 Eros flyby (Veverka et al., 2000). The open triangleslabeled "C&M JHK Average" are the telescopic JHK photometry results of Chapman andMorrison (1976). Error bars shown on the NIS spectrum include instrumental and calibrationuncertainties as well as uncertainties in the photometric modeling. The average spectrum wasscaled to 1.0 at 902 nm to facilitate comparison with the telescopic and MSI spectra.


55

Figure 11. (a) Change in observed NIS radiance factor (I/F) at 900 nm as a function of phaseangle for the NIS low phase flyby (LPF) dataset. (b) Same data as in (a), but after thephotometric correction of Clark et al. (2001) has been applied to convert the data to normalalbedo (radiance factor at 0° phase angle). The asterisk at 0° phase angle is the normal albedo at900 nm of the average NIS LPF spectrum in Figure 10. See text for details.


56

Figure 12. Band I center wavelength vs. Band II center wavelength for (a) I/F spectra withoutphotometric correction, and (b) Normal albedo spectra after application of the Clark et al. (2001)photometric correction. The crosses indicate the average 1σ uncertainty in each parameter foreach data point in the cloud.

Figure 13. Band I depth vs. Band II depth for (a) I/F spectra without photometric correction, and(b) Normal albedo spectra after application of the Clark et al. (2001) photometric correction. Thecrosses indicate the average 1σ uncertainty in each parameter for each data point in the cloud.


57

Figure 14. (a) Band I depth vs. phase angle; (b) Band II depth vs. phase angle. Both of thesebands decrease in strength at lower phase angles, consistent with the theoretical predictions ofGradie and Veverka (1981, 1986). Viewing geometry variations appear to explain much of thesubtle variability in band strengths seen in Figure 13.


58

Figure 15. (a) Correlation between continuum slope across Band I and Band II within the NISLPF dataset. (b) Band I continuum slope vs. photometrically-corrected I/F (normal albedo) at900 nm. (c) Band I continuum slope vs. photometrically-corrected Band I depth.


59

Figure 16. Normal albedo map of Eros at 900 nm from NIS low phase flyby observations.Albedo was generated using the photometric correction routine of Clark et al. (2001). Theaverage albedo value for each plate in the shape model is shown, overlaid onto a sketch map ofmajor geologic features (Thomas et al., 2001).


60

Figure 17. (a) Map of 1000 nm (Band I) depth from NIS low phase flyby observations. (b) Mapof 2000 nm (Band II) depth from NIS low phase flyby observations. The average band depthvalues for each plate in the shape model are shown, overlaid onto a sketch map of major geologicfeatures (Thomas et al., 2001).


61

Figure 18. Depth of Band I (Figure 17a) in the NIS low phase flyby data, projected onto the Erosshape model of Thomas et al. (2001) and viewed from different directions. The MSI base map ofBussey et al. (2001) has also been draped over the shape model to provide geologic context.Areas of gray are regions lacking NIS low phase flyby coverage.


62

Figure 19. As in Figure 16, except for the slope of the continuum across Band I.


63

Figure 20. Plots of Band Area Ratio (BAR) vs. Band I center wavelength for (a) NIS low phaseflyby spectra in photometrically-uncorrected I/F units, and (b) photometrically-corrected NISnormal albedo, corrected using the model of Clark et al. (2001). The NIS data are plotted overthe fields defined by Gaffey et al. (1993) for olivine-rich meteorites (OL); pyroxene-rich basalticachondrite meteorites (BA); and ordinary chondrites (OC), which also provide the definition ofthe S(IV) asteroid class. H, L, and LL show the general locations of the OC field occupied bythose specific petrographic grades (Gaffey and Gilbert, 1998). The dashed line shows theolivine-orthopyroxene mixing line of Cloutis et al. (1986). The crosses indicate the average 1σuncertainty in each parameter for each data point in the cloud.


64

Figure 21. Map of BAR values derived from photometrically corrected NIS low phase flybydata and the 22,540 plate shape model of Thomas et al. (2001). The average BAR value for eachplate in the shape model is shown, overlaid onto a sketch map of major geologic features(Thomas et al., 2001) and the MSI photometrically-normalized base map of Bussey et al. (2001).


65

Figure 22. Examples of linear mixing models results attempting to match the average NIS LowPhase Flyby (LPF) spectrum. The NIS spectrum (crosses) is the LPF average spectrumphotometrically corrected to i=30°, e=0° viewing geometry (Clark et al., 2001), for comparisonto laboratory spectra measured at similar geometry. The endmember spectra include an intimatemixture of 50% olivine (Fa10Fo90) and 50% meteoritic metal (MIX261 from Cloutis et al. 1990),Orthopyroxene (En55Fs41Wo4), troilite, and anorthite. The olivine+metal, opx, and FeS mixture(solid line) does a good job of matching the approximate Band I and Band II center positions anddepths, but not the overall spectral shape or spectral slope. The second mixture, with anorthiteadded and artificially reddened by an additional 50% over the NIS wavelength range, provides acloser match to the data, but still does not fit Band I or Band II particularly well.


66

Figure 23. Near-infrared spectrum of the LL6 ordinary chondrite Manbhoom (crosses) and amodel fit to the Manbhoom spectrum as described in the text (solid line). The model spectrumreproduces the major features of the chondrite spectrum, including overall reflectance, spectralslope, and detailed band structure.


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Figure 24. (a) Spectra of LL6 ordinary chondriteManbhoom (crosses), Eros (squares) and modelspectra of a Manbhoom-like assemblage withincreasing amounts of sub-microscopic iron (solidlines). The solid line closely overlaying theManbhoom spectrum includes no modelsubmicroscopic iron and is the same as the model fitshown in Fig. 23. In descending order,the four solid lines with decreasing reflectanceinclude 0.05, 0.1, 0.2 and 0.4 wt.% submicroscopiciron. These model spectra suggest that nanophaseiron alone, presumably accumulated during a spaceweathering process, cannot account for thedifference between the spectrum of LL ordinarychondrites and Eros. (b) Like (a) except for modelspectra of a Manbhoom-like assemblage withincreasing amounts of dark component. The darkcomponent has a flat 5% reflectance, a 5 µm grainsize, and the solid curves represent 0, 1, 2 and 4wt.% dark material added. This suggests that simpledarkening of an LL-chondrite, for example viashock, cannot account for the difference betweenLL-chondrites and the spectrum of Eros. (c)Spectrum of a Manbhoom-like assemblage with acombination of nanophase iron and a 5% darkcomponent to match the reflectance and slopeof Eros. This match includes 0.16 wt.%submicroscopic iron and 2 wt.% dark component.The model reflectance and slope are similar to thoseof Eros, but the spectral contrast is much lower. (d)Spectrum of a Manbhoom-like assemblage with acombination of nanophase iron and a 5% darkcomponent to match the reflectance and slopeof Eros, with the grain size of the Manbhoom-likeassemblage adjusted to match the spectral contrastof Eros. In this case the grain sizes of the modelsilicates are 200 µm, the abundance of nanophaseiron is 0.2 wt.% and the abundance of the darkcomponent is 1.8 wt.%. The match to the Erosspectrum is not formally within the noise, butgenerally reproduces the features of the Erosspectrum.

Documents

Near-IR Reflectance Spectroscopy of 433 Eros from the NIS