Visible and near-infrared reflectance spectral analysis model and the applied approachfor Asteroid.
Tokuhiro Nimura
It is believed that because most asteroids were destroyed in the process of evolution
and remained small, they did not experience as strong erosion, thermal metamorphism,or remelting as the Earth, Mars, or Moon did.

By examining asteroidal mineralogy

and the degrees of secondary processes such as aqueous alteration, thermal
metamorphism, and space weathering, the initial distribution of materials in the Solar
System and the environment (temperature, pressure, etc.) they experienced can beunderstood, and constraints can be given to the Solar System formation model
calculations.
Visible and near-infrared reflectance spectroscopy has been a useful method forremotely detecting mineralogy of planetary surface materials. However, there havebeen two problems in the analysis.
(1) On airless bodies such as asteroids, there exists a phenomenon called space
weathering which is a process of alteration due to exposure to the harsh space environment
including solar wind and micrometeorite bombardments (Fig.1.).

Their surface reflectancespectra show reddened continua, lowered albedos, and attenuated absorption features[Pieters et al., 1993], which makes it more difficult to analyze their spectra(Fig.2.).

Fig.1. Transmission electron microscopebright-field image of a lunar grain witha npFe0-rich coating.

Fig. 2. Space weathering effect on visible andnear-infrared reflectance spectra. Space
weathering alters the surfaces of airless bodies insuch a manner that their reflectance spectra show
lowered albedos (I), reddened continua (II), andattenuated absorption features (III) [Pieters et al.,1993]. Thus, it becomes more difficult to use
important diagnostic features for detecting theircomponent minerals and chemical compositions.

(2) In the reflectance spectra of solid planetary surfaces, we often find that eachcomponent mineral shows multiple broad bands which overlap with one another andwith those of other minerals, making it very difficult to deconvolve them and assign thedeconvolved bands into individual mineral components(Fig. 3).

Fig. 3. Visible and near-infrared spectra of olivine (Fa#=11, Fo#=89) [Sunshine and Pieters, 1998], low-Ca
pyroxene (Fs#=30, En#=70) [Kilma et al., 2007] and high-Ca pyroxenes (Wo#=40, Fs#=30, En#=30), andplagioclase [Pieters, 1996], plotted in green, blue, red, and black lines, respectively. Each mineral species has
multiple absorption bands indicated with arrows.

The purpose of this study is to solve these two problems.
First, this study made a progress toward solving the problem (1) by modeling the light-scattering
property of a regolith particle having a vapor coating containing nanophase reduced iron (npFe0)
particles, including not only the effect on absorption coefficient as in Hapke [2001] but also thechange in boundary reflectivities.

This model can provide reasonable estimates of the volume0

concentration of npFe particles and thickness of the coating layer from the visible and near-infrared
reflectance spectra even in cases such as lunar regoliths wherein their reflectance spectra change
drastically just by the difference in the degree of space weathering even if their compositions are
similar to one another (Fig. 4.).
1dw

Hostparticle

dhDQSdw

npFe 0QT
Fig. 4. A schematic view of a space-weathered particle. Denoted as D, dh, and dw are the diameters of the wholeparticle, the host particle, and the weathering layer, respectively. Inset: Transmission electron microscopebright-field image of a lunar plagioclase grain with a npFe0-rich coating [Noble, 2002]. Denoted as QS and QT arescattering and transmitting efficiencies, respectively.

Next, the absorption spectra of silicates were studied.

As a method of deconvolving thecomplex absorption spectra of silicates into individual absorption bands, the modified
Gaussian model (MGM) is commonly used [Sunshine et al., 1990] (For example Fig. 5.).
In this study, we investigated the relationships between the chemical composition (Fe,Mg, and Ca content) and the absorption band parameters (band center, width, andrelative strength) of major rock-froming minerals: olivine, low-Ca pyroxene, andhigh-Ca pyroxene, and also determined the band center, width, and relative strength ofplagioclase (For example Fig. 6.). These relationships were utilized in MGM
calculations.

In this way, we solved the problem (2).
Band 3

Band 2
Band 1

Band 3Fig. 5. MGM fits of olivine absorption coefficient
spectra. In each plot, measured spectrum isplotted in a red line, calculated fit pectrum in adashed blue line, continuum background in adashed black line, and modified Gaussians in blue
(M1 site), green (M2 site), orange (M1), and blacklines. A red line in the top portion represents theerror spectrum in the fitting process.

Band 2
Band 1

Fig. 6. Band centers of three olivine absorption
bands. Bands 1 and 3 are assigned to the M1
site, and the band 2 the M2 site.

Utilizing the above two models and a mineral mixing model, we have constructed aunified model for estimating the mineral assemblage, chemical compositions of the component
minerals, mineral grain size, and the degree of space weathering from the visible andnear-infrared reflectance spectrum of a given airless celestial body.
This new model has been applied to the visible and near-infrared reflectance spectra ofasteroids 6 Hebe, 433 Eros, and 25143 Itokawa (For example Fig. 7.).

The results

indicate that their surface compositions correspond to those of H chondrites (6 Hebe)and LL chondrites (433 Eros and 25143 Itokawa) which are abundant in our meteorite
collections.

Although such results had been also given by past studies, this study has

also determined their Mg numbers, mineral mixing ratios of four major minerals ofolivine, low-Ca pyroxene, high-Ca pyroxene, and plagioclase, mineral grain sizes, andthe degree of space weathering from their visible and near-infrared reflectance spectra
only (Table 1). These model calculations have demonstrated that there are differencesin the thickness of vapor coating layer and the concentration of npFe0 in the coatinglayer among the asteroids. The former indicates that airless celestial bodies having
boulders and coarse regoliths may be more space-weathered than those having fineregoliths. The reason behind this may be that the surface of larger asteroids having
stronger gravity are gardened and renewed more frequently. The latter is a new findingthat the Fe concentration in the impact vapor varies among the three asteroids, which
can be explained by the difference in the metallic iron abundance on these ordinarychondrite like asteroid surfaces. This is consistent with the fact that H chondrites
contain significantly more metallic iron than LL chondrites, and the recognition that433 Eros which is much larger than 25143 Itokawa should have a regolith which is fineenough to separate metallic iron particles from the remaining silicates in the LL
chondrite mineral assemblage and the separated metallic iron may be concentrated on
the top of the regolith, which may constitute the ponds on 433 Eros observed by NEAR
spacecraft.

Fig. 7. Model fit of the reflectance spectra of asteroids 6 Hebe [Chapman and Gaffey, 1979; McFadden 1984], Ineach plot, observed and model spectra are shown in red diamonds and a green line. The blue line shows themodel spectrum with space weathering effect removed. The top black squares represent the residual error
spectrum.
Table 1. Results of analyzing reflectance spectra of asteroids. Parameters dh, dw, φw, and Φw denote the diameter ofhost material, thickness of space weathering layer, and volume fraction of npFe0 in this layer, , respectively. The
Mg# is defined as Mg 100/(Mg+Fe). OL : LCP : HCP : PL denotes the volume ratio of olivine, low-Ca pyroxene,
high-Ca pyroxene, and plagioclase.
6 Hebe433 Eros25143 Itokawa

25143 Itokawa(Telescopic)(Telescopic)(Telescopic)(Hayabusa NIRS)

dh (µm)40405060dw (nm)6798Mg#

83

7575

75

OL : LCP : HCP : PL

43:39:6:12

58:19:5:18

62:21:5:11

58:21:6:15

Meteorite type

HLL

LL

LL

This study is a new, ambitious attempt to determine the composition of an unknown mixture ofminerals from its reflectance spectrum by significantly reducing the number of unknown variables
through restricting the visible and near-infrared absorption coefficient spectra of the end member
minerals utilizing Gaussians with the characteristics of mineral absorption bands built in and by
simultaneously applying a mineral mixing model and a space weathering model.

This is the firstattempt in the world and is a method having a potential of continuing to develop in the future.