- Email: [email protected]
Midinfrared spectra and optical constants of bulk hematite: Comparison with particulate hematite spectra
Icarus 211 (2011) 839–848
Contents lists available at ScienceDirect
Icarus journal homepage: www.elsevier.com/locate/icarus
Midinfrared spectra and optical constants of bulk hematite: Comparison with particulate hematite spectra A.C. Marra a,⇑, M.D. Lane b, V. Orofino c, A. Blanco c, S. Fonti c a
Istituto di Scienze dell’Atmosfera e del Clima, Consiglio Nazionale delle Ricerche, U.O.S. Lecce, Strada Provinciale Lecce-Monteroni Km 1.200, 73100 Lecce, Italy Planetary Science Institute, 1700 E. Fort Lowell, Suite 106, 85719 Tucson, AZ, USA c Dipartimento di Fisica, Università del Salento, Via per Arnesano, 73100 Lecce, Italy b
a r t i c l e
i n f o
Article history: Received 26 February 2010 Revised 15 September 2010 Accepted 23 September 2010 Available online 7 October 2010 Keywords: Infrared observations Mars Mineralogy Spectroscopy
a b s t r a c t Hematite is an iron oxide that is very important for the study of climatic evolution of Mars. It can occur in three forms: nanophase (dark purple), fine-grained (red) and coarse-grained (gray). In a previous work, we studied the influence of particle size and shape on the infrared spectra (in the wavelength range 6.25–50 lm) of submicron red hematite particles and found that bulk optical constants did not fit the spectra of very fine particles with several classes of models. In the present paper, we derive bulk optical constants of a sample of the same parent material of hematite already used in a previous work in order to determine the particulate optical constants. As a first result we find that, also in this case, bulk and particulate optical constants are different from each other. Furthermore, we show that these bulk optical constants, although derived starting from the same parent material of hematite and used with a model adopting the laboratory measured grain size distribution of the sample, cannot be used to reproduce the spectra of submicron particles. Our results can help the scientific community to appropriately model the contribution of hematite submicron grains to the martian dust for a better understanding of the geologic evolution of the planet. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Among the materials of martian interest, hematite is certainly, for various reasons, one of the most important. Hematite is a ferric oxide (a-Fe2O3), naturally occurring in three forms, mainly depending on the particle size of the samples (Lane et al., 1999): nanophase (dark purple, <0.01 lm-diameter), fine-grained (red, diameter between about 0.01 and 10 lm), and coarse-grained (gray, >10 lmdiameter). On Earth the red variety is widespread; together with goethite and other phases, it is one of the main components of rust, which forms readily whenever iron is exposed to air and is oxidized. Pure crystalline hematite particulate samples with grain sizes between 0.01 lm and 10 lm (hereafter, normal red hematite samples) exhibit reflectance spectra that are saturated (near zero) in the violet and ultraviolet and steeply increase throughout the visible spectral range. This behavior explains the typical red color of this kind of hematite. In addition, superimposed on this steep slope, three Fe3+ crystal-field bands are present at the wavelengths of about 0.53, 0.63 and 0.86 lm. All laboratory and observational works indicate that the oxide component of bright soil deposits on Mars appear to consist largely
⇑ Corresponding author. E-mail address: [email protected] (A.C. Marra). 0019-1035/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2010.09.021
of nanocrystalline dark purple hematite with a smaller amount of admixed red hematite (Morris et al., 1989; Morris and Lauer, 1990). The minor amounts of red hematite in the dusty regions of Mars do not preclude the occurrence of more abundant red hematite in some other regions of Mars. In addition, in some scattered spots of the martian surface hematite particles with size even greater than those of normal red hematite (typically >10 lm) are present. This kind of coarser-grained hematite has a spectral behavior, both in the visible and in the infrared (IR), that is very different from that of red hematite (Lane et al., 1999). For this reason, due to its color, it is called gray hematite. Christensen et al. (2000, 2001) found evidence for a large deposit of gray hematite in a low albedo area in the equatorial Sinus Meridiani region of Mars as well as in Valles Marineris, Aram Chaos, Aureum Chaos and a few other minor areas (Glotch and Christensen, 2005; Glotch and Rogers, 2007; Weitz et al., 2008; Roach et al., 2010). Detectable crystalline gray hematite regions larger than 100 km2 are lacking elsewhere on Mars from 60°N to 60°S (Christensen et al., 2001). The presence of gray hematite in Sinus Meridiani has been confirmed, based on its thermal infrared spectral signatures, by the Mini-TES spectrometer onboard the rover MER-Opportunity, landed in that region (Christensen et al., 2004), and by the Mössbauer instrument on the rover arm (Klingelhöfer et al., 2004). According to the previous discussion, red hematite is an important component of martian surface materials and, due to the strong
840
A.C. Marra et al. / Icarus 211 (2011) 839–848
Fig. 1. Comparison between our emissivity spectra of bulk hematite converted to reflectance via Kirchhoff’s law and those by Querry (1985). The upper panel shows the ordinary ray, the lower panel the extraordinary ray spectra.
winds blowing on the surface, it is probable that hematite is also present in the atmospheric dust, making the skies appear reddish. For this reason the knowledge of the optical constants of hematite will be very useful for modeling the surficial sediment and the airborne dust on Mars.
2. Previous results For radiative transfer computations needed to model martian spectra, optical constants serve as the basis for the calculation of the necessary absorption and scattering efficiencies for different size and shape distributions of the dust particles. In a previous paper (Marra et al., 2005, hereafter Paper I), we calculated, following the approach described in previous papers (Orofino et al., 1998, 2002; Marzo et al., 2004), the optical constants of a particulate sample of hematite with an average grain size of about 0.1 lm. The sample (HMMG1) analyzed in that work was kindly provided by R. Morris at NASA JSC and it was previously studied by Lane et al. (2002). The procedure, which uses both Mie theory and Lorentz dispersion theory, allows the derivation of the dielectric constant starting from transmission measurements and it has been applied successfully by the Astrophysics Group of the
University of Salento in order to calculate the complex refractive index of various minerals, such as calcite and gypsum (Orofino et al., 1998, 2002; Marzo et al., 2004). In Paper I, synthetic extinction spectra of hematite particles were calculated using the optical constants of bulk hematite derived by Onari et al. (1977) and Querry (1985). The results of these calculations clearly indicated that both position and spectral contrast of the main features of the experimental extinction spectrum of particulate hematite cannot be well reproduced by means of the bulk optical constants and Mie theory (see Fig. 3 in Paper I). These results are not surprising, since SEM images of our sample showed irregular grain shapes, so that the classical Mie theory, which is the theory of light scattering by a sphere, cannot be used to accurately calculate the extinction properties of our hematite particles. For the case of hematite when small irregularly-shaped particles are produced by mechanical grinding, a model based on a collection of randomly oriented ellipsoids (continuous distribution of ellipsoids – CDE) can reproduce the experimental extinction spectrum better than the Mie theory spherical approximation (see Fig. 4 in Paper I). It is worthwhile to note that the extinction cross section per unit volume (Cext(k)/V) spectra for CDEs were calculated within the Rayleigh limit and this can explain some differences between the experimental spectrum and the synthetic ones. In any
A.C. Marra et al. / Icarus 211 (2011) 839–848
841
Fig. 2. Comparison between our experimental and modeled reflectance spectra of bulk hematite. The upper panel shows the ordinary ray, the lower panel the extraordinary ray spectra.
case, in the part of the spectral range where the Rayleigh limit is valid, one can state that CDEs reproduce the spectral behavior of fine-grained non-spherical hematite better than Mie theory for spherical particles. Although Paper I showed that this strategy was promising, the resulting CDE model-lab fits were not yet satisfactory. In this paper, we investigate whether better results can be obtained by using optical constants derived from both bulk and particulate samples of the same parent materials. The main goal of the present work is to calculate the optical constants of bulk hematite starting from emission measurements performed on the same sample (HMMG1) previously studied by Lane et al. (2002). In this way we shall use these data in order to reproduce the extinction spectra of finer hematite particles coming from the same parent bulk sample.
3. Bulk optical constants of hematite Hematite is a uniaxial crystal which crystallizes into the trigonal system, whose optic axis corresponds to the crystallographic c-axis. Perpendicular to the c-axis are three radial a axes. The dielectric constants of bulk hematite must therefore be measured
for two principal polarizations of the incident light, namely one with the electric vector in any direction perpendicular to the c-axis (the so-called ordinary ray – O ray), and the other with the electric vector along the crystalline c-axis (the extraordinary ray – E ray). In this work we use emissivity measurements of the hematite sample HMMG1 acquired by one of us (MDL) using the thermal emission laboratory at Arizona State University and presented previously in Lane et al. (2002). We convert these emissivity (e) data into hemispherical reflectance (R) according to Kirchhoff’s law (R = 1 e) and then we calculate optical constants by means of an approach described and validated for uniaxial crystals by Wenrich and Christensen (1996) and Lane (1999). 3.1. Emissivity spectra of hematite The hematite sample for this study was analyzed in thermal emission at ambient pressure using Arizona State University’s Mars Space Flight Facility. The spectrometer is a modified Nicolet Nexus 670 E.S.P. FT-IR interferometric spectrometer attached to an external glove box containing a temperature-stabilized sample chamber. This spectrometer is equipped with a thermoelectrically stabilized DTGS detector and a CsI beam splitter that allows the measurement of emitted radiation over the midinfrared range
842
A.C. Marra et al. / Icarus 211 (2011) 839–848
Fig. 3. Comparison between the optical constants of the ordinary ray of hematite, obtained in the present work and by other authors (real part in the upper panel, imaginary part in the lower panel).
from 5 to 45 lm (2000 to 220 cm1). To reduce and maintain the amount of atmospheric water and CO2 vapor inside the spectrometer, external sample chamber, and glove box (and to reduce the degradation of the hydrophilic CsI beam splitter) the entire system is continuously purged with air scrubbed of water and CO2. Details of the data calibration are presented in Christensen and Harrison (1993), Wenrich and Christensen (1996), and Ruff et al. (1997). The hematite sample HMMG1 has been described in detail by Lane et al. (2002). The measurements analyzed in this paper were performed on a cube 1 cm on a side. Two opposite faces are natural ‘‘c’’ faces (normal to c-axis) and they have a natural ‘‘mirror’’ finish. The other four faces, obtained by sawing the single crystal hematite perpendicular to the ‘‘c’’ faces, are approximately perpendicular to the ‘‘c’’ faces and have a planar but visibly dull finish. However, we believe the faces all to be smooth relative to thermal infrared wavelengths. For each measurement, the cube was placed in a sample cup, heated from below to a setpoint of 80 °C until equilibrium was reached. The actual surface temperature of the cube whose radiance is measured by the detector was derived for each sample and was approximately 10–20 °C less than the 80 °C setpoint adjusted to the bottom of the cube. Multiple spectral analyses of each face were obtained during the study. Each spectral analysis pro-
duced a spectrum that was an average of 160 scans of the sample cube at 2 cm1 spectral sampling. The best of these spectra (as determined by their spectral contrast) were selected for averaging to produce the two spectra required to derive the E ray according to Wenrich and Christensen (1996) and Lane (1999), where
E ray ¼ 2 ða axisÞ c axis The c-axis spectrum (i.e., the spectrum acquired of the hematite crystal c-face) was determined by averaging the three best spectra, and the a-axis spectrum (i.e., the spectrum acquired of the cube faces perpendicular to the c-faces) was determined by averaging the two best spectra. The O-ray spectrum is equivalent to the c-axis spectrum. The emissivity spectrum of the ordinary ray has three broad bands centered at 17.7, 21.6 and 31.4 lm (565, 463 and 318 cm1, respectively), while that of the extraordinary ray has only two bands at 18.0 and 30.5 lm (556 and 328 cm1). Once converted to reflectance by means of Kirchhoff’s law, we compared our spectra to those reported by Querry (1985). The comparison is quite encouraging, since our derived reflectance spectra are in good agreement with the original reflectance spectra by Querry (1985) (see Fig. 1).
843
A.C. Marra et al. / Icarus 211 (2011) 839–848
Fig. 4. Same as in Fig. 3, but for the extraordinary ray of hematite.
3.2. Dispersion analysis and derivation of optical constants
nk ¼
Dispersion theory consists of interpreting the vibrations of a crystal lattice as due to the vibrations of one or more classical harmonic oscillators. Each oscillator is described by three parameters: its characteristic band strength 4pq, frictional force (i.e., damping coefficient) C, and position k. A further parameter, e1, the high-frequency dielectric constant, is related to the bulk property of the mineral – i.e., the polarizability of the electrons with the atomic centers rigidly fixed (Spitzer and Kleinman, 1961). Dispersion theory has been widely applied in order to determine the optical constants of minerals (Roush et al., 1991 and references therein). The method (see Marra, 2005) consists in determining the oscillator parameters and using them in dispersion equations that link those parameters to the real (n) and imaginary (k) parts of the refractive index. In the literature there are slightly different expressions of the dispersion equations. Here we adopt those suggested by Roush et al. (1991), which are
2 2 2 n 4 p q k k k X j j 2 n2 k ¼ e1 þ 2 j¼1 k2 k2j þ C2j k2 and
n X j¼1
ð2Þ
Here, each oscillator j is described by its characteristic wavelength kj, width Cj, and strength 4pqj, while e1 is the high-frequency dielectric constant. By substituting kj with 1/mj, k with 1/m and Cj with cj/mj in Eqs. (1) and (2), one obtains the same equations given by Spitzer and Kleinman (1961). The derived values for the optical constants for the O and E rays may be inserted into the simplified Fresnel equation to calculate reflectivity R as a function of wavelength for specular surfaces. For normal incidence, the Fresnel equation has a particularly simple expression: 2
R¼
ðn 1Þ2 þ k 2
2
ðn þ 1Þ þ k
ð3Þ
Reflectivity is then converted to emissivity e according to Kirchhoff’s law:
e¼1R ð1Þ
2pqj k3 Cj 2 k2 k2j þ C2j k2
ð4Þ
The results of the fit procedure are shown in Fig. 2, where our measured and modeled reflectance spectra are compared. The oscillator parameters used to fit the experimental spectra are listed in Table 1.
844
A.C. Marra et al. / Icarus 211 (2011) 839–848
Table 1 Best-fit oscillator parameters of ordinary and extraordinary rays of hematite.
mi (cm1)
Ci (lm)
4pqi
Ordinary ray (e1 = 6.05) 1 19.11 2 22.88 3 33.82
523 437 256
0.822 0.735 1.314
0.072 0.195 0.700
Extraordinary ray (e1 = 5.02) 1 19.41 2 34.31
515 291
1.806 2.037
0.135 0.741
Oscillator
ki (lm)
These values are different from those reported by Onari et al. (1977) and Glotch et al. (2006), and are not directly comparable because they use a different expression for the dispersion equations. In any case, even if converted for comparison, the values of the dispersion parameters differ because they have been obtained starting from different experimental spectra. Unfortunately the paper by Querry (1985) does not report the values of the oscillator parameters, so no comparison is possible in this case. However, apart from some discrepancies in the height and wavelength position, probably due to different samples and preparation techniques, the derived optical constants are in good agreement with each other (see Figs. 3 and 4).
On the contrary, even if calculated for the same parent material, bulk optical constants of hematite are very different from the particulate ones; in particular, the spectral contrast of the bands at about 35 lm (286 cm1) is greatly reduced for the refractive index of the particulate sample as compared to the bulk refractive index (see Fig. 5).
4. Model-lab comparison As already mentioned, the main purpose of this study is to try to reproduce the experimental extinction spectrum of fine hematite particles (repeating all the measurements described in Paper I), by means of the bulk optical constants of the same parent material. The experimental approach, described in detail in Paper I, is here briefly described. In order to obtain very fine grains of hematite, we ground our sample in an agate mill and we also performed wet sedimentation. The analysis of images, taken in our laboratory by means of a Philips XL20 Scanning Electron Microscope, indicates an average size of hematite particles of about 0.1 lm. All transmission measurements were performed by means of the Perkin Elmer Spectrum 2000 in the spectral range 6.25–50 lm (1600– 200 cm1).
Fig. 5. Comparison between the optical constants of bulk hematite, obtained in the present work, and those obtained for particulate hematite in Paper I (real part in the upper panel, imaginary part in the lower panel).
A.C. Marra et al. / Icarus 211 (2011) 839–848
845
Fig. 6. Size distribution of our particulate hematite sample. The average radius of the grain distribution is about 0.16 lm, while rmin ffi 0.07 lm and rmax ffi 1.02 lm.
Before going on with the theoretical work, we checked with great accuracy the grain size of the particulate sample (previously simply estimated by SEM analyses, as already stated), by means of a Malvern Mastersizer 2000 Laser Diffractometer. This instrument measures the particle size distribution of a sample, based on a volume or a number percentage. In this case we show only the results concerning the number distribution, since the extinction measurements are more sensitive to this parameter. The results of these measurements, reported in Fig. 6, indicate that the average radius of the distribution is about 0.16 lm. By means of the diffractome-
ter, we have also determined the minimum (rmin) and maximum (rmax) radius of the particles: we found rmin ffi 0.07 lm and rmax ffi 1.02 lm. First, starting from the previous derived bulk optical constants, we attempted fitting the extinction spectrum by means of Mie theory, under the simple assumption that hematite particles are homogeneous spheres with a radius of 0.16 lm. We obviously know that this is not true, because, as already observed in Paper I, the hematite particles produced by mechanical grinding are very irregularly shaped. To the best of our knowledge, this is the first
Fig. 7. Experimental extinction cross section of hematite grains compared with those calculated by means of the optical constants of bulk hematite derived in this study, for spheres, for a flat CDE and a peaked CDE. In the case of spheres, calculations were performed by means of Mie theory for particles with an average radius of 0.16 lm.
846
A.C. Marra et al. / Icarus 211 (2011) 839–848
Fig. 8. SEM image of the hematite particles of the sample under study.
study where extinction spectra of hematite are modeled by means of bulk optical constants calculated starting from the same sample from which fine particles were obtained (a similar test has been performed only by Fabian et al. (2001) for olivine samples). As expected, the results of this calculation, shown in Fig. 7, are not good. The synthetic spectrum obtained for spheres shows features whose profiles and peak strengths are quite different from the laboratory data of small particles. Moreover, as stressed by Fabian et al. (2001) for olivine spectra, the continuum obtained from transmission measurements is enhanced compared to the values based on the reflection measurements in regions where the mass absorption coefficients are small, because of particle agglomeration/boundaries in the matrix or a non-optimal calibration of the scattering of the pellet (Fabian et al., 2001). Because SEM images show particles with various shapes (see Fig. 8), we consider two continuous distributions of ellipsoids in the Rayleigh limit as an approximation of shape distribution of
the hematite sample particles. The continuous distributions of ellipsoids are characterized either by equal probability of all shapes (the so-called flat CDE) or by a greater probability of spherical shape of particles (the so-called peaked CDE). As it is evident in Fig. 7, CDE calculations reproduce the experimental spectra much better than the spherical approximation given by Mie theory. In particular, flat CDE calculations qualitatively agree with the bands at 18 lm (556 cm1), while peaked CDE results agree relatively well with the experimental spectrum in the region of the 21 lm (476 cm1) and 29 lm (345 cm1) bands. We already mentioned that Cext(k) spectra for CDEs have been calculated in the Rayleigh limit and this can explain some differences between the experimental spectrum and the synthetic ones. Moreover, even if CDEs are more realistic than spheres in the attempt to reproduce the experimental spectra, they are always artificial shape distributions, very different from the real grains present in mineral samples. For this reason, the following step in order to improve our results was to look for other shape distributions of particles, which could reproduce the experimental spectra of hematite grains better than Mie theory or CDE approximations. In this respect an aid is provided by a T-matrix approach, which allows us to attempt to reproduce the spectra by means of particle shapes other than spheres or ellipsoids. A very comprehensive treatment of the theory can be found in Waterman (1965, 1971), Mishchenko et al. (2002) and references therein. The T-matrix approach is one of the most powerful and widely used tools for accurately computing scattering by single and compounded particles. In this work we used a Fortran T-matrix code (available at the web site http://www.giss.nasa.gov/’crmim) for computing the far-field scattering and absorption characteristics of a polydisperse ensemble of randomly oriented, homogeneous, rotationally symmetric particles. After an accurate analysis of our samples, we decided to perform our calculations for spheroids, which are formed by the rotation of an ellipse about its minor axis (oblate spheroid) or major axis (prolate spheroid). The shape and size of spheroids can be conveniently specified by the axis ratio la/lb (with la > lb for oblate spheroids and la < lb for prolate spheroids) and by
Fig. 9. Experimental extinction cross section of hematite particles compared with those calculated for a peaked CDE and for two log-normal distributions of spheroids with different aspect ratios, starting from the experimental bulk optical constants of the same parent material.
A.C. Marra et al. / Icarus 211 (2011) 839–848
the radius rs of a sphere having the same surface area. The code also allows us to use several types of particle sizes distributions. In our tests we used a log normal size distribution, whose behavior is very similar to that measured by the granulometer (see Fig. 6). As far as the shape distribution is concerned, we tried to reproduce the experimental spectrum by means of different ratios of the spheroids axes. It is important to note that similar results are obtained by using both oblate and prolate spheroids. For this reason in Fig. 9 only results for oblate spheroids are shown. Results become worse when the axis ratio increases. In fact, if we compare the simulation for spheroids with la/lb = 1.3 and la/lb = 2.0, it is evident that the increasing axis ratio produces a double maximum for the band at 18 lm (556 cm1), while the band at 30 lm (333 cm1) is reproduced neither for maximum position nor shape.
847
tory measurements on a real particulate sample (Huffman, 1977; Bohren and Huffman, 1983; Jurewicz et al., 2003). Indeed, although such quantities are not, strictly speaking, characteristic of the material (since they vary from sample to sample), they have the advantage to reproduce, by means of the simple models currently used, the extinction spectra of submicron particles. Actually, different grain samples of the same material have different particulate optical constants. However, the discrepancies are generally much smaller than those between bulk and particulate optical constants. Therefore, the latter represent a practical way to obtain reliable extinction spectra of grains, until new models will be able to take into account additional effects such as surface modes, packing, porosity, and various different shape distributions.
Acknowledgments 5. Conclusions Red hematite is an important component of martian surface materials (Morris et al., 1989) and, probably also of the atmospheric dust, making the skies appear reddish. For this reason the knowledge of the optical constants of hematite will be very useful for modeling the surficial sediment and the airborne dust on Mars. To the best of our knowledge, this is the first study where extinction spectra of hematite are modeled by means of bulk optical constants derived from the same parent material from which the fine particles were obtained. In the case of hematite, where small and very irregularlyshaped particles are produced by mechanical grinding, application of a model based on a collection of irregularly-shaped particles can reproduce the experimental extinction spectrum better than the Mie theory spherical approximation. As it is evident in Fig. 9, our attempt to use a more realistic shape distribution of the grains failed, as evidenced by the best results being obtained by using a peaked CDE rather than a distribution of spheroids. Moreover, because a decreasing value of the axis ratio means that spheroids are very close to spheres, for which the axis ratio is obviously 1, and because our best results for spheroids are obtained with a small axis ratio, this makes us more confident in the peaked CDE approach. However, it is important to note that there are other possibilities for grain shapes that have not yet been tried. CDE models and T-matrix codes are not the only option or highest level of sophistication for modeling irregularly-shaped particles. As an example a fractal or different agglomeration code (see Levasseur-Regourd et al., 2007 and references therein) used to model astrophysical dust might give improved results. In any case the bulk optical constants of hematite, even if obtained from the same parent material from which the particulate sample is obtained, do not seem able to reproduce satisfactorily the spectra of submicron particles. The reason for this mismatch is that the models currently used are not able to take into account all the effects introduced by the radiation-matter interaction due to the particulate nature of the sample. In fact models should account for many grain properties such as different size and shape distributions, packing, porosity and the so-called surface modes (Bohren and Huffman, 1983). The latter are lattice vibrations, induced by the incoming radiation and specific to the shape and size of the grains, occurring in small particles (2pr/k < 0.1, where r is the grain radius and k is the wavelength) and often disregarded in the literature. For this reason particular caution must be exercised in extrapolating laboratory results concerning bulk samples to physical conditions where particles smaller than the wavelength are expected to occur (Bohren and Huffman, 1983). To overcome these difficulties a practical approach is to derive a sort of ‘‘effective’’ optical constants directly starting from labora-
The authors are indebted to Dr. M. Mishchenko for providing his T-matrix code. The authors also warmly thank Marcella D’Elia for SEM analysis and Ted Roush and Bob Hogan for IDL code. Thanks are extended to Phil Christensen of Arizona State University for the use of his thermal emission laboratory and to Tim Glotch and Karly Pitman for their constructive work as reviewers of the manuscript. This work has been partially supported by the Italian Space Agency (ASI) and the Euro-Mediterranean Centre for Climate Changes (CMCC).
References Bohren, C.E., Huffman, D.R., 1983. Absorption and Scattering of Light by Small Particles. Wiley, New York. Christensen, P.R., Harrison, S.T., 1993. Thermal infrared emission spectroscopy of natural surfaces: Application to desert varnish coatings on rocks. J. Geophys. Res. 98, 19819–19834. Christensen, P.R. et al., 2000. Detection of crystalline hematite mineralization on Mars by the Thermal Emission Spectrometer: Evidence for near-surface water. J. Geophys. Res. 105, 9623–9642. Christensen, P.R., Morris, R.V., Lane, M.D., Bandfield, J.L., Malin, M.C., 2001. Global mapping of martian hematite mineral deposits: Remnants of water-driven processes on early Mars. J. Geophys. Res. 106, 23873–23886. Christensen, P.R. et al., 2004. Mineralogy at Meridiani Planum from the Mini-TES experiment on the Opportunity rover. Science 306, 1733–1739. Fabian, D., Henning, T., Jager, C., Mutschke, H., Dorschner, J., Wehrhan, O., 2001. Steps toward interstellar silicate mineralogy. VI. Dependence of crystalline olivine IR spectra on iron content and particle shape. Astron. Astrophys. 378, 228–238. Glotch, T.D., Christensen, P.R., 2005. Geologic and mineralogic mapping of Aram Chaos: Evidence for a water-rich history. J. Geophys. Res. 110. E09006. doi:10.1029/2004JE002389. Glotch, T.D., Rogers, A.D., 2007. Evidence for aqueous deposition of hematite- and sulfate-rich light-toned layered deposits in Aureum and Iani Chaos, Mars. J. Geophys. Res. 112. E06001. doi:10.1029/2006JE002863. Glotch, T.D., Christensen, P.R., Sharp, T., 2006. Fresnel modeling of hematite crystal surfaces and application to martian hematite spherules. Icarus 181, 408–418. Huffman, D.R., 1977. Interstellar grains. The interaction of light with a smallparticle system. Adv. Phys. 26, 129–230. Jurewicz, A., Orofino, V., Marra, A.C., Blanco, A., 2003. Optical constants of powdered limestone obtained by taking into account the grain shapes: Applicability to martian studies. Astron. Astrophys. 410, 1055–1062. Klingelhöfer, G. et al., 2004. Jarosite and hematite at meridiani planum from Opportunity’s Mössbauer spectrometer. Science 306 (5702), 1740–1745. Lane, M.D., 1999. Midinfrared optical constants of calcite and their relationship to particle size effects in thermal emission spectra of granular calcite. J. Geophys. Res. 104, 14099–14108. Lane, M.D., Morris, R.V., Christensen, P.R., 1999. Spectral behavior of hematite at visible/near infrared and midinfrared wavelengths. In: The Fifth International Conference on Mars, LPI Contribution No. 972. Lunar and Planetary Institute, Houston. Abstract #6085 (CD-ROM). Lane, M.D., Morris, R.V., Mertzman, S.A., Christensen, P.R., 2002. Evidence for platy hematite grains in Sinus Meridiani, Mars. J. Geophys. Res. 107, 5126. doi:10.1029/2001JE001832. Levasseur-Regourd, A.C., Mukai, T., Lasue, J., Okada, Y., 2007. Physical properties of cometary and interplanetary dust. Planet. Space Sci. 55, 1010–1020. Marra, A.C., 2005. Spectral Properties of Particulate Materials of martian Interest. Ph.D. Thesis, University of Lecce, Italy.
848
A.C. Marra et al. / Icarus 211 (2011) 839–848
Marra, A.C., Blanco, A., Fonti, S., Jurewicz, A., Orofino, V., 2005. Fine hematite particles of martian interest: Absorption spectra and optical constants. J. Phys. Conf. S. 6, 132–138 (Paper I). Marzo, G.A., Blanco, A., De Carlo, F., D’Elia, M., Fonti, S., Marra, A.C., Orofino, V., Politi, R., 2004. The optical constants of gypsum particles as analog of martian sulfates. Adv. Space Res. 33, 2246–2251. Mishchenko, M.I., Travis, L.D., Lacis, A.A., 2002. Scattering, Absorption and Emission of Light by Small Particles. Cambridge University Press, Cambridge. Morris, R.V., Lauer, H., 1990. Matrix effects for reflectivity spectra of dispersed nanophase (superparamagnetic) hematite with application to martian spectral data. J. Geophys. Res. 95, 5101–5109. Morris, R.V., Agresti, D.G., Lauer, H.V., Newcomb, J.A., Shelfer, T.D., Murali, A.V., 1989. Evidence for pigmentary hematite on Mars based on optical, magnetic, and Moessbauer studies of superparamagnetic (nanocrystalline) hematite. J. Geophys. Res. 94, 2760–2778. Onari, S., Arai, T., Kudo, K., 1977. Infrared lattice vibrations and dielectric dispersion in a-Fe2O3. Phys. Rev. 16, 1717–1721. Orofino, V., Blanco, A., Fonti, S., Proce, R., Rotundi, A., 1998. The infrared optical constants of limestone particles and implications for the search of carbonates on Mars. Planet. Space Sci. 46, 1659–1669. Orofino, V., Blanco, A., Fonti, S., Marra, A.C., Polimeno, N., 2002. The complex refractive index of limestone particles: An extension to the FIR range for Mars applications. Planet. Space Sci. 50, 839–847.
Querry, M.R., 1985. Optical Constants. US Army CRDEC, Contractor Report, Aberdeen Proving Ground, Md. Roach, L.H., Mustard, J.F., Lane, M.D., Bishop, J.L., Murchie, S.L., 2010. Diagenetic hematite and sulfate assemblages in Valles Marineris. Icarus 207, 659–674. Roush, T., Pollack, J., Orenberg, J., 1991. Derivation of midinfrared (5–25 lm) optical constants of some silicates and palagonite. Icarus 94, 191–208. Ruff, S.W., Christensen, P.R., Barbera, P.W., Anderson, D.L., 1997. Quantitative thermal emission spectroscopy of minerals: A laboratory technique for measurement and calibration. J. Geophys. Res. 102, 14899–14913. Spitzer, W.G., Kleinman, D.A., 1961. Infrared lattice bands of quartz. Phys. Rev. 121, 1324–1335. Waterman, P.C., 1965. Matrix formulation of electromagnetic scattering. Proc. IEEE 53, 805–812. Waterman, P.C., 1971. Symmetry, unitarity, and geometry in electromagnetic scattering. Phys. Rev. D 3, 825–839. Weitz, C.M., Lane, M.D., Staid, M., Noe Dobrea, E., 2008. Gray hematite distribution and formation in Ophir and Candor Chasmata. J. Geophys. Res. 113. E02016. doi:10.1029/2007JE002930. Wenrich, M.L., Christensen, P.R., 1996. Optical constants of minerals derived from emission spectroscopy: Application to quartz. J. Geophys. Res. 101, 15921– 15931.
Contents lists available at ScienceDirect
Icarus journal homepage: www.elsevier.com/locate/icarus
Midinfrared spectra and optical constants of bulk hematite: Comparison with particulate hematite spectra A.C. Marra a,⇑, M.D. Lane b, V. Orofino c, A. Blanco c, S. Fonti c a
Istituto di Scienze dell’Atmosfera e del Clima, Consiglio Nazionale delle Ricerche, U.O.S. Lecce, Strada Provinciale Lecce-Monteroni Km 1.200, 73100 Lecce, Italy Planetary Science Institute, 1700 E. Fort Lowell, Suite 106, 85719 Tucson, AZ, USA c Dipartimento di Fisica, Università del Salento, Via per Arnesano, 73100 Lecce, Italy b
a r t i c l e
i n f o
Article history: Received 26 February 2010 Revised 15 September 2010 Accepted 23 September 2010 Available online 7 October 2010 Keywords: Infrared observations Mars Mineralogy Spectroscopy
a b s t r a c t Hematite is an iron oxide that is very important for the study of climatic evolution of Mars. It can occur in three forms: nanophase (dark purple), fine-grained (red) and coarse-grained (gray). In a previous work, we studied the influence of particle size and shape on the infrared spectra (in the wavelength range 6.25–50 lm) of submicron red hematite particles and found that bulk optical constants did not fit the spectra of very fine particles with several classes of models. In the present paper, we derive bulk optical constants of a sample of the same parent material of hematite already used in a previous work in order to determine the particulate optical constants. As a first result we find that, also in this case, bulk and particulate optical constants are different from each other. Furthermore, we show that these bulk optical constants, although derived starting from the same parent material of hematite and used with a model adopting the laboratory measured grain size distribution of the sample, cannot be used to reproduce the spectra of submicron particles. Our results can help the scientific community to appropriately model the contribution of hematite submicron grains to the martian dust for a better understanding of the geologic evolution of the planet. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Among the materials of martian interest, hematite is certainly, for various reasons, one of the most important. Hematite is a ferric oxide (a-Fe2O3), naturally occurring in three forms, mainly depending on the particle size of the samples (Lane et al., 1999): nanophase (dark purple, <0.01 lm-diameter), fine-grained (red, diameter between about 0.01 and 10 lm), and coarse-grained (gray, >10 lmdiameter). On Earth the red variety is widespread; together with goethite and other phases, it is one of the main components of rust, which forms readily whenever iron is exposed to air and is oxidized. Pure crystalline hematite particulate samples with grain sizes between 0.01 lm and 10 lm (hereafter, normal red hematite samples) exhibit reflectance spectra that are saturated (near zero) in the violet and ultraviolet and steeply increase throughout the visible spectral range. This behavior explains the typical red color of this kind of hematite. In addition, superimposed on this steep slope, three Fe3+ crystal-field bands are present at the wavelengths of about 0.53, 0.63 and 0.86 lm. All laboratory and observational works indicate that the oxide component of bright soil deposits on Mars appear to consist largely
⇑ Corresponding author. E-mail address: [email protected] (A.C. Marra). 0019-1035/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2010.09.021
of nanocrystalline dark purple hematite with a smaller amount of admixed red hematite (Morris et al., 1989; Morris and Lauer, 1990). The minor amounts of red hematite in the dusty regions of Mars do not preclude the occurrence of more abundant red hematite in some other regions of Mars. In addition, in some scattered spots of the martian surface hematite particles with size even greater than those of normal red hematite (typically >10 lm) are present. This kind of coarser-grained hematite has a spectral behavior, both in the visible and in the infrared (IR), that is very different from that of red hematite (Lane et al., 1999). For this reason, due to its color, it is called gray hematite. Christensen et al. (2000, 2001) found evidence for a large deposit of gray hematite in a low albedo area in the equatorial Sinus Meridiani region of Mars as well as in Valles Marineris, Aram Chaos, Aureum Chaos and a few other minor areas (Glotch and Christensen, 2005; Glotch and Rogers, 2007; Weitz et al., 2008; Roach et al., 2010). Detectable crystalline gray hematite regions larger than 100 km2 are lacking elsewhere on Mars from 60°N to 60°S (Christensen et al., 2001). The presence of gray hematite in Sinus Meridiani has been confirmed, based on its thermal infrared spectral signatures, by the Mini-TES spectrometer onboard the rover MER-Opportunity, landed in that region (Christensen et al., 2004), and by the Mössbauer instrument on the rover arm (Klingelhöfer et al., 2004). According to the previous discussion, red hematite is an important component of martian surface materials and, due to the strong
840
A.C. Marra et al. / Icarus 211 (2011) 839–848
Fig. 1. Comparison between our emissivity spectra of bulk hematite converted to reflectance via Kirchhoff’s law and those by Querry (1985). The upper panel shows the ordinary ray, the lower panel the extraordinary ray spectra.
winds blowing on the surface, it is probable that hematite is also present in the atmospheric dust, making the skies appear reddish. For this reason the knowledge of the optical constants of hematite will be very useful for modeling the surficial sediment and the airborne dust on Mars.
2. Previous results For radiative transfer computations needed to model martian spectra, optical constants serve as the basis for the calculation of the necessary absorption and scattering efficiencies for different size and shape distributions of the dust particles. In a previous paper (Marra et al., 2005, hereafter Paper I), we calculated, following the approach described in previous papers (Orofino et al., 1998, 2002; Marzo et al., 2004), the optical constants of a particulate sample of hematite with an average grain size of about 0.1 lm. The sample (HMMG1) analyzed in that work was kindly provided by R. Morris at NASA JSC and it was previously studied by Lane et al. (2002). The procedure, which uses both Mie theory and Lorentz dispersion theory, allows the derivation of the dielectric constant starting from transmission measurements and it has been applied successfully by the Astrophysics Group of the
University of Salento in order to calculate the complex refractive index of various minerals, such as calcite and gypsum (Orofino et al., 1998, 2002; Marzo et al., 2004). In Paper I, synthetic extinction spectra of hematite particles were calculated using the optical constants of bulk hematite derived by Onari et al. (1977) and Querry (1985). The results of these calculations clearly indicated that both position and spectral contrast of the main features of the experimental extinction spectrum of particulate hematite cannot be well reproduced by means of the bulk optical constants and Mie theory (see Fig. 3 in Paper I). These results are not surprising, since SEM images of our sample showed irregular grain shapes, so that the classical Mie theory, which is the theory of light scattering by a sphere, cannot be used to accurately calculate the extinction properties of our hematite particles. For the case of hematite when small irregularly-shaped particles are produced by mechanical grinding, a model based on a collection of randomly oriented ellipsoids (continuous distribution of ellipsoids – CDE) can reproduce the experimental extinction spectrum better than the Mie theory spherical approximation (see Fig. 4 in Paper I). It is worthwhile to note that the extinction cross section per unit volume (Cext(k)/V) spectra for CDEs were calculated within the Rayleigh limit and this can explain some differences between the experimental spectrum and the synthetic ones. In any
A.C. Marra et al. / Icarus 211 (2011) 839–848
841
Fig. 2. Comparison between our experimental and modeled reflectance spectra of bulk hematite. The upper panel shows the ordinary ray, the lower panel the extraordinary ray spectra.
case, in the part of the spectral range where the Rayleigh limit is valid, one can state that CDEs reproduce the spectral behavior of fine-grained non-spherical hematite better than Mie theory for spherical particles. Although Paper I showed that this strategy was promising, the resulting CDE model-lab fits were not yet satisfactory. In this paper, we investigate whether better results can be obtained by using optical constants derived from both bulk and particulate samples of the same parent materials. The main goal of the present work is to calculate the optical constants of bulk hematite starting from emission measurements performed on the same sample (HMMG1) previously studied by Lane et al. (2002). In this way we shall use these data in order to reproduce the extinction spectra of finer hematite particles coming from the same parent bulk sample.
3. Bulk optical constants of hematite Hematite is a uniaxial crystal which crystallizes into the trigonal system, whose optic axis corresponds to the crystallographic c-axis. Perpendicular to the c-axis are three radial a axes. The dielectric constants of bulk hematite must therefore be measured
for two principal polarizations of the incident light, namely one with the electric vector in any direction perpendicular to the c-axis (the so-called ordinary ray – O ray), and the other with the electric vector along the crystalline c-axis (the extraordinary ray – E ray). In this work we use emissivity measurements of the hematite sample HMMG1 acquired by one of us (MDL) using the thermal emission laboratory at Arizona State University and presented previously in Lane et al. (2002). We convert these emissivity (e) data into hemispherical reflectance (R) according to Kirchhoff’s law (R = 1 e) and then we calculate optical constants by means of an approach described and validated for uniaxial crystals by Wenrich and Christensen (1996) and Lane (1999). 3.1. Emissivity spectra of hematite The hematite sample for this study was analyzed in thermal emission at ambient pressure using Arizona State University’s Mars Space Flight Facility. The spectrometer is a modified Nicolet Nexus 670 E.S.P. FT-IR interferometric spectrometer attached to an external glove box containing a temperature-stabilized sample chamber. This spectrometer is equipped with a thermoelectrically stabilized DTGS detector and a CsI beam splitter that allows the measurement of emitted radiation over the midinfrared range
842
A.C. Marra et al. / Icarus 211 (2011) 839–848
Fig. 3. Comparison between the optical constants of the ordinary ray of hematite, obtained in the present work and by other authors (real part in the upper panel, imaginary part in the lower panel).
from 5 to 45 lm (2000 to 220 cm1). To reduce and maintain the amount of atmospheric water and CO2 vapor inside the spectrometer, external sample chamber, and glove box (and to reduce the degradation of the hydrophilic CsI beam splitter) the entire system is continuously purged with air scrubbed of water and CO2. Details of the data calibration are presented in Christensen and Harrison (1993), Wenrich and Christensen (1996), and Ruff et al. (1997). The hematite sample HMMG1 has been described in detail by Lane et al. (2002). The measurements analyzed in this paper were performed on a cube 1 cm on a side. Two opposite faces are natural ‘‘c’’ faces (normal to c-axis) and they have a natural ‘‘mirror’’ finish. The other four faces, obtained by sawing the single crystal hematite perpendicular to the ‘‘c’’ faces, are approximately perpendicular to the ‘‘c’’ faces and have a planar but visibly dull finish. However, we believe the faces all to be smooth relative to thermal infrared wavelengths. For each measurement, the cube was placed in a sample cup, heated from below to a setpoint of 80 °C until equilibrium was reached. The actual surface temperature of the cube whose radiance is measured by the detector was derived for each sample and was approximately 10–20 °C less than the 80 °C setpoint adjusted to the bottom of the cube. Multiple spectral analyses of each face were obtained during the study. Each spectral analysis pro-
duced a spectrum that was an average of 160 scans of the sample cube at 2 cm1 spectral sampling. The best of these spectra (as determined by their spectral contrast) were selected for averaging to produce the two spectra required to derive the E ray according to Wenrich and Christensen (1996) and Lane (1999), where
E ray ¼ 2 ða axisÞ c axis The c-axis spectrum (i.e., the spectrum acquired of the hematite crystal c-face) was determined by averaging the three best spectra, and the a-axis spectrum (i.e., the spectrum acquired of the cube faces perpendicular to the c-faces) was determined by averaging the two best spectra. The O-ray spectrum is equivalent to the c-axis spectrum. The emissivity spectrum of the ordinary ray has three broad bands centered at 17.7, 21.6 and 31.4 lm (565, 463 and 318 cm1, respectively), while that of the extraordinary ray has only two bands at 18.0 and 30.5 lm (556 and 328 cm1). Once converted to reflectance by means of Kirchhoff’s law, we compared our spectra to those reported by Querry (1985). The comparison is quite encouraging, since our derived reflectance spectra are in good agreement with the original reflectance spectra by Querry (1985) (see Fig. 1).
843
A.C. Marra et al. / Icarus 211 (2011) 839–848
Fig. 4. Same as in Fig. 3, but for the extraordinary ray of hematite.
3.2. Dispersion analysis and derivation of optical constants
nk ¼
Dispersion theory consists of interpreting the vibrations of a crystal lattice as due to the vibrations of one or more classical harmonic oscillators. Each oscillator is described by three parameters: its characteristic band strength 4pq, frictional force (i.e., damping coefficient) C, and position k. A further parameter, e1, the high-frequency dielectric constant, is related to the bulk property of the mineral – i.e., the polarizability of the electrons with the atomic centers rigidly fixed (Spitzer and Kleinman, 1961). Dispersion theory has been widely applied in order to determine the optical constants of minerals (Roush et al., 1991 and references therein). The method (see Marra, 2005) consists in determining the oscillator parameters and using them in dispersion equations that link those parameters to the real (n) and imaginary (k) parts of the refractive index. In the literature there are slightly different expressions of the dispersion equations. Here we adopt those suggested by Roush et al. (1991), which are
2 2 2 n 4 p q k k k X j j 2 n2 k ¼ e1 þ 2 j¼1 k2 k2j þ C2j k2 and
n X j¼1
ð2Þ
Here, each oscillator j is described by its characteristic wavelength kj, width Cj, and strength 4pqj, while e1 is the high-frequency dielectric constant. By substituting kj with 1/mj, k with 1/m and Cj with cj/mj in Eqs. (1) and (2), one obtains the same equations given by Spitzer and Kleinman (1961). The derived values for the optical constants for the O and E rays may be inserted into the simplified Fresnel equation to calculate reflectivity R as a function of wavelength for specular surfaces. For normal incidence, the Fresnel equation has a particularly simple expression: 2
R¼
ðn 1Þ2 þ k 2
2
ðn þ 1Þ þ k
ð3Þ
Reflectivity is then converted to emissivity e according to Kirchhoff’s law:
e¼1R ð1Þ
2pqj k3 Cj 2 k2 k2j þ C2j k2
ð4Þ
The results of the fit procedure are shown in Fig. 2, where our measured and modeled reflectance spectra are compared. The oscillator parameters used to fit the experimental spectra are listed in Table 1.
844
A.C. Marra et al. / Icarus 211 (2011) 839–848
Table 1 Best-fit oscillator parameters of ordinary and extraordinary rays of hematite.
mi (cm1)
Ci (lm)
4pqi
Ordinary ray (e1 = 6.05) 1 19.11 2 22.88 3 33.82
523 437 256
0.822 0.735 1.314
0.072 0.195 0.700
Extraordinary ray (e1 = 5.02) 1 19.41 2 34.31
515 291
1.806 2.037
0.135 0.741
Oscillator
ki (lm)
These values are different from those reported by Onari et al. (1977) and Glotch et al. (2006), and are not directly comparable because they use a different expression for the dispersion equations. In any case, even if converted for comparison, the values of the dispersion parameters differ because they have been obtained starting from different experimental spectra. Unfortunately the paper by Querry (1985) does not report the values of the oscillator parameters, so no comparison is possible in this case. However, apart from some discrepancies in the height and wavelength position, probably due to different samples and preparation techniques, the derived optical constants are in good agreement with each other (see Figs. 3 and 4).
On the contrary, even if calculated for the same parent material, bulk optical constants of hematite are very different from the particulate ones; in particular, the spectral contrast of the bands at about 35 lm (286 cm1) is greatly reduced for the refractive index of the particulate sample as compared to the bulk refractive index (see Fig. 5).
4. Model-lab comparison As already mentioned, the main purpose of this study is to try to reproduce the experimental extinction spectrum of fine hematite particles (repeating all the measurements described in Paper I), by means of the bulk optical constants of the same parent material. The experimental approach, described in detail in Paper I, is here briefly described. In order to obtain very fine grains of hematite, we ground our sample in an agate mill and we also performed wet sedimentation. The analysis of images, taken in our laboratory by means of a Philips XL20 Scanning Electron Microscope, indicates an average size of hematite particles of about 0.1 lm. All transmission measurements were performed by means of the Perkin Elmer Spectrum 2000 in the spectral range 6.25–50 lm (1600– 200 cm1).
Fig. 5. Comparison between the optical constants of bulk hematite, obtained in the present work, and those obtained for particulate hematite in Paper I (real part in the upper panel, imaginary part in the lower panel).
A.C. Marra et al. / Icarus 211 (2011) 839–848
845
Fig. 6. Size distribution of our particulate hematite sample. The average radius of the grain distribution is about 0.16 lm, while rmin ffi 0.07 lm and rmax ffi 1.02 lm.
Before going on with the theoretical work, we checked with great accuracy the grain size of the particulate sample (previously simply estimated by SEM analyses, as already stated), by means of a Malvern Mastersizer 2000 Laser Diffractometer. This instrument measures the particle size distribution of a sample, based on a volume or a number percentage. In this case we show only the results concerning the number distribution, since the extinction measurements are more sensitive to this parameter. The results of these measurements, reported in Fig. 6, indicate that the average radius of the distribution is about 0.16 lm. By means of the diffractome-
ter, we have also determined the minimum (rmin) and maximum (rmax) radius of the particles: we found rmin ffi 0.07 lm and rmax ffi 1.02 lm. First, starting from the previous derived bulk optical constants, we attempted fitting the extinction spectrum by means of Mie theory, under the simple assumption that hematite particles are homogeneous spheres with a radius of 0.16 lm. We obviously know that this is not true, because, as already observed in Paper I, the hematite particles produced by mechanical grinding are very irregularly shaped. To the best of our knowledge, this is the first
Fig. 7. Experimental extinction cross section of hematite grains compared with those calculated by means of the optical constants of bulk hematite derived in this study, for spheres, for a flat CDE and a peaked CDE. In the case of spheres, calculations were performed by means of Mie theory for particles with an average radius of 0.16 lm.
846
A.C. Marra et al. / Icarus 211 (2011) 839–848
Fig. 8. SEM image of the hematite particles of the sample under study.
study where extinction spectra of hematite are modeled by means of bulk optical constants calculated starting from the same sample from which fine particles were obtained (a similar test has been performed only by Fabian et al. (2001) for olivine samples). As expected, the results of this calculation, shown in Fig. 7, are not good. The synthetic spectrum obtained for spheres shows features whose profiles and peak strengths are quite different from the laboratory data of small particles. Moreover, as stressed by Fabian et al. (2001) for olivine spectra, the continuum obtained from transmission measurements is enhanced compared to the values based on the reflection measurements in regions where the mass absorption coefficients are small, because of particle agglomeration/boundaries in the matrix or a non-optimal calibration of the scattering of the pellet (Fabian et al., 2001). Because SEM images show particles with various shapes (see Fig. 8), we consider two continuous distributions of ellipsoids in the Rayleigh limit as an approximation of shape distribution of
the hematite sample particles. The continuous distributions of ellipsoids are characterized either by equal probability of all shapes (the so-called flat CDE) or by a greater probability of spherical shape of particles (the so-called peaked CDE). As it is evident in Fig. 7, CDE calculations reproduce the experimental spectra much better than the spherical approximation given by Mie theory. In particular, flat CDE calculations qualitatively agree with the bands at 18 lm (556 cm1), while peaked CDE results agree relatively well with the experimental spectrum in the region of the 21 lm (476 cm1) and 29 lm (345 cm1) bands. We already mentioned that Cext(k) spectra for CDEs have been calculated in the Rayleigh limit and this can explain some differences between the experimental spectrum and the synthetic ones. Moreover, even if CDEs are more realistic than spheres in the attempt to reproduce the experimental spectra, they are always artificial shape distributions, very different from the real grains present in mineral samples. For this reason, the following step in order to improve our results was to look for other shape distributions of particles, which could reproduce the experimental spectra of hematite grains better than Mie theory or CDE approximations. In this respect an aid is provided by a T-matrix approach, which allows us to attempt to reproduce the spectra by means of particle shapes other than spheres or ellipsoids. A very comprehensive treatment of the theory can be found in Waterman (1965, 1971), Mishchenko et al. (2002) and references therein. The T-matrix approach is one of the most powerful and widely used tools for accurately computing scattering by single and compounded particles. In this work we used a Fortran T-matrix code (available at the web site http://www.giss.nasa.gov/’crmim) for computing the far-field scattering and absorption characteristics of a polydisperse ensemble of randomly oriented, homogeneous, rotationally symmetric particles. After an accurate analysis of our samples, we decided to perform our calculations for spheroids, which are formed by the rotation of an ellipse about its minor axis (oblate spheroid) or major axis (prolate spheroid). The shape and size of spheroids can be conveniently specified by the axis ratio la/lb (with la > lb for oblate spheroids and la < lb for prolate spheroids) and by
Fig. 9. Experimental extinction cross section of hematite particles compared with those calculated for a peaked CDE and for two log-normal distributions of spheroids with different aspect ratios, starting from the experimental bulk optical constants of the same parent material.
A.C. Marra et al. / Icarus 211 (2011) 839–848
the radius rs of a sphere having the same surface area. The code also allows us to use several types of particle sizes distributions. In our tests we used a log normal size distribution, whose behavior is very similar to that measured by the granulometer (see Fig. 6). As far as the shape distribution is concerned, we tried to reproduce the experimental spectrum by means of different ratios of the spheroids axes. It is important to note that similar results are obtained by using both oblate and prolate spheroids. For this reason in Fig. 9 only results for oblate spheroids are shown. Results become worse when the axis ratio increases. In fact, if we compare the simulation for spheroids with la/lb = 1.3 and la/lb = 2.0, it is evident that the increasing axis ratio produces a double maximum for the band at 18 lm (556 cm1), while the band at 30 lm (333 cm1) is reproduced neither for maximum position nor shape.
847
tory measurements on a real particulate sample (Huffman, 1977; Bohren and Huffman, 1983; Jurewicz et al., 2003). Indeed, although such quantities are not, strictly speaking, characteristic of the material (since they vary from sample to sample), they have the advantage to reproduce, by means of the simple models currently used, the extinction spectra of submicron particles. Actually, different grain samples of the same material have different particulate optical constants. However, the discrepancies are generally much smaller than those between bulk and particulate optical constants. Therefore, the latter represent a practical way to obtain reliable extinction spectra of grains, until new models will be able to take into account additional effects such as surface modes, packing, porosity, and various different shape distributions.
Acknowledgments 5. Conclusions Red hematite is an important component of martian surface materials (Morris et al., 1989) and, probably also of the atmospheric dust, making the skies appear reddish. For this reason the knowledge of the optical constants of hematite will be very useful for modeling the surficial sediment and the airborne dust on Mars. To the best of our knowledge, this is the first study where extinction spectra of hematite are modeled by means of bulk optical constants derived from the same parent material from which the fine particles were obtained. In the case of hematite, where small and very irregularlyshaped particles are produced by mechanical grinding, application of a model based on a collection of irregularly-shaped particles can reproduce the experimental extinction spectrum better than the Mie theory spherical approximation. As it is evident in Fig. 9, our attempt to use a more realistic shape distribution of the grains failed, as evidenced by the best results being obtained by using a peaked CDE rather than a distribution of spheroids. Moreover, because a decreasing value of the axis ratio means that spheroids are very close to spheres, for which the axis ratio is obviously 1, and because our best results for spheroids are obtained with a small axis ratio, this makes us more confident in the peaked CDE approach. However, it is important to note that there are other possibilities for grain shapes that have not yet been tried. CDE models and T-matrix codes are not the only option or highest level of sophistication for modeling irregularly-shaped particles. As an example a fractal or different agglomeration code (see Levasseur-Regourd et al., 2007 and references therein) used to model astrophysical dust might give improved results. In any case the bulk optical constants of hematite, even if obtained from the same parent material from which the particulate sample is obtained, do not seem able to reproduce satisfactorily the spectra of submicron particles. The reason for this mismatch is that the models currently used are not able to take into account all the effects introduced by the radiation-matter interaction due to the particulate nature of the sample. In fact models should account for many grain properties such as different size and shape distributions, packing, porosity and the so-called surface modes (Bohren and Huffman, 1983). The latter are lattice vibrations, induced by the incoming radiation and specific to the shape and size of the grains, occurring in small particles (2pr/k < 0.1, where r is the grain radius and k is the wavelength) and often disregarded in the literature. For this reason particular caution must be exercised in extrapolating laboratory results concerning bulk samples to physical conditions where particles smaller than the wavelength are expected to occur (Bohren and Huffman, 1983). To overcome these difficulties a practical approach is to derive a sort of ‘‘effective’’ optical constants directly starting from labora-
The authors are indebted to Dr. M. Mishchenko for providing his T-matrix code. The authors also warmly thank Marcella D’Elia for SEM analysis and Ted Roush and Bob Hogan for IDL code. Thanks are extended to Phil Christensen of Arizona State University for the use of his thermal emission laboratory and to Tim Glotch and Karly Pitman for their constructive work as reviewers of the manuscript. This work has been partially supported by the Italian Space Agency (ASI) and the Euro-Mediterranean Centre for Climate Changes (CMCC).
References Bohren, C.E., Huffman, D.R., 1983. Absorption and Scattering of Light by Small Particles. Wiley, New York. Christensen, P.R., Harrison, S.T., 1993. Thermal infrared emission spectroscopy of natural surfaces: Application to desert varnish coatings on rocks. J. Geophys. Res. 98, 19819–19834. Christensen, P.R. et al., 2000. Detection of crystalline hematite mineralization on Mars by the Thermal Emission Spectrometer: Evidence for near-surface water. J. Geophys. Res. 105, 9623–9642. Christensen, P.R., Morris, R.V., Lane, M.D., Bandfield, J.L., Malin, M.C., 2001. Global mapping of martian hematite mineral deposits: Remnants of water-driven processes on early Mars. J. Geophys. Res. 106, 23873–23886. Christensen, P.R. et al., 2004. Mineralogy at Meridiani Planum from the Mini-TES experiment on the Opportunity rover. Science 306, 1733–1739. Fabian, D., Henning, T., Jager, C., Mutschke, H., Dorschner, J., Wehrhan, O., 2001. Steps toward interstellar silicate mineralogy. VI. Dependence of crystalline olivine IR spectra on iron content and particle shape. Astron. Astrophys. 378, 228–238. Glotch, T.D., Christensen, P.R., 2005. Geologic and mineralogic mapping of Aram Chaos: Evidence for a water-rich history. J. Geophys. Res. 110. E09006. doi:10.1029/2004JE002389. Glotch, T.D., Rogers, A.D., 2007. Evidence for aqueous deposition of hematite- and sulfate-rich light-toned layered deposits in Aureum and Iani Chaos, Mars. J. Geophys. Res. 112. E06001. doi:10.1029/2006JE002863. Glotch, T.D., Christensen, P.R., Sharp, T., 2006. Fresnel modeling of hematite crystal surfaces and application to martian hematite spherules. Icarus 181, 408–418. Huffman, D.R., 1977. Interstellar grains. The interaction of light with a smallparticle system. Adv. Phys. 26, 129–230. Jurewicz, A., Orofino, V., Marra, A.C., Blanco, A., 2003. Optical constants of powdered limestone obtained by taking into account the grain shapes: Applicability to martian studies. Astron. Astrophys. 410, 1055–1062. Klingelhöfer, G. et al., 2004. Jarosite and hematite at meridiani planum from Opportunity’s Mössbauer spectrometer. Science 306 (5702), 1740–1745. Lane, M.D., 1999. Midinfrared optical constants of calcite and their relationship to particle size effects in thermal emission spectra of granular calcite. J. Geophys. Res. 104, 14099–14108. Lane, M.D., Morris, R.V., Christensen, P.R., 1999. Spectral behavior of hematite at visible/near infrared and midinfrared wavelengths. In: The Fifth International Conference on Mars, LPI Contribution No. 972. Lunar and Planetary Institute, Houston. Abstract #6085 (CD-ROM). Lane, M.D., Morris, R.V., Mertzman, S.A., Christensen, P.R., 2002. Evidence for platy hematite grains in Sinus Meridiani, Mars. J. Geophys. Res. 107, 5126. doi:10.1029/2001JE001832. Levasseur-Regourd, A.C., Mukai, T., Lasue, J., Okada, Y., 2007. Physical properties of cometary and interplanetary dust. Planet. Space Sci. 55, 1010–1020. Marra, A.C., 2005. Spectral Properties of Particulate Materials of martian Interest. Ph.D. Thesis, University of Lecce, Italy.
848
A.C. Marra et al. / Icarus 211 (2011) 839–848
Marra, A.C., Blanco, A., Fonti, S., Jurewicz, A., Orofino, V., 2005. Fine hematite particles of martian interest: Absorption spectra and optical constants. J. Phys. Conf. S. 6, 132–138 (Paper I). Marzo, G.A., Blanco, A., De Carlo, F., D’Elia, M., Fonti, S., Marra, A.C., Orofino, V., Politi, R., 2004. The optical constants of gypsum particles as analog of martian sulfates. Adv. Space Res. 33, 2246–2251. Mishchenko, M.I., Travis, L.D., Lacis, A.A., 2002. Scattering, Absorption and Emission of Light by Small Particles. Cambridge University Press, Cambridge. Morris, R.V., Lauer, H., 1990. Matrix effects for reflectivity spectra of dispersed nanophase (superparamagnetic) hematite with application to martian spectral data. J. Geophys. Res. 95, 5101–5109. Morris, R.V., Agresti, D.G., Lauer, H.V., Newcomb, J.A., Shelfer, T.D., Murali, A.V., 1989. Evidence for pigmentary hematite on Mars based on optical, magnetic, and Moessbauer studies of superparamagnetic (nanocrystalline) hematite. J. Geophys. Res. 94, 2760–2778. Onari, S., Arai, T., Kudo, K., 1977. Infrared lattice vibrations and dielectric dispersion in a-Fe2O3. Phys. Rev. 16, 1717–1721. Orofino, V., Blanco, A., Fonti, S., Proce, R., Rotundi, A., 1998. The infrared optical constants of limestone particles and implications for the search of carbonates on Mars. Planet. Space Sci. 46, 1659–1669. Orofino, V., Blanco, A., Fonti, S., Marra, A.C., Polimeno, N., 2002. The complex refractive index of limestone particles: An extension to the FIR range for Mars applications. Planet. Space Sci. 50, 839–847.
Querry, M.R., 1985. Optical Constants. US Army CRDEC, Contractor Report, Aberdeen Proving Ground, Md. Roach, L.H., Mustard, J.F., Lane, M.D., Bishop, J.L., Murchie, S.L., 2010. Diagenetic hematite and sulfate assemblages in Valles Marineris. Icarus 207, 659–674. Roush, T., Pollack, J., Orenberg, J., 1991. Derivation of midinfrared (5–25 lm) optical constants of some silicates and palagonite. Icarus 94, 191–208. Ruff, S.W., Christensen, P.R., Barbera, P.W., Anderson, D.L., 1997. Quantitative thermal emission spectroscopy of minerals: A laboratory technique for measurement and calibration. J. Geophys. Res. 102, 14899–14913. Spitzer, W.G., Kleinman, D.A., 1961. Infrared lattice bands of quartz. Phys. Rev. 121, 1324–1335. Waterman, P.C., 1965. Matrix formulation of electromagnetic scattering. Proc. IEEE 53, 805–812. Waterman, P.C., 1971. Symmetry, unitarity, and geometry in electromagnetic scattering. Phys. Rev. D 3, 825–839. Weitz, C.M., Lane, M.D., Staid, M., Noe Dobrea, E., 2008. Gray hematite distribution and formation in Ophir and Candor Chasmata. J. Geophys. Res. 113. E02016. doi:10.1029/2007JE002930. Wenrich, M.L., Christensen, P.R., 1996. Optical constants of minerals derived from emission spectroscopy: Application to quartz. J. Geophys. Res. 101, 15921– 15931.